( PDF ) Seasonal variability of the heat and mass transport along the western boundary of tropical Atlantic

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UNIVERSIDADE FEDERAL DE PERNAMBUCO-UFPE

CENTRO DE TECNOLOGIA E GEOCIÊNCIAS-CTG

DEPARTAMENTO DE OCEANOGRAFIA-DOCEAN

PROGRAMA DE PÓS-GRADUAÇÃO EM

OCEANOGRAFIA

Seasonal variability of the heat and mass transport

along the western boundary of tropical Atlantic

Marcus André Silva

Recife/Brasil

2009

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Marcus André Silva

Seasonal variability of the heat and mass transport

along the western boundary of tropical Atlantic

Tese apresentada ao Programa de

Pós-graduação em Oceanografia da

Universidade Federal de Pernambuco como

requisito parcial para obtenção do título de

Doutor em Ciências, na área de

Oceanografia Física.

Orientador: Dr. Moacyr Araújo

Recife/Brasil

2009

ads:

S586s Silva, Marcus André

Seasonal variablity of the heat and mass transport along

the western boundary of tropical Atlantic / Marcus André

Silva. - Recife: O Autor, 2009.

xv, 121f.; il., gráfs., tabs.

Tese (Doutorado) – Universidade Federal de Pernambuco.

CTG. Programa de Pós-Graduação em Oceanografia, 2009.

Inclui Referências.

1. Oceanografia. 2. Atlântico tropical. 3. Correntes de

borda oeste. 4. Bifurcação da CSE. I. Título.

551.46 CDD (22. ed.) UFPE/BCTG/2009-256

Para minha filha

Mariana.

Agradecimentos

Aos meus pais, à Renata e minha filhinha Mariana pelo companheirismo,

amor e motivação que me dedicaram neste período.

Ao amigo e professor Dr. Moacyr Araújo pela sua orientação, dedicação e

principalmente por acreditar sempre no meu trabalho.

Ao Dr. Jacques Servain pelo imenso suporte dado na interpretação dos

resultados obtidos neste meu trabalho. Sem o qual este trabalho não seria o

mesmo.

Ao Dr. Pierrick Penven pela presteza em colaborar com códigos

computacionais bem como, com contribuições importantes na compreensão

dos resultados numéricos obtidos.

A todos os professores do Departamento de Oceanografia da UFPE, que

sempre me motivaram a seguir na carreira de pesquisador.

À CAPES pela concessão de Bolsa de Pesquisa, sem a qual este trabalho não

teria sido possível.

Agradeço aos meus amigos do LOFEC, Dóris, Marcelo, Patrícia, Fabiana,

Rodolfo, Isaac e Márcio, pela amizade e companheirismo.

Agradeço a Márcio e Dóris (LOFEC) pelo apoio em diferentes etapas deste

trabalho e à Miriam Calland pelo empenho e dedicação na revisão gramatical

deste documento.

Agradeço a Christina, Tiago, Pedro, Marcelo, Adilma e todos os amigos dos

momentos de descontração, que tornaram esta jornada mais alegre.

A todos os funcionários do DOCEAN, pela presteza e amizade.

E a todos aqueles que, de uma forma ou outra, contribuíram para a

realização deste trabalho.

III

Resumo

O Atlântico tropical compreendido entre 20ºN e 20ºS apresenta-se hoje como

chave para o entendimento das mudanças climáticas na Terra. Duas regiões

despertam particular interesse: A banda equatorial do Atlântico onde o

sistema de correntes interage com forçantes térmicos superficiais e

ressurgência de Ekman, como a área sudoeste do Atlântico tropical (05°S-

25°S / 20°W-47°W), onde parte da Corrente Sul Equatorial (CSE) penetra

pela borda leste e contribui com muitas das correntes de fronteira oeste ao

longo da plataforma continental brasileira. Entretanto, a variabilidade da

dinâmica nestas regiões, que se mostra importante por sua contribuição

sobre o clima da região nordeste do Brasil, apresenta-se pouco estudada. O

presente trabalho investigou estes importantes sistemas do Atlântico tropical

utilizando o ROMS (Regional Ocean Model System). A primeira área de

estudo compreendida entre 20°S-20°N e 42°W–15°E, com resolução

horizontal de 1/6º e 30 camadas sigma (que acompanham o terreno).

Variações sazonais do transporte zonal, estrutura das correntes e

distribuição da TSM (Março e Agosto) obtidos numericamente foram

avaliados e comparados com dados: de literatura, experimentais do PIRATA e

observados por satélite. Os resultados desta simulação mostram que o

modelo é capaz de reproduzir os principais aspectos da Subcorrente

Equatorial (SE), Contra-corrente Norte Equatorial (CNE), Corrente do Golfo

(CG) e os ramos central e norte dos sistemas de Corrente Sul Equatorial

(cCSE/nCSE), em diferentes seções ao longo do equador. A comparação

entre a estrutura térmica nos primeiros 500 m simulada e do Programa

PIRATA mostra uma Camada de Mistura (MLD) bem reproduzida,

particularmente, a ressurgência que induz uma MLD mais rasa verificada

nas boias mais à leste durante o inverno austral até o final da primavera

austral. A evolução sazonal do sistema Piscina Quente do Atlântico Sul

(SAWP) – Língua Fria (Cold Tongue) foi bem representado, que é importante

para futuras previsões de variabilidade climática sobre as fronteiras

continentais da parte sudoeste do Atlântico tropical. Do lado sudoeste do

Atlântico tropical (05°S-25°S / 20°W-47°W), O ROMS (Regional Ocean Model

System) foi utilizado pela primeira vez nesta área para simular a circulação

oceânica utilizando uma malha de resolução horizontal de 1/12º com 40

camadas sigma, que acompanham o terreno, para resolução vertical. Para

avaliação preliminar da configuração do ROMS adotada foram analisadas as

distribuições superficiais e verticais de temperatura, além de calculadas as

evoluções sazonais da camada bem misturada e dos balanços, atmosféricos e

oceânicos, envolvendo a troca de calor dentro da camada bem misturada. A

ordem de grandeza das componentes oceânicas (principalmente a difusão

vertical e a advecção horizontal) é da mesma ordem de grandeza dos

forçantes atmosféricos e quase sempre opostos entre si, com alguma

diferença de fase e transporte dentro das camadas mais superficiais.

Resultados de variabilidade interanual foram comparados com os primeiros

dois anos de perfis de temperatura observados advindos dos três fundeios do

programa PIRATA-SWE (Projeto PIRATA, Extensão Sudoeste). A estrutura

térmica simulada nas camadas mais superficiais do oceano está em

concordância com os resultados obtidos in situ. Resultados de simulação

apontam para uma larga e relativamente fraca CSE, composta por uma

sequência de núcleos não bem definidos e próximos a superfície. O

transporte que flui para oeste da CSE nos primeiros 400 m de profundidade

ao longo da seção que atravessa as boias PIRATA-SWE, calculado para

simulação do ROMS entre 2005-2007, apresenta um volume médio

transportado de 14,9 Sv, com um máximo observado em JFM (15,7 Sv) e um

mínimo durante MJJ (13.8 Sv). Os resultados de simulação indicam que em

2005-2007 o transporte para oeste da CSE foi modulado pela variabilidade

da componente zonal do vento. Três seções zonais, posicionadas do

continente até a posição da boia PIRATA, confirmam transporte mais intenso

da Sub-corrente do Norte do Brasil (SNB), fluindo para norte, e uma

diminuição no transporte da Corrente do Brasil (CB),que flui para sul,

durante maio de 2006 e maio de 2007, quando a bifurcação do ramo sul da

CSE alcança sua posição mais ao sul. Por outro lado, o máximo escoamento

da CB foi registrado durante janeiro de 2006, janeiro de 2007 e março de

2007, com um mínimo da SNB fluindo para norte em dezembro de 2005 e

outubro/dezembro 2006, correspondendo ao período em que a bifurcação do

ramo sul da CSE alcança sua posição mais ao norte (OND). A Elevação da

Superfície do Mar (ESM) e a Energia Cinética turbulenta (ECT) superficial

calculada a partir das simulações e dos produtos AVISO Rio05 apontam na

superfície para os mais altos níveis de energia de meso-escala ao longo do

ramo central da CSE e da SNB/CB. Resultados de modelagem ecológica

usando o modelo NPZD acoplado com o ROMS confirmam esta região como

uma área oligotrófica. Resultados do modelo ecológico são comparados com

SeaWifs dataset e a dinâmica e a produção primária são localmente

discutidos. Estes resultados preliminares disponibilizam mais informações

diante da complexidade da região de divergência da SCE e encoraja-nos a

conduzir estudos mais detalhados a respeito da dinâmica e do transporte de

massa nessa região utilizando o ROMS. Este trabalho também apresenta a

necessidade de continuação, ampliação e extensão vertical para o sistema de

observação PIRATA-SWE, especialmente com medidas de salinidade em mais

níveis de profundidade, além da instalação de medidores de correntes.

Palavras-chave: Atlântico tropical, modelagem matemática, conteúdo

térmico, bifurcação da CSE, correntes de borda oeste.

Abstract

The Tropical Atlantic comprised between 20ºN and 20ºS presents today an

important key to the knowledge of climatic changes in the world. Two areas

are of particular interest: the zonal band of the Equatorial Atlantic ocean

where the current system interacts with surface thermal forcing and Ekman

upwelling, and the Southwestern tropical Atlantic (05°S-25°S / 20°W-47°W),

where part of the South Equatorial Current (SEC) enters at its eastern

border, feeding many western boundary currents along the Brazilian

continental shelf. However, the long-term variability of the dynamics in these

regions, which is also important for their contribution to the climate over

northeastern Brazil, is largely unknown. This present work investigated

these important systems of the South tropical Atlantic using the Regional

Ocean Model System (ROMS). The first study area was comprised between

20°S-20°N and 42°W–15°E, with an horizontal grid resolution of 1/6º and 30

vertical sigma (terrain-following) layers. Seasonal variations of zonal

transport, current structure and SST distribution (March and August)

obtained numerically were evaluated and compared with: literature, PIRATA

array and satellite data. Results show that the model is capable of

reproducing the mean features of the EUC/NECC/GC and cSEC/nSEC

systems, at different sections along the equator. The comparison between the

simulated thermal structure, upper 500 m ocean layer, and PIRATA Program

dataset shows a very well reproduced Mixing Layer Depth (MLD). In

particular, the upwelling induced shallower MLD verified in eastern buoys

during austral winter to late spring. The seasonal evolution of the

Southwestern Atlantic Warm Pool (SAWP) – Cold Tongue equatorial system

was quite represented, which is important for future forecasting of climate

variability along the continental boundaries in Southwestern tropical

Atlantic. The second part of this work has the first use of ROMS to simulate

the ocean circulation in the Southwestern tropical Atlantic. This modelling

used a isotropic horizontal grid resolution of 1/12

and 40 sigma layers. For

preliminary evaluation of this ROMS configuration, surface and vertical

thermal structures were explored. Indeed, seasonal evolutions of the surface

mixed layer, atmospheric and oceanic balances involving the mixed layer

heat budget were calculated. The magnitude of oceanic components (mainly

the vertical diffusion and the horizontal advection) is in the same order as

the atmospheric forcing, and practically always opposes to it, with some local

and seasonal timing differences and mass transports within the upper levels.

Interannual variability results were compared with the first two-year series

of observed thermal profiles derived from the three PIRATA-SWE moorings.

The simulated thermal structure in the upper ocean layers agrees well with

in-situ data. ROMS simulations point out a broad and relatively weak SEC

flow composed of a sequence of more or less defined near-surface cores. The

westward SEC transport for the upper 400 m along the PIRATA-SWE section,

calculated from the ROMS simulation for 2005-2007, shows an average

volume transport of 14.9 Sv, with a maximum observed in JFM (15.7 Sv),

and a minimum during MJJ (13.8 Sv). ROMS results indicate that the 2005-

2007 seasonal near-surface westward SEC transport is modulated by the

zonal wind variability. Three zonal sections extending from the American

continent to the PIRATA buoy sites confirm that stronger northward NBUC

transport and decreasing BC transport were achieved during May 2006 and

May 2007, i.e. at the time the sSEC bifurcation reached its southernmost

position. On the other hand, the maximum southward BC flow was verified

during January 2006, January 2007 and March 2007, with a minimum

northward NBUC flow in December 2005 and October/December 2006,

corresponding to the period when the sSEC bifurcation reached its lowest

latitude (OND). Sea Surface Height (SSH) and the surface Eddy Kinetic

Energy (EKE) derived from simulations and AVISO Rio05 product point out

the highest surface meso-scale activity (EKE ≥ 50 cm

-2

) along the cSEC

and NBUC/BC patches. Results of an ecological modelling using NPZD

model coupled with the ROMS confirm this region as an oligotrophic area.

NPZD outputs were compared with SeaWifs dataset and dynamic and

primary production are locally discussed. Preliminary results provide

additional ingredients in the complexity of the SEC divergence region, and

encourages us to conduct a more detailed exploration of the dynamics and

mass transport of this region using the ROMS. This also shows the need to

continue, extend, and vertically upgrade the observational PIRATA-SWE

array system, especially with more levels of salinity measurements and the

installation of current measurements.

Keywords: Tropical Atlantic, mathematical modeling, SEC bifurcation, heat

content, western boundary current.

VII

Figure List

Figure 1.1. Chart of study area with domain limited by 10°N-40°S and 20°E-

60°W with a schematic representation of mean zonal and meridional

currents: The upper ocean northward NBUC flow (red solid line), and the

deepest southward transport by the DWBC (blue dashed line). The sSEC

bifurcation at 100m depth (solid line); between 200m and 500m depth

(dashed line); between 500m and 1200m depth (pointed line), according with

Stramma and England (1999). ................................................................... 18

Figure 3.1. Region of model integrations, solid lines indicate domain of 1/6º

climatological equatorial modelling. Dashed lines indicate southwestern

tropical Atlantic domain limits of 1/12º horizontal grid. ............................. 31

Figure 4.1. Study area and model integration domain. The sections A, B and

C are dashed lines and the squares over the equator line are the PIRATA

mooring sites. Bathymetry is represented by gray shading contour. ........... 35

Figure 4.3. Zonal transport (u-component) obtained from model simulation

(ROMS) along Section A, 25°W. Dashed lines represent the isotherm 20°C.

Monthly-averaged values for March and August. ........................................ 38

Figure 4.4. Seasonal variability of EUC, NECC, SECc and SECn transports

at 25W° section, obtained in simulations. .................................................. 38

Figure 4.5. Zonal transport (u-component) obtained from model simulation

(ROMS) along Section B, 15°W. Dashed lines represent the isotherm 20°C.

Monthly-averaged values for March and August. ........................................ 40

Figure 4.6. Seasonal variability of EUC, NECC, SECc and SECn transports

at 15W° section, obtained in simulations. .................................................. 41

Figure 4.7. Zonal transport (u-component) obtained from model simulation

(ROMS) along Section C, 5°W. Dashed lines represent the isotherm 20°C.

Monthly-averaged values for March and August. ........................................ 42

Figure 4.8. Seasonal variability of EUC, NECC, SECc and SECn transports

at 5W° section, obtained in simulations. .................................................... 43



Figure 4.9. Comparison of the vertical temperature distribution provided by

ROMS (black dashed contours) with the three PIRATA buoys (colour shading)

extending from surface down to 500 m. Monthly-averaged distributions for

the PIRATA period 1997-2007. ................................................................... 44



Figure 4.10. Monthly-averaged SST obtained from ROMS model (above) and

from satellite observations (AVHRR-NOAA) (below) for March and August. . 45

Figure 5.1. Model domain (dashed lines) with the PIRATA-SWE locations

(filled triangles) and three sites located along the western boundary region

closer to the Brazilian edge (filled circles). Near shore 100m and 1000m

isobaths are plotted. .................................................................................. 48

Figure 5.2. Horizontal distribution of SST obtained from ROMS in mid-

September (a) and mid March (b). PIRATA-SWE locations (filled triangles)

and three sites located along the western boundary region closer to the

Brazilian edge (filled circles). ...................................................................... 50

Figure 5.3. Horizontal distribution of SST obtained from observed satellite

data on 15 September 2005 (a) and 15 March 2006 (b). PIRATA-SWE

locations (filled triangles) and three sites located along the western boundary

region closer to the Brazilian edge (filled circles). ........................................ 51

Figure 5.4. Comparison of the monthly mean vertical temperature

VIII

distribution (ºC) provided by ROMS (black dashed contours) with monthly

mean for the period of September 2005 to February 2008 of the three

PIRATA-SWE buoys (colour shading and white lines) extending from surface

down to 500m. The four vertical black thin lines correspond to September

, December 15

, March 15

and June 15

,. (a) 8°S-30°W; (b) 14°S-

32°W; (c) 19°S-34°W. ................................................................................. 53

Figure 5.5. September (a), December (b), March (c) and June (d) monthly

averaged vertical profiles of temperature 0-500m at the three PIRATA-SWE

sites provided by climatic ROMS and by PIRATA monthly averaged profiles

from September 2005 to February 2008 ..................................................... 55

Figure 5.6. Comparison of temporal evolution of seasonal heat content in

the mixed layer at 8°S-30°W (a), 14°S-32°W (b) and 19°S-34°W (c), provided

by PIRATA in-situ observations (dashed line) and ROMS (solid line) for the

period of September to August. .................................................................. 57

Figure 5.7. Seasonal evolution of SST (°C) (black dashed line), atmospheric

(red line) and oceanic (blue line) contributions to the local change of SST

(°C/Month) (black continuous line) provided by ROMS at 8°S-34°W (a), 8°S-

30°W (b), 14°S-38°W (c), 14°S-32°W (d), 19°S-38°W (e) and 19°S-34°W (f)

locations. ................................................................................................... 61

Figure 5.8. Mean annual volume transport averaged across three zonal

sections at 8°S (a), 14°S (b) and 19°S (c), and across the section along the

PIRATA-SWE array (d). Positive (negative) values indicated by solid (dashed)

white lines correspond to northward (southward) currents (panels 8a, 8b,

and 8c), while positive (negative) values indicated by solid (dashed) white

lines for the section along the PIRATA array correspond to eastward

(westward) currents (panel 8d). The three horizontal solid black lines indicate

the 24.5, 26.8 and 32.15 sigma-t values (in kg.m-3). ................................. 63

Figure 6.1. Comparison between ROMS-derived SST (a and c) and GHRSST-

PP-derived SST (b and d) for September 15

2005 and March 15

2006. ... 67

Figure 6.2. Comparison between simulated and observed temperature (

variations for the upper 500m from September 2005 to June 2007 for the

three PIRATA-SWE locations at (a) 08°S-30°W, (b) 14°S-32°W and (c) 19°S-

34°W. Model-derived temperature is represented by black contours and the

PIRATA-SWE observed temperature appears as white contours and shaded

colors. ....................................................................................................... 71

Figure 6.3. Comparison of monthly averaged vertical profiles of temperature

for the first 500m between the PIRATA-SWE in-situ observations (dashed

line) and the ROMS simulation (solid line) for: (a) September 2005, (b)

December 2005, (c) March 2006 and (d) June 2006. Colours are associated

to each PIRATA-SWE buoy: 08°S-30°W (blue), 14°S-32°W (black) and 19°S-

34°W (red). ................................................................................................ 73

Figure 6.4. Comparison of temporal evolution of seasonal mixed layer depth

(MLD) at: (a) 8°S-30°W, (b) 14°S-32°W and (c) 19°S-34°W, provided by

PIRATA-SWE in-situ observations (dashed line) with ROMS (solid line) for the

period of September 2005 to June 2007. .................................................... 75

Figure 6.5. Averaged (2005-2007) SSH and EKE comparisons between

ROMS simulations and AVISO Rio05 data: (a) ROMS - SSH (cm), (b) ROMS -

EKE (cm

-2

), (c) RIO05+AVISO - SSH (cm) and (d) AVISO - EKE (cm

-2

). . 77

Figure 6.6. Transport function (Sv) for the modelled seasonal averaged

(three months, 2005-2007) currents integrated from 600 m to the sea

surface for a) MJJ and b) OND. The 1000 m isobath is represented by a red

line. ........................................................................................................... 78

Figure 6.7. Model simulation of meridional current and transport values

averaged from September 2005 to July 2007 along zonal sections (0-1500 m)

at: (a) 8°S, (b) 14°S and (c) 19°S. Positive (negative) values indicated by solid

(dashed) lines correspond to northward (southward) currents. ................... 80

Figure. 6.8. Annual averaged (2005-2007) meridional current (cm s-1) and

transport (in Sv) obtained by ROMS for: (a) the along zonal section (0-1500

m) at 7°S. Positive (negative) values indicated by solid (dashed) lines

correspond to northward (southward) currents, (b) the depth averaged range

200-1000 m, where CFLOW is stronger ...................................................... 83

Figure. 6.9. Interannual variability of the North Brazil Undercurrent (NBUC)

transport along 8oS (from the surface to 400 m), and Brazil Current (BC)

transport along 19oS (from the surface to 400 m), obtained from ROMS

simulations for the period of September 2005 to July 2007. Positive

(negative) values indicated by the black (gray) line correspond to northward

(southward) transports. ............................................................................. 85



Figure. 6.10. Seasonal averaged (three months, 2005-2007) meridional

velocity (m s-1) obtained from the ROMS simulation for a) MJJ and b) OND.

The velocities are averaged within a 1° longitude band off of the Brazilian

coast. The white line is the contour of zero velocity that represents the

bifurcation of the sSEC. The white areas represent the Vitoria-Trindade

Ridge and Abrolhos Bank. ......................................................................... 87

Figure 7.1. Model domain (dashed gray lines) with the PIRATA-SWE

locations (filled triangles). Section along three PIRATA-SWE sites (solid lines).

Thin dashed lines are 100m and 1000m isobaths, respectively. ................. 94

Figure 7.2. Chlorophyll a distribution on surface to SeaWifs on: a) November

b) July and ROMS results on: c) November d) July ..................................... 95

Figure 7.3. Vertical salinity distribution of ROMS along PIRATA-SWE sites

on: a) November b) July. ............................................................................ 97



Figure 7.4. Vertical nitrate distribution of ROMS along PIRATA-SWE sites

on: a) November b) July. ............................................................................ 99



List of Abbreviations

AABW Atlantic Antarctic Bottom Water

AAIW Atlantic Antarctic Intermediate Water

AC Agulhas Current

ACC Antarctic Circumpolar Current

ACM Acoustic Current Meter

ADCP Acoustic Doppler Current Profile

AMSR-E Advanced Microwave Scanning Radiometer

BC Brazil Current

Brazilian CHM Brazilian Navy Hydrographic Center

CC Coefficient of Correlation

CLIVAR Climate Variability and Predictability Program

COADS Comprehensive Ocean-Atmosphere Data Set

cSEC central band of the South Equatorial Current

CTW Coastal Trapped Wave

CW Central Water

CW Coastal Water

DMSP Defense Meteorological Satellites Program

DWBC Deep Western Boundary Current

EOF Empirical Orthogonal Functions

ERA-40 Reanalysis of the Global Atmosphere for 45 years

ERS European Remote Sensing Satellite

ETOPO-2 Digital Terrain Data with 2’’ resolution

EUC Equatorial Undercurrent

Guinea C. Guinea Current

GC Gulf Current

GEOSAT Geodetic Satellite

GFDL Geophysical Fluid Dynamics Laboratory

GLOSS/IOC Global Sea Level Observing System

GOES Geostationary Operational Environmental Satellites

HDF-EOS Hierarchical Data Format - Earth Observing System

IBGE Institute of Geography and Statistics

IO/USP Institute of Oceanography of São Paulo University

IRD Institut de recherche pour le développement

ISV Intraseasonal Variability

ITCZ Intertropical Convergence Zone

IWBC Intermediate Western Boundary Current

MLD Mixed Layer Depth

MOC Meridional Overturning Cell

MODIS Moderate-resolution Imaging Spectroradiometer

MOM Modular Ocean Model

MSLA Mean Sea Level Anomaly

MSW Maximum Salinity Water

NADW North Atlantic Deep Water

NBC North Brazil Current

NBUC North Brazil Undercurrent

NCAR National Center for Atmospheric Research

NCEP National Centers for Environmental Prediction

NEC North Equatorial Current

nSEC northern band of the South Equatorial Current

NOAA National Oceanic and Atmospheric Administration

OCCAM Ocean Circulation and Climate Advanced Modeling

Project

PCA Principal Component Analysis

POD Proper Orthogonal Decomposition

POP Parallel Ocean Program

QuikSCAT Quick Scatterometer

RCM Recording Current Meter

RMPG Geodetic Permanent Tide Gauge Network

ROMS Regional Ocean Modeling System

SAC South Atlantic Current

SACW South Atlantic Central Water

SACZ South Atlantic Convergence Zone

SASH South Atlantic Subtropical High

XII

SBB Southeast Brazil Bight

SCF Squared Covariance Fraction

SEC South Equatorial Current

SECC South Equatorial Countercurrent

SEUC South Equatorial Undercurrent

SICC Southern Intermediate Countercurrent

SMW Salinity Maximum Water

SODA Simple Ocean Data Assimilation

sSEC southern band of the South Equatorial Current

SSM/I Special Sensor Microwave

SST Sea Surface Temperature

STC Subtropical Cell

Sv Sverdrup

SVD Singular Value Decomposition

TMI Microwave Imager

TOGA-TAO Tropical Ocean Global Atmosphere - Tropical

Atmosphere Ocean

TRMM Tropical Rainfall Measuring Mission

TSW Tropical Surface Water

TW Tropical Water

uCDW upper Circumpolar Deep Water

WOA World Ocean Atlas

ZCAS Convergence Zone of the South Atlantic

XIII

Contents

AGRADECIMENTOS ................................................................................................. III

RESUMO .................................................................................................................. IV

ABSTRACT .............................................................................................................. VI

FIGURE LIST ........................................................................................................ VIII

LIST OF ABBREVIATIONS .................................................................................. XI

CONTENTS ........................................................................................................... XIV

CHAPTER 1 ............................................................................................................ 16

1. MOTIVATION AND OBJECTIVE ................................................................ 16

CHAPTER 2 ............................................................................................................ 23

2. OBSERVATIONAL AND DERIVED DATA ................................................. 23

CHAPTER 3 ............................................................................................................ 29

3. THE MODELLING TOOLS ........................................................................... 29

CHAPTER 4 ............................................................................................................ 34

4. THE PATHWAYS AND THERMAL STRUCTURE OF SURFACE

CURRENTS IN THE EQUATORIAL ATLANTIC OCEAN .................................. 34



CHAPTER 5 ............................................................................................................ 46

5. NEAR SURFACE TRANSPORT AND HEAT BUDGET IN THE

SOUTHWESTERN TROPICAL ATLANTIC USING A HIGH-RESOLUTION

NUMERICAL MODELLING SYSTEM .................................................................. 46

CHAPTER 6 ............................................................................................................ 65

6.1. THE HIGH-RESOLVED INTERANNUAL ROMS APPROACH ................. 66

XIV

CHAPTER 7 ............................................................................................................ 92

7. REGIONAL BIOGEOCHEMICAL MODELLING IN THE

SOUTHWESTERN TROPICAL ATLANTIC ......................................................... 92

CHAPTER 8 .......................................................................................................... 100

8. CONCLUSIONS AND PERSPECTIVES .................................................... 100

REFERENCES ...................................................................................................... 108

Chapter 1 – Motivation and objective

Chapter 1

1. Motivation and objective

The equatorial Atlantic Ocean is a very complex area where zonal currents

and counter-currents coexist with strong ocean-atmosphere heat exchanges

as well as local upwelling of cold waters. It is not surprising that several

research programs have been proposed to study the transport and heat

variability in this area, as for example the Global Atmospheric Research

Program (GARP), the Atlantic Tropical Experiment (GATE), the Francais

Ocean Climat Atlantique Equatorial (FOCAL), and the Pilot Research Moored

Array in the Tropical Atlantic (PIRATA). Most of these efforts focus on air-sea

interaction processes related to climate change investigations.

The importance of sea surface temperature (SST) variability in the

tropical Atlantic, for example, as a response to the seasonal varying winds

has been the subject of numerous studies. SST in the tropics is largely

determined by upper ocean dynamics including advection. Therefore, it is

necessary to understand and model its influence in climate, as well as, to

know the surface currents and their variability (Lumpkin and Garzoli, 2005).

Different work-groups have presented results from modeling and

Chapter 1 – Motivation and objective

observations confirming that the tropical Atlantic is a critical region for

processes associated to the Meridional Overturning Circulation (MOC), such

as cross-equatorial exchanges, and sea surface temperature variability that

impacts on climate variability of the coupled tropical ocean/atmosphere

system - see Stramma and Schott (1999), Lee and Csanady (1999a,b),

Stramma et al. (2003), for an overview. In the tropical ocean, cold

thermocline waters of non-equatorial origin from both hemispheres are

transformed into warm water masses via entrainment and subsequent

surface heat flux, and then escape from this region northward as well as

southward, although the net heat transport is northward. The tropical

Atlantic warm water pool stores heat during May-October and permits heat

escape in November-April.

The warm component of the ‘‘conveyor belt’’ in the equatorial Atlantic

is sensitive to this global circulation cell, but also to smaller horizontal scale

movements related to the subtropical subduction and to the equatorial

recirculation cell, for which Ekman divergence partially compensates

geostrophic convergence (Blanke et al, 1999). According to Stramma and

Schott (1999), the water mass exchange, including net northward meridional

heat transport across the equator, occurs on three layers (surface, central

and bottom). In the surface layer, it is accomplished by warm Tropical

Surface Water (TSW). In the central layer, Antarctic Intermediate Water

(AAIW) and upper Circumpolar Deep Water (uCDW) move northward in the

upper 1200 m and is compensated by cold North Atlantic Deep Water

(NADW), which moves southward between 1200 and 4000 m. In the bottom

layer, the northward transport of Antarctic Bottom Water (ABW) also carries

a small amount of cold water into the northern hemisphere.

The main zonal currents that act on the equatorial surface layer are

the North Equatorial Current (NEC), the North Equatorial Countercurrent

(NECC), the South Equatorial Current (SEC), and the Equatorial

Undercurrent (EUC) near the equator. These standards were more recently

detailed from drifter observations as analyzed by Lumpkin and Garzoli

(2005). In the South tropical Atlantic, the SEC flows westward on the upper

ocean circulation. Molinari (1982) described the SEC as divided into three

Chapter 1 – Motivation and objective

bands in the tropical Atlantic. The northern branch of SEC was called

northern SEC (nSEC), the flow between the SEUC and the SECC is known as

central SEC (cSEC), and finally, flowing south of the SECC, the southern

SEC (sSEC). On the western boundary of the tropical Atlantic, the sSEC

bifurcates into the North Brazilian Undercurrent (NBUC) that flows

northward, and the Brazilian Current (BC) that flows southward along the

Brazilian coast. In the beginning of the northern Brazilian shelf, near 4ºS,

the NBUC joins the cSEC to form the shallower NBC. After crossing the

equator, a part of NBC retroflects and contributes to eastward flows of the

NECC and EUC. Therefore the west border of the tropical Atlantic, near 35°

W, has particular importance for the mixture of water masses because it

contains a large portion of the Atlantic Meridional Overturning Circulation

(MOC) passing through it (Schott et al., 2003).

( Veleda, 2008)

Figure 1.1. Chart of study area with domain limited by 10

N-40

S and 20

E-60

with a schematic representation of mean zonal and meridional currents: The upper

ocean northward NBUC flow (red solid line), and the deepest southward transport by

the DWBC (blue dashed line). The sSEC bifurcation at 100m depth (solid line);

between 200m and 500m depth (dashed line); between 500m and 1200m depth

(pointed line), according with Stramma and England (1999).

Inside the southwestern border of the tropical Atlantic, the Northern

Atlantic Deep Water (NADW), originated by the cold water plunged from the

North Atlantic, which runs towards the South Atlantic, are relatively well-

Chapter 1 – Motivation and objective

known (Arhan et al., 1998; Stramma et al., 2005). However, in the southern

hemisphere, the formation of shallow warm water by subduction, and its

northward transfer, to close the heat budget of MOC across the equator, is

less understood. The southern Atlantic Ocean presents prime importance for

global climate change due to the number of key zones where oceanic signals,

on various scales (from intra-seasonal to decadal), crosses through. In the

southern subtropical Atlantic basin, mainly subjected to both cyclonic and

anticyclonic gyres strongly controlled by the surface wind (Stramma and

Schott, 1999; Lumpkin and Garzoli, 2005), the following key zones can be

listed, from South Africa to South America successively:

(i) The Agulhas Current off South Africa, coming up from the

Indian Ocean and propagating northwards while skirting the

African coast;

(ii) The Benguela Current system, including the continuation

towards the north of the previous current, and the regional

cyclonic circulation within the Angola Dome region;

(iii) The south subtropical Atlantic gyre (15°S-20°S, 40°W-20°W),

corresponding to an important area of warm-salted water

formation by subduction, and feeding the South Equatorial

Current (SEC), which is itself formed by at least three easterly

branches separated by narrow regions of less evident counter-

currents (Stramma, 1991);

(iv) And finally, more to the west, near South America, the zone of

divergence of the southern SEC while approaching the

Brazilian edge.

This last region is in part at the origin of the feeding of numerous current

systems bordering the Brazilian coast, either towards the north or south.

Towards the north, the northern branch of the SEC termination forms the

North Brazilian Current-North Brazilian Under Current (NBC-NBUC) system,

one of the most powerful western boundary currents in the world. This

system participates in the feeding of a few other currents (Stramma et al.,

Chapter 1 – Motivation and objective

2005); among them, the northward Guyana Current, the eastward North

Equatorial Counter Current with its associated complex retroflection system

(Goes et al., 2005), and finally the eastward Equatorial Under Current (EUC).

It is believed that NBC accounts for approximately one-third of the net

warm-water transported across the equatorial tropical gyre boundary into

the North Atlantic, compensating for the southward export of NADW

(Stramma, 1991; Stramma and England, 1999) as previously cited. More to

the south, the southern branch of SEC forms the Brazil Current (BC)

propagating southward along the coast of Brazil meeting the Malvinas

Current (Gordon and Greengrove, 1986) at about 35°S, which is itself fed

partially by cold water coming from the Pacific Ocean via the Drake’s

passage. The latitude where the SEC bifurcation occurs is not yet very well

known, although it has been demonstrated that the North Brazil Under

Current (NBUC) originates south of 10°S (Stramma et al., 2003, 2005; Schott

et al., 2002, 2005), where a branch of the SEC flows northward and merges

within the NBUC. The BC, which flows southwesterly, merges into the

Atlantic Meridional Overturning Cell (MOC) system (Silveira et al., 1994;

Stramma et al., 1990, 1995; Schott et al., 1998) in the vicinity of the Brazil-

Malvinas confluence (Olson et al., 1988; Garzoli and Garraffo, 1989).

The interactions between sea surface temperature (SST) and the easterly

atmospheric circulation may play a significant role in local climate

fluctuations of Northeast Brazil, a region affected by intermittence of severe

droughts and floods (Moura and Shukla, 1981; Rao et al., 1993).

In the oligotrophic regions of the ocean, the supply of inorganic nutrients

to the euphotic layer may limit the concentration of microalgal biomass, the

rate of phytoplankton growth, or both (Marañón et al., 2000). Several

Mechanisms have been proposed to explain the formation and maintenance

of the deep chlorophyll maximum (DCM), that is a oceanographic feature of

tropical and subtropical oceans. Higher in-situ growth at the nutricline than

in the upper mixed layer, and acclimation of phytoplankton to low

irradiance, has been suggested (Cullen,1982) as cause of DCM formation at

subtropical gyres.

Very early reports of low productivity in the subtropical gyres

Chapter 1 – Motivation and objective

(Steeman-Nielsen and Jansen, 1957; Thomas, 1970; Eppley et al., 1973)

have been considered to be in error due to methodological inadequacies

(Fitzwater et al., 1982). Nevertheless, recent work carried out in oligotrophic

regions, using new techiniques (Malone et al. 1993; Letelier et al. 1996),

presents a large range in the rates of phytoplankton production and growth

inside these areas. In fact, this controversy will be maintained, while the lack

of knowledge about temporal and spatial variability in the biology of open

ocean is being kept (Marañón et al., 2000). Consequently, any uncertainties

in the productivity estimates in these areas will have significant effects on

the prediction of global biogeochemical models (Marañón et al., 2000). In

other hand, there are few studies about the physical processes related with

DCM formation inside the southwestern tropical Atlantic. Montes (2003)

suggests that the support of nutrient inside euphotic zone, in the

southwestern tropical Atlantic, is maintained by diffusion process in

nutricline basis.

Supported by the ideas stated above, the goal of this work is to investigate

the seasonal and intraseasonal variability of the western boundary regime of

the South tropical Atlantic. I will conduct this research following these

pathways:

(a) The use of a numerical model in the Tropical Atlantic to investigate the

seasonal and intraseasonal variability of main currents and SST along

equatorial band.

(b) The investigation of the near surface ocean heat budget in the

Southwestern tropical Atlantic from high resolved modelling results.

forcing for the intraseasonal variability of the southern branch of SEC

regime.

(d) The influence of sSEC on the distribution of phytoplankton biomass

along the southwestern boundary of the tropical Atlantic.

This manuscript is organized as follows: the next chapter brings an

Chapter 1 – Motivation and objective

overview of the study area and the used datasets. The main characteristics

of ROMS and conditions of the simulations are presented in Chapter 3.

Chapter 4 shows illustrations of the tropical Atlantic simulation, where,

simulated and observed thermal structures in upper ocean layer are

compared. Also in Chapter 4, intensities of simulated oceanic currents, as

well as the mass transport across three zonal sections along the equator are

presented. Chapter 5 presents simulated and observed thermal structures in

the upper layer to improve the climatological high resolved simulation of

Southwestern tropical Atlantic. Chapter 5 also provides a simulated

seasonal evolution of oceanic components that compound the heat content

in the mixed layer, as well as mass and heat transport across three zonal

sections and a fourth section along the PIRATA-SWE sites. Chapter 6

presents the results of an interannual simulation in the same region of the

Southwestern tropical Atlantic (seen in chapter 5). In Chapter 6 the ROMS

evaluation is performed by comparing outputs of the model with observed

high-frequency SST fields estimated by satellite, thermal structures and

mixed layer depths provided by the PIRATA-SWE dataset. As a first

dynamical application of ROMS, it shows examples of simulated variation of

mass transports across three zonal transects, and a section along the

PIRATA-SWE sites. In Chapter 7, results of biogeochemical modelling of the

oligotrophic zone in the Southwestern tropical Atlantic are presented. The

last Chapter presents the conclusion and perspectives.

Chapter 2 – Observational and derived data

Chapter 2

2. Observational and derived data

This thesis concentrates on the tropical Atlantic with specific focus at the

upper layer of the western boundary of the Southern tropical Atlantic along

the Brazilian shelf. The purpose here is to introduce the data necessary to

feed surface and open boundaries of the modelling package chosen, The

Regional Ocean Model system (ROMS). The data presented in this chapter

can describe the seasonal and intraseasonal mechanisms of the coupling

ocean and atmosphere variability. Thus, results from the ROMS simulation

were obtained to evaluate this modelling tool representing these regions.

Chapter 2 – Observational and derived data

2.1. Surface forcing datasets

2.1.1. Comprehensive Ocean-Atmosphere Data Set (COADS)

The Comprehensive Ocean-Atmosphere Data Set (COADS) is a

extensive and widely used set of sea surface data. The COADS is maintained

by U.S. inter-agency cooperation since January of 1981, formed by NOAA’s

Environmental Research Labs. (ERL), Cooperative Institute for Research in

Environmental Sciences (CIRES), NCDC and NCAR. The COADS data is

actually composed by observational data from a variety of ship cruises,

drifting and moored buoys from Canada’s Marine Environmental Data

Service (MEDS) and Pacific Marine Environmental Laboratory (PMEL) and

the National Data Buoy Center (NDBC) (Wooddruff et al., 1998)

The basic observed variables in COADS include sea surface and air

temperatures, wind, humidity (dew point or wet bulb temperature),

barometric pressure, cloudiness weather, and wave and swell fields. The

observations have been quality controlled, and monthly statistics calculated

for the period of 1854 to 1995, for resolution of 2º x 2º boxes and 1º x 1º

from 1960 to 1993 (Wooddruff et al., 1998; daSilva et al., 1994).

2.1.2. QuikSCAT daily wind data

The wind stress data were obtained from the CERSAT/IFREMER

(Centre ERS d’Archivage et de Traitement/Institut Français de Recherche

pour l’Exploitation de la Mer ) available on line at http://www.ifremer.fr/

using OPENDAP tool (http://www.opendap.org

), to be used in interannual

runs. The scatterometer winds are retrieved from Quick Scatterometer

(QuikSCAT) the version 3 (Penven et al., 2008, Liu, 2002). It consists of daily

averaged gridded values of scalar wind speed, meridional and zonal

components of the wind velocity and wind speed squared. The grid has 1.8º

latitude x 1.6º longitude resolution (Tsai et. al., 2000).

Chapter 2 – Observational and derived data

2.2. PIRATA data set

Entering now in its permanent phase, PIRATA is an ocean-

meteorological observing system which contributes to the operational climate

survey of the global ocean (Servain et al., 1998; Bourlés et al., 2008). PIRATA

is the result of an international scientific cooperation gathering Brazil,

France and the United States of America. Its main scientific motivation is to

monitor, describe and understand the two main modes of climatic variability

over the tropical Atlantic which are the equatorial and the meridional modes

(Servain et al., 1998). Based on this rational, the original array configuration

is presently composed by 10 ATLAS moorings (Hayes et al., 1991) along the

equatorial line (35°W, 23°W, 10°W and 0°E) and two meridional lines, one

along the 38°W meridian (4°N, 7°N, 12°N and 15°N), and the other along the

10°W meridian (0°N, 6°S and 10°S). Brazil and France are responsible for the

yearly sea maintenance of the array, while USA provides the ATLAS material

and process the raw data. Daily transmissions of the first 500m of ocean

temperature (11 levels) and salinity (4 levels), and the meteorological main

variables at the sea surface, are ensured thanks to the ARGOS satellite

system, and immediately available on the Web after first validation

(http://www.pmel.noaa.gov/ pirata/).

Once assured the success of the PIRATA original array (Bourlés et al.,

2008), and in order to complete the necessary set of observations to

understand more in details the seasonal and inter-annual variability over

specific areas in the tropical Atlantic, it was decided to extend the original

mooring array. Three extensions were proposed: a first one over the

southwest tropical basin (along the Brazilian coast), a second one over the

southeast basin (along the African coast, south of the Equator), and a third

one over the northern and northeast basin (again along the African coast).

Brazil took alone the responsibility of implementing the south-western

extension. Obviously, the main goal of this project, the PIRATA South West

Extension (PIRATA-SWE), is primarily to help in the forecasting of the

Brazilian climate, especially over the Northeast Brazilian region.

Three sites for the ATLAS mooring were chosen for the PIRATA-SWE

(Figure 5.1) according to scientific arguments (Nobre et al., 2005, 2008): (i)

Chapter 2 – Observational and derived data

8ºS-30ºW in connection with the 0ºN-10ºW site of the PIRATA’s backbone,

(i.e. the equatorial Atlantic warm pool vs. the cold tongue complex); (ii) 14ºS-

32ºW in connection with the SEC flow (properties and heat flux transport

variability); (iii) 19ºS-34ºW in connection with the SACZ (relations to the

oceanic heat flux changes). Obviously, the PIRATA-SWE array only addresses

the changes in the thermohaline structure within the SEC's flow into the

bifurcation region. The understanding of changes in the partition between

the NBC and the BC at the SEC bifurcation is out of the PIRATA-SWE

objectives.

The three ATLAS buoys of the PIRATA-SWE were launched during the

second half of August 2005, thus the time series analysed here cover the

first year of dataset, from September 2005 to August 2006. With the

exception of a four-month interruption of the 8°S-32°W site due to anchoring

problems (the buoy’s nylon cable sheared off after ten months of

deployment), the buoys reported daily with nearly 100% of data return. In

this study we are using the daily observed PIRATA-SWE data for temperature

(0m, 20m, 40m, 60m, 80m, 100m, 120m, 140m, 180m, 300m, 500m)

operationally obtained on the Web at the PMEL/NOAA address

(http://www.pmel.noaa.gov/pirata/).

2.3. Hydrology and biogeochemical datasets

2.3.1. WOA2005 data

The World Ocean Atlas (WOA2005) is a data product of the Ocean

Climate Laboratory of National Oceanographic Data Center (NODC). The

WOA consists a climatology with global grids at 1º spatial resolution of three-

dimensional fields (Conkright et al., 2002). The data are normally

interpolated onto 33 vertical layers from surface to seafloor. WOA fields

include eight ocean properties (temperature, salinity, dissolved oxygen,

apparent oxygen utilization, percent oxygen saturation, phosphate, silic acid

and nitrate). WOA fields are produced for annual, seasonal and monthly

time-scales. This data product is distributed with ROMSTOLS (Penven et al.,

2008) package version used in this thesis.

Chapter 2 – Observational and derived data

2.3.2. ECCO interannual model data

The ECCO Ocean Data Assimilation is a project which study seasonal

to interannual changes in ocean circulation from assimilating observations

with a global circulation model. The model assimilates altimetric data

provide from TOPEX/POSEIDON data, CTD and XBT’s profiles, current

meter, drifters, floats and air sea-flux from NCEP reanalyses. The ECCO

model domain has 46 vertical levels and a telescopic horizontal grid of 1º

covering the globe (80ºN to 80ºS) and 1/3º within 20º of the equator.

Hydrology ECCO results from 2000 to 2007 were obtained through the use

of OPENDAP tool (http://www.opendap.org) to feed the boundary and initial

conditions of interannual run of high resolution ROMS simulation of the

southwestern tropical Atlantic.

2.3.3. SeaWifs data

The SeaWifs chlorophyll a data is a derived product from images from

SeaWifs scanner and algorithms (O’ Reilly et al., 1998). The processed data

can be obtained at different time-scales (monthly, 8-day, daily). It is available

in http://las.pfeg.noaa.gov/OceanWatch/ocean_watch_safari.php

with

horizontal resolution of 0.1º. The Climatological run of biogeochemical model

used seasonal Seawifs data of Chlorophyll a to build boundary and initial

conditions is seasonal and was obtained with ROMSTOOLS (Penven, et al.,

2008).

Chapter 2 – Observational and derived data

2.4. MLD Criteria

Most criteria used to determine the MLD in the ocean require that the

deviation of the temperature T (or density,

) from its surface value be

smaller then a certain fixed value (Sprintall and Tomczak, 1990; Brainerd

and Gregg, 1995). The MLD is estimated as the depth at which density is

equal to the sea surface value plus an increment

equivalent to a desired

net decrease in temperature. For instance, Miller (1976) and Spall (1991) use

()

0125.0

σ=σΔ to determine the mixed layer depth, while Sprintall and

Tomczak (1992) and Ohlmann et al. (1996) adopt

(

T/C5.0

∂σ∂=σΔ

)

, where

∂σ∂ is the coefficient of thermal expansion. Following Sprintall and

Tomczak (1992), we evaluate the MLD in terms of temperature and density

steps ( TΔ =0.5°C and

(

)

T/C0.5

∂

°=σΔ ) from the SST and density (

(

)

and

()

σ ) obtained from the PIRATA and ROMS profiles:

()

⎟

⎠

⎞

⎜

⎝

⎛

∂

σ∂

+σ=σ= T

0zMLD

(A.1)

where

∂σ∂ is calculated as a function of the surface temperature and

salinity (Blanck, 1999).

Chapter 3 – The modelling tools

Chapter 3

3. The modelling tools

To evaluate interactions between ocean and atmosphere, its variability, and

distribution transport, inside a study area, a numerical modelling related to

low operational cost and to the possibility of studying large geographic areas

were chosen. Based on the use of a numerical modelling tool, the focus of

this work is to investigate a seasonal and intraseasonal variability of the

main transport systems, heat budget and biogeochemical cycles. In this

chapter, the Regional Ocean Modeling System (ROMS) is presented, with

more details on definitions of grids, boundary and stability conditions.

Chapter 3 – The modelling tools

3.1 The Regional Ocean Modeling System - ROMS

ROMS is an ocean model (Shchepetkin and McWilliams, 2005)

previously adapted to different regions of the world (Haidvogel et al., 2000;

Malanotte-Rizzoli et al., 2000; She and Klinck, 2000; Penven et al., 2000,

2001a,b; McCready and Geyer, 2001; Lutjeharms et al., 2003). The model

solves the free surface primitive equations in an Earth-centered rotating

environment based on the classical Boussinesq approximation and

hydrostatic vertical momentum balance. ROMS is discretized in terrain-

following vertical coordinates. The model grid, forcing, initial and boundary

conditions were built using the ROMSTOOLS package developed by the

Institut de Recherche pour le Développement (IRD) (Penven et al., 2008).

Upstream advection is treated with a third-order scheme that enhances the

solution by generating steep gradients as a function of a given grid size

(Shchepetkin and McWilliams, 1998). Unresolved vertical subgrid-scale

processes are parameterized by an adaptation of the non-local K-profile

planetary boundary layer scheme (Large et al., 1994). A complete description

of the model can be found in Haidvogel et al. (2000) and Shchepetkin and

McWilliams (2005).

3.2 Model Grids

Reproducing ocean features along the Equatorial band required a

horizontal regular grid of 341 x 245 cells with a resolution of 1/6° in an area

that goes from 20ºS to 20ºN and between 42°W and 15°E (Figure 3.1). The

vertical discretization used had 30 sigma levels (terrain-following). For

application in the Southwestern tropical Atlantic, near the NE Brazilian

coast, the integration domain used was from 25°S to 5°S and from 47°W to

20°W (dashed line in Figure. 3.1), with a isotropic 1/12° horizontal grid with

323 x 249 horizontal mesh cells and a vertical discretization of 40 levels.

Chapter 3 – The modelling tools

Figure 3.1. Region of model integrations, solid lines indicate domain of 1/6º

climatological equatorial modelling. Dashed lines indicate southwestern tropical

Atlantic domain limits of 1/12º horizontal grid.

At both presented cases, bottom topography was derived from a 2’

resolution database ETOPO2 (Smith and Sandwell, 1997). A slope parameter

20.0<∇= hhr

was used to prevent errors in the computation of the pressure

gradient (Haidvogel et al., 2000).

3.3 Boundary Conditions

At the open boundaries, an active, implicit, upstream-biased radiation

condition connects the model solution to the surrounding ocean

(Marchesiello et al., 2001). Horizontal Laplacian diffusivity inside the

integration domain is zero, and 1 degree relaxation scheme is imposed (up to

-1

) in the sponge layers near the open ocean boundaries. The model

equations are subjected to no-slip boundary conditions along the coastline.

Chapter 3 – The modelling tools

The oceanic circulation was forced on sea surface by monthly-averaged

winds, heat and fresh water fluxes derived from COADS (daSilva et al., 1994)

for the climatological simulations (Chapters 4, 5 and 7). For the interannual

run (Chapter 6) a daily-mean (2005-2007) wind stress by QuikSCAT (Liu,

2002) replaces the COADS wind dataset. To compose the lateral forcing of

climatological run, hydrography and geostrophic velocities derived from

WOA2005 (Conkright et al., 2002) were used. On the other hand, a basin-

scale daily-mean (2005-2007) hydrography and currents derived from

OGCM–ECCO (http://ecco.jpl.nasa.gov/) was used to infer thermohaline

properties and water volume exchanges at the open horizontal boundaries.

3.3 Biogeochemical model coupled to ROMS

The ROMS version used in this thesis presents the possibility of being

coupled with a biogeochemical model NCPZD (Moore, 2002). This model is

based on limitant nutrient. Its hypothesis considers twelve variables of state:

nitrate, ammonia, Chlorophyll a, phytoplankton, zooplankton, micro and

macro detritus (for nitrogen and carbon), oxygen, inorganic dissolved carbon

and pH. These variables are balanced during each time step according to

biogeochemical reactions and to horizontal and vertical advectives transport,

as well as to vertical diffusion transport calculated by the physical model.

More details about equations can be found in ROMS documentation

(http://roms.mpl.ird.fr/documentation.html).

3.4 Model Simulation

The climatological simulations ran from a state of rest during 10 years.

After a spin-up period of one year, the model had achieved a statistically

steady state. All the numerical results from these runs examined correspond

to the last year of simulation (year 10).

The interannual simulation was performed in two steps. First, the

Chapter 3 – The modelling tools

rested ocean was gradually forced by applying ramp functions to the

boundary conditions for the period Jan.-Dec. 2001. After this, the full sets of

forcing conditions for 2001-2007 were considered for the interannual

simulation. Only results from 2005-2007 were used.

Chapter 4 – The pathways and thermal structure of surface currents in the…

Chapter 4

4. The pathways and thermal structure

of surface currents in the equatorial

Atlantic Ocean

This chapter aims to use a regional well-solved circulation model to

investigate the subsurface zonal currents and the thermal structure of the

upper 500 m equatorial ocean layer. The simulations were performed with

the Regional Ocean Modeling System (ROMS), which improves the numerical

resolution of strong currents and counter-currents systems like those

observed in equatorial Atlantic.

Nowadays, the knowledge of the pathways and the dynamics of the

oceans through prediction models can contribute to solve many questions on

climate and oceanography sciences. Modeling constitutes an essential tool

for evaluation of the standings and variations presented in a complex

hydrodynamic regime, just like the equatorial Atlantic Ocean. Using the

available records of observed PIRATA data, Foltz et al. (2003) showed that

the latent heat loss and the absorbed shortwave radiation represent the

largest contributions for the seasonal SST variability in the northwest and

southeast tropical Atlantic basin. Over the equator, however, the authors

Chapter 4 – The pathways and thermal structure of surface currents in the…

suggest that the contributions from the latent heat loss are less important

than the horizontal temperature advection and the vertical entrainment.

Chaves and Nobre (2004) by observational and numerical modeling results

suggest that atmospheric variability over the southwest tropical Atlantic also

influences the SST variability locally, thus forming a coupled system with

multiple feedback processes.

4.1 The ROMS Approach

The simulation performed here consists in a meso-scale scenario that

reproduced the equatorial currents system. The model domain results in 341

x 245 horizontal mesh cells in a regular horizontal grid with a resolution of

1/6

covering an ocean area from latitude 20ºS to 20ºN and from 42ºW to

15ºE of longitude. The vertical discretization used had 30 levels. Bottom

topography was derived from a 2’ resolution database ETOPO2 (Smith and

Sandwell, 1997). At the three open boundaries (north, west and south) an

active, implicit, upstream biased radiation condition connects the model

solution to the surrounding ocean (Marchesiello et al., 2001).

Figure 4.1. Study area and model integration domain. The sections A, B and C are

dashed lines and the squares over the equator line are the PIRATA mooring sites.

Bathymetry is represented by gray shading contour.

Chapter 4 – The pathways and thermal structure of surface currents in the…

Horizontal Laplacian diffusivity inside the integration domain is zero,

and a 5-point smooth increasing is imposed (up to 800 m

-1

) in sponge

layers near open ocean boundaries. The model domain was forced at the sea

surface by winds, heat and fresh water fluxes derived from the

Comprehensive Ocean-Atmosphere Data Set (COADS) monthly fluxes data at

0.5°x0.5° resolution (da Silva et al., 1994). The database World Ocean Atlas

2001 (WOA2001), monthly climatology at 1°x1° resolution (Conkright et al.,

2002), is used to infer thermodynamics (temperature and salinity) and

geostrophic currents at the open boundaries.

4.2 Experimental dataset

The horizontal distributions of monthly-averaged Sea Surface

Temperature (SST) obtained in simulations were compared to satellite

observations. The Advanced Very High Resolution Radiometer (AVHRR)

aboard the National Oceanic Atmospheric Administration (NOAA) satellite

was employed in comparisons. The data set from AVHRR-SST has a 1/18°

resolution and the observation period was 1985-2006. This dataset is

available on <http://dods.jpl.nasa.gov/>.

4.3 ROMS Simulation Results

Using the forcing boundary conditions presented in previous section

4.1, the model ran from a state of rest during 10 years. Figure 4.2 shows a

time series of the integration domain kinetic energy (KE) from the model. It

indicates that after a spin-up period of two years, the model has achieved a

statistically steady state. After stabilization, the model had only small

oscillations, standing a mean kinetic energy around 1,4 J.m

-3

. These

oscillations represent the seasonal changes in each year. All the numerical

results correspond to the last year of simulation (year 10).

Chapter 4 – The pathways and thermal structure of surface currents in the…

Figure 4.2. Mean kinetic energy (J.m

-3

) of model (ROMS) during 10 simulation years.

4.3.1 Current structure and zonal transport

The equatorial currents obtained in numerical simulations were

identified and characterized along the three sections located at 25°W, 15°W

and 5°W (see Figure 4.1). According to Lumpkin and Garzoli (2005), the SEC

system is part of the equatorial gyre. This flow is clearly divided in two

westward jets, the SECc and the SECnc, separated at 0-2°S by a band of

weak southward flow, which is induced by the equatorial divergence.

Numerical results confirm these observations, with the westward SECc flow

occurring between 1°S–8°S, as well as the westward SECn transport near

surface in the range ~1°N–4°N, between the equator and the NECC (Figure

4.3 to 4.7).

Model results show that the eastward flow of NECC occurs at ~4°N–

8°N, with an indistinct core in function to the time of the year (Figure 4.3 to

4.7). These results corroborate with the description of Richardson and

McKee (1984). They found an eastward NECC flow close to surface in

latitudes 3°N–10°N with velocities reaching up to 0.5 m.s

-1

Chapter 4 – The pathways and thermal structure of surface currents in the…

Figure 4.3. Zonal transport (u-component) obtained from model simulation (ROMS)

along Section A, 25°W. Dashed lines represent the isotherm 20°C. Monthly-averaged

values for March and August.

The EUC was present in simulations below the equator line on all

ocean basin. These results agree with the findings of Stramma and Schott

(1999), which state that the EUC crosses the entire Atlantic, feeding more or

less the equatorial upwelling.

Figure 4.4. Seasonal variability of EUC, NECC, SECc and SECn transports at 25W°

section, obtained in simulations.

Chapter 4 – The pathways and thermal structure of surface currents in the…

Numerical results presented in Figure 4.3 show at 25°W (section A,

Figure 4.1) a core of the EUC located closer to sea surface in March (~80 m,

velocity of 0.8 m.s

-1

) than in August (~100m, velocity of 1.0 m.s

-1

). Giarolla et

al. (2005) pointed out that the EUC is shallower during January to May and

deeper in other parts of the year. By the same time, the sea measurements of

Stramma et al (2003), obtained during March-April 2000 in a 23°W section

show a EUC core velocity of 0.75 m.s

-1

at ~80 m depth. Simulation results

are also in good agreement with the recent analysis of Brandt et al. (2006).

These authors measured a EUC core (0.7 m.s

-1

) positioned at 85 m along the

26°W section. The associated EUC transport calculated from model results

also presented a good fit to values estimated from sea measurements. While

Brandt et al. (2006) found 13.8 Sv (1 Sv ≡ 10

-1

) in the section 26°W,

ROMS model pointed out 12.3 Sv (March) and 15 Sv (August) at the 25°W

section (Figure 4.3).

The two main equatorial SEC branches were sharply defined in

simulations, separated by the EUC. Numerical velocities of SECc reach -0.5

m.s

-1

and SECn reach -0.6 m.s

-1

in March. In August the SECc and the

SECn were more extended, with velocities reaching -0.2 m.s

-1

. s

Measurements (Brandt et al., 2006) at 26°W furnished westward transports

of -4.9 Sv (SECc) and -3.5 Sv (SECn). Model issues show good agreement

with these estimations, especially for the central SEC.

Simulations showed that the NECC is more defined during March,

with a core velocity of 0.4 m.s

-1

located near 5°N. In August, a deeper current

core was present in this same latitude (Figure 4.3), with velocities of 0.3 m.s

. According to Richardson and McKee (1984), the NECC responds strongly

to seasonal variations on atmospheric forcing. Around March, when the

Intertropical Convergence Zone (ITCZ) is at its southernmost position, the

equatorial trade-winds are weakest while in August, when the ITCZ is at its

northernmost position, the equatorial winds are strongest (Weisberg, 1985).

The seasonal variability of the transport close to equator due to main

zonal currents at 25°W (section A) is presented in Figure 4.4. In general one

may say that the transport along the 25°W section did not show strong

seasonal variability. EUC presented maximum values during April–August

Chapter 4 – The pathways and thermal structure of surface currents in the…

(21.8 Sv), and lowest transport in the period September to March. The NECC

variability shows two maxima in April and July, 10.1 Sv and 10 Sv,

respectively. These numerical results mismatch with Richardson and McKee

(1984) contributions, which found that NECC has peaks of transport at

25°W-30°W profiles during July-September. The equatorial branches of the

SEC (SECc and SECn) presented minimum westward transports between

May and September, which is in contrast to the period of maximum

eastward transport by EUC.

Figure 4.5. Zonal transport (u-component) obtained from model simulation (ROMS)

along Section B, 15°W. Dashed lines represent the isotherm 20°C. Monthly-averaged

values for March and August.

The comparison between mean zonal current structures obtained

numerically for March and August at 15°W (section B) is shown in Figure

4.5. The seasonal variability of the zonal transport at this longitude is

presented in Figure 4.6. At 15°W the SECn core is deeper than the SECc

core, although both westward currents are sharply in March when compared

to the weaker and sparser structures found in August. The SECc reaches

velocities of -0.5 m.s

-1

during March and drops to -0.15 m.s

-1

in August. The

SECn showed velocity core of -0.7 m.s

-1

in March, which is reduced to -0.4

m.s

-1

during August. Figure 4.5 shows that both currents have almost the

same mean net transport in March (-11.8 Sv, SECn) and August (-11.9 Sv,

SECc). During the austral winter (August), weakest westward transports are

verified in simulations, when the SECn drops to -4.1 Sv and SECc to -2.8 Sv

(Figure 4.5 and 4.6).

Chapter 4 – The pathways and thermal structure of surface currents in the…

Figure 4.6. Seasonal variability of EUC, NECC, SECc and SECn transports at 15W°

section, obtained in simulations.

Numerical results indicate that the EUC core at 15°W has the same

maximum velocity (0.8 m.s

-1

) in both months of comparison, but it presents

an increase in transport flow that rises from 7.9 Sv in March to 13.4 Sv in

August (Figure 4.5). Simulations also indicate that month-averaged NECC

transport of 5.4 Sv in March is reduced to 2.6 Sv in August. The EUC

presented the highest values of transport in the period April–September. The

SEC branches have the highest values of flow during the months of October

to April, always in phase with the period of reduced values of EUC transport.

The velocity structure, core position and zonal transports at the 5°W

cross-equatorial transect (section C, Figure 4.1) are presented in Figures 4.7

and 4.8. In Figure 4.7 it is possible to identify the Guinea Current (GC), the

equatorial SEC branches and the EUC. The NECC merges with GC near the

Africa Coast and flows into the Guinea Gulf.

Simulations also indicated that although the core velocities reach

almost the same maximum intensity (0.8 m.s

-1

, not shown here) in both

periods, the EUC core is deeper in August (80-110m depth) than in March

(50–70m depth). In contrast, the SEC branches were more defined in March.

During this period the SECc at 5°W is shallower and more extended than

Chapter 4 – The pathways and thermal structure of surface currents in the…

SECn, both with cores velocity of -0.5 m.s

-1

. In August, the SECc and SECn

present the weakest values along the last simulated year, with core velocities

around -0.1 m.s

-1

and -0.3 m.s

-1

, respectively. Model results showed more

important GC velocities in March, although the overall transport had been

superior in August (3.8 Sv). In March (1.6 Sv) the GC was pushed against

the continent due to the enlargement of the SECn. Due to reduction of the

current area, in March, the GC gains on velocity, but loses in transport

(Figure 4.7). The simulations show that the mean SECc transport was

sensible reduced in August when compared to March, dropping from -8.2 Sv

to -4 Sv. The contribution of EUC to equatorial mean eastward transport did

not show important differences for both compared periods (March 11.3 Sv

and August 10.3 Sv). As verified in 25°W and 15°W sections, the EUC core is

slightly deeper during August when compared to its position in March

(Figure 4.7).

The seasonal variation of the equatorial transport along the 5°W

section presented in Figure 4.8 shows highest values of the EUC and GC in

the period April–August, and lowest values occurring in the period

September-March. In opposition, the SEC seasonality presents inverse

standards, with weakest SECc and SECn transports in April–August and

strongest westward flows from September to March.

Figure 4.7. Zonal transport (u-component) obtained from model simulation (ROMS)

along Section C, 5°W. Dashed lines represent the isotherm 20°C. Monthly-averaged

values for March and August.

Chapter 4 – The pathways and thermal structure of surface currents in the…

Figure 4.8. Seasonal variability of EUC, NECC, SECc and SECn transports at 5W°

section, obtained in simulations.

4.3.2 Thermal structure

The vertical thermal structure of surface layers in Equatorial Atlantic

obtained from ROMS simulations is compared with observations of the

PIRATA Program. Monthly-averaged temperature profiles from these

moorings (1997-2007, 500 m depth) and climatological model results were

used to construct the Hovmüller diagrams presented in Figure 4.9. The

vertical distribution of the simulated temperature fairly agrees with the field

measurements. In subsurface (z > 100 m) the best resemblance (Δ < |0.5°C|)

occurs for the western PIRATA sites (35°W and 23°W). Such weak difference

between model and observed data is noted in March for those sites in the

shallow (0-100 m) and deeper (300-500 m) waters. Larger differences (~ 1 to

3°C), noted between 100 and 300 m for eastern PIRATA sites (10°W and 0°W)

in austral winter continue to be acceptable considering that the ROMS is

running here according to a climatic forcing. Of special interest is the very

well reproduced MLD for the equatorial PIRATA sites, in particular the

Chapter 4 – The pathways and thermal structure of surface currents in the…

upwelling induced shallower MLD verified in eastern buoys during austral

winter and late spring (Figure 4.9).

SST results obtained from the climatological run are compared in Fig.

10 with satellite observations for both months, March and August. SST

values provided by the model are close to the satellite observation, especially

taking in consideration that results from ROMS were obtained with 1/6°

horizontal resolution while the AVHRR images are generated with a 1/18°

grid.

Figure 4.9. Comparison of the vertical temperature distribution provided by ROMS

(black dashed contours) with the three PIRATA buoys (colour shading) extending from

surface down to 500 m. Monthly-averaged distributions for the PIRATA period 1997-

2007.

Chapter 4 – The pathways and thermal structure of surface currents in the…

The warm belt on equatorial region observed in March is well

reproduced numerically, as well as the meridional loss of ocean surface heat

in North/South directions. In particular, one may see the presence of a well

defined pool of warm waters on the southwestern equatorial Atlantic, which

is also observed in August. This so-called Southwestern Atlantic Warm Pool

(SAWP), is marked by high SST values (> 27°C), that extends off the coastline

from the equator to about 12

S during austral winter. According to Huang et

al. (1995), the SAWP is one of the regions of warm waters accumulation. In

contrast to these warm waters a Cold Tongue region can be defined as a

region of relatively cold water (< 24°C) that enters the tropical mixed layer

west of 30°W in the Atlantic in late boreal spring in response to intensified

winds and a shallowing of the thermocline (Figure 4.9 and 4.10).

Figure 4.10. Monthly-averaged SST obtained from ROMS model (above) and from

satellite observations (AVHRR-NOAA) (below) for March and August.

Chapter 5 – Near surface transport and heat budget in the Southwestern …

Chapter 5

5. Near surface transport and Heat

budget in the Southwestern Tropical

Atlantic using a high-resolution

numerical modelling system

The Southwestern tropical Atlantic is an interesting ocean area, acting as a

cradle of multiple oceanic-weather forcings of great importance. In this

region, part of the South Equatorial Current (SEC) feeds many western

boundary currents along the eastern Brazilian edge, thus contributing to the

climatic variability over Northeast Brazil. In this chapter, thermal structures,

heat budget in the surface mixing layer and mass transports were obtained

from climatological ROMS ocean simulations, and analyzed in connection

with scarce available long-term observations of PIRATA-SWE dataset.

Chapter 5 – Near surface transport and heat budget in the Southwestern …

5.1 The near surface current system along the area

The South Equatorial Current (SEC) divergence occurs in the

Southwestern tropical Atlantic, producing a northern and a southern branch

along the Brazilian coastline. Towards the north, the northern branch of the

SEC termination forms the North Brazil Under Current-North Brazil Current

(NBUC-NBC) system (Silveira et al., 1994; Stramma et al., 1995; Rodrigues

et al., 2007), one of the most powerful western boundary current in the

world. This is the preferential way of connecting the subduction regions of

the subtropical South Atlantic, the eastern equatorial and off-equatorial

undercurrents, as part of the Atlantic Subtropical Cell (STC), which in turn

feeds the equatorial upwelling systems Malanote-Rizolli et al, 2000; Schott et

al., 2005). This region is also an important highway for the Atlantic

Meridional Overturning Circulation (MOC), where the southward flow of

deeper North Atlantic Deep Water (NADW) is compensated by the northward

transfer of near surface warm and intermediate waters, as well as by the

Antarctic Bottom Water (Lumpkin & Speer, 2003; Ganachaud, 2003).

After the bifurcation close to the Brazilian shelf, the SEC also supplies

the Brazil Current (BC) propagating southward along the coast of Brazil

(Stramma, 1991; Peterson & Stramma, 1991; Stramma et al., 1995). Being

in the region of the southeast trade winds and the South Atlantic

Convergence Zone (SACZ), interactions between sea surface temperatures

(SST) and the easterly atmospheric circulation may play a significant role in

local climate fluctuations of Northeast Brazil, a region affected by

intermittence of severe droughts or floods (Moura & Shukla, 1981; Rao et al.,

1993).

5.2 The ROMS approach

The study case presented here involves the open ocean area near the

Brazilian coast. The integration domain is comprised between 5

S and 25

W and 47

W (Figure 5.1). An isotropic 1/12

horizontal grid results in 323

x 249 horizontal mesh cells. Vertical discretization has 40 levels. Bottom

topography was derived from a 2’ resolution database ETOPO2 (Smith &

Chapter 5 – Near surface transport and heat budget in the Southwestern …

Sandwell, 1997), and a slope parameter

20.0hhr

∇

is used to prevent

errors in the computation of pressure gradient (Haidvogel et al., 2000).

Figure 5.1. Model domain (dashed lines) with the PIRATA-SWE locations (filled

triangles) and three sites located along the western boundary region closer to the

Brazilian edge (filled circles). Near shore 100m and 1000m isobaths are plotted.

In three open boundaries (north, east and south) an active, implicit,

upstream biased radiation condition connects the model solution to the

surrounding ocean (Marchesiello et al., 2001). Horizontal Laplacian

diffusivity inside the integration domain is zero, and a 12-point smooth

increasing is imposed (up to 10

-1

) in sponge layers near open ocean

boundaries. The model equations are subjected to no-slip boundary

conditions along the coastline. A basin scale seasonal hydrology derived from

World Ocean Atlas 2005 WOA2005 database (monthly climatology at 1

x1°

resolution) (Conkright et al., 2002) is used to infer thermodynamics

(temperature and salinity) and geostrophic currents in the open boundaries.

The oceanic circulation was forced at the sea surface by winds, heat and

fresh water fluxes derived from the Comprehensive Ocean-Atmosphere Data

Chapter 5 – Near surface transport and heat budget in the Southwestern …

Set (COADS) monthly fluxes data at 0.5°x0.5° resolution (daSilva et al.,

1994). The model runs from a state of rest for 10 years. The stability is

achieved after a spin-up period of about one year. All the numerical results

examined in this chapter correspond to averages for the last two years of

simulation, except for the snapshots of high-frequency simulations (Section

5.3.1).

5.3 High-resolved climatological ROMS results

Due to its refined spatial resolution (1/12° in latitude and longitude;

40 vertical levels), I should expect the ROMS to resolve the meso-scale

dynamics in the region, which isn’t sufficiently allowed when OGCMs is

used. Thus, as a preliminary test (Section 5.3.1), the high-resolution spatial

model accuracy was evaluated by comparing two extreme seasonal model

SST snapshots with examples of high-frequency SST patterns provided by

satellite measurements. A second type of verification (Section 5.3.2)

presented, was to validate the simulated annual cycle of temperature and

heat content in the mixing layer depth (MLD). The PIRATA-SWE observations

were then used for a rough local thermal evaluation of the model within the

first 500m depth. As an early application of ROMS, a third type of interest

(Section 5.3.3) is to use the simulated results in order to give insight to our

knowledge of the seasonal variation of the main oceanic dynamics

components which are at the origin of the annual SST change in that area.

Finally, a fourth type of interest (Section 5.3.4) is to look at the yearly

averaged oceanic transport simulated by ROMS across zonal sections at the

three PIRATA buoy latitudes, as well as a fourth section along these buoys

locations.

Chapter 5 – Near surface transport and heat budget in the Southwestern …

5.3.1 Instantaneous SST evaluation

The SST patterns for the two mid-month September and March, i.e.

during two extreme seasonal conditions, is presented as the first

illustrations of the model simulation. These figures show mainly a seasonal

meridional change in SST along the South Atlantic western boundary. The

so-called South Atlantic Warm Pool (SAWP), marked by high SST values (>

27°C), extends off the coastline from the equator to about 12

S during

austral winter (e.g. September, Figure 5.2a) which only includes the region of

the northern PIRATA buoy. Six months later, during austral summer (e.g.

March, Figure. 5.2b), these high SST values invade the whole studied zone,

and warmer waters now bathe the three PIRATA buoys. In connection with

seasonal meridional extension, the SAWP pattern records seasonal zonal

changes, with a more alongshore location during summer. The seasonal

colder waters (< 22°C) extension at the southern limit of the study domain

follows the same meridional progression as the SAWP. These cold waters

cross the southern limit of the studied domain during March. However, along

the coastline, they remain stationary south of 16°S during all the year (Ikeda

& Stevenson, 1978; Carbonel, 2003). It is also noted, as illustrated by these

figures, that ROMS succeeds in resolving meso-scale dynamical processes

such as frontal structures or filaments.

Figure 5.2. Horizontal distribution of SST obtained from ROMS in mid-September (a)

and mid March (b). PIRATA-SWE locations (filled triangles) and three sites located

along the western boundary region closer to the Brazilian edge (filled circles).

Recife

Salvador

Rio de Janeiro

W 40

W 35

W 30

W 25

W 20

SST - ROMS12 (

C) - 09/15

08S-30W

14S-32W

19S-34W

08S-34W

14S-38W

19S-38W

20 22 24 26 28 30

(a)

Recife

Salvador

Rio de Janeiro

W 40

W 35

W 30

W 25

W 20

08S-30W

14S-32W

19S-34W

08S-34W

14S-38W

19S-38W

SST - ROMS12 (

C) - 03/15

(b)

20 22 24 26 28 30

Chapter 5 – Near surface transport and heat budget in the Southwestern …

These phenomena, which are particularly visible at the frontier

between warm and cold waters on the ROMS results (Figures. 5.2a,b) when

using a 1/12° resolution (= 9.25 km), also appear (slightly more diffuse) in

two examples of daily observed SST by satellite also using 9,25 km

resolution (GODAE High Resolution Sea Surface Temperature Pilot Project -

GHRSST-PP, 2007): one in September 15, 2005 (Figures 5.3a), the other one

in March 15, 2006 (Figure 5.3b). Due to several individual images used to

estimate the satellite SST (process brought about cloud covers) the

mesoscale structures seem damped when compared to the model

instantaneous SST outputs.

(b)

(a)

Figure 5.3. Horizontal distribution of SST obtained from observed satellite data on 15

September 2005 (a) and 15 March 2006 (b). PIRATA-SWE locations (filled triangles)

and three sites located along the western boundary region closer to the Brazilian edge

(filled circles).

5.3.2 Seasonal evaluation of the heat content

Figures 5.4a,b,c provide the seasonal variations of the 0-500m

temperature for the three PIRATA-SWE locations (08°S-30°W, 14°S-32°W

and 19°S-34°W). These Hovmüller diagrams show the ROMS simulated

temperature (dashed contours), superimposed by daily observed in-situ data

(shaded colors) during the initial 2.5 year, approximately, recording of

PIRATA-SWE network (September 2005 to February 2008). No-shaded parts

Chapter 5 – Near surface transport and heat budget in the Southwestern …

mean missing PIRATA values. A reasonable agreement between ROMS and

PIRATA observations is noted, definitely more satisfactory than using an

OGCM, even when forced by inter-annual surface fluxes (e.g. Nobre et al.,

2008). ROMS reproduces the tightening of the thermocline for the most

equatorial PIRATA site (08°S-30°W). The opposite situation, the relaxation of

vertical gradient, occurs for the two southern sites.

Also of great interest is the apparent capacity of ROMS to simulate

intraseasonal variations of the thermocline depth, which clearly occurs for

the central and southernmost locations on the in-situ data within a 3-4

month periodicity. This is particularly evident on the 19°S-34°W site (Figure

5.4c). These observed variabilities can be partially explained in terms of

ocean adjustment to disturbances in the buoyancy field due to the

propagation of barotropic Kelvin waves. It causes vertical displacement of

isopycnals and propagates with the coastal boundary on the left in the

southern hemisphere. Keeping in mind that the present run is only forced by

climatological fluxes, let us consider a promising accuracy of the intra-

seasonal phenomena if ROMS is forced into inter-annual mode (showed in

Chapter 6).

Chapter 5 – Near surface transport and heat budget in the Southwestern …

(c)

(b)

(a)

Figure 5.4. Comparison of the monthly mean vertical temperature distribution (ºC)

provided by ROMS (black dashed contours) with monthly mean for the period of

September 2005 to February 2008 of the three PIRATA-SWE buoys (colour shading

and white lines) extending from surface down to 500m. The four vertical black thin

lines correspond to September 15

, December 15

, March 15

and June 15

,. (a) 8°S-

30°W; (b) 14°S-32°W; (c) 19°S-34°W.

Chapter 5 – Near surface transport and heat budget in the Southwestern …

Detailing the preceding discussion, monthly mean temperature profiles

(0-500m) observed by PIRATA-SWE array during September, December,

March and June (i.e. as related on Figures 5.4a,b,c) are compared to ROMS

outputs for the same calendar months (Figures 5.5a,b,c,d). As shown

previously, and though climatologically forced, the vertical distribution of the

simulated temperature fairly agrees with the field measurements. In

subsurface (z >100m) the best resemblance (Δ < |0.5°C|) occurs for the

PIRATA –SWE northernmost site (8°S-30°W) in March.

Such a small difference between model and observation is also noted,

in December, for that site in the shallow (0-100m) and deeper (300-500m)

waters, as well as for the 14ºS-32ºW site in the same month (December, Fig

5.5b). Larger differences (~ 1-to-2°C), noted below 200 m for the southern

site (19ºS-34ºW) in March, for the two other sites for all months plotted, as

well as upper 100m to 8°S-30°W in June, only, continue to be very

acceptable considering that the ROMS here is running according to a

climatological forcing. Of special interest is the very well reproduced MLD for

the three PIRATA sites, whether during warm season (~20-30m in March), or

cold season (~80-100m in September). SST values provided by the model are

close to the PIRATA observations, except in September for the southern

location, obviously subjected to larger inter-annual variability (see also Fig.

5.3a).

Chapter 5 – Near surface transport and heat budget in the Southwestern …

(b)

(a)

Figure 5.5. September (a), December (b), March (c) and June (d) monthly averaged

vertical profiles of temperature 0-500m at the three PIRATA-SWE sites provided by

climatic ROMS and by PIRATA monthly averaged profiles from September 2005 to

February 2008

Chapter 5 – Near surface transport and heat budget in the Southwestern …

(a)

(b)

(Continue)

Chapter 5 – Near surface transport and heat budget in the Southwestern …

(c)

Figure 5.6. Comparison of temporal evolution of seasonal heat content in the mixed

layer at 8°S-30°W (a), 14°S-32°W (b) and 19°S-34°W (c), provided by PIRATA in-situ

observations (dashed line) and ROMS (solid line) for the period of September to

August.

Seasonal evolutions of the heat content integrated inside the mixing

layer (MLD defined in Appendix) are computed for the three PIRATA-SWE

sites (Figure 5.6) according to the ROMS outputs (solid lines), and the in-situ

PIRATA measurements from September 2005 to February 2008 (dashed

lines). Like the previous discussion about the thermal profile comparisons

(Figure 5.5), the agreement between simulation and observation here is also

very satisfactory, especially for the site 14°S-32°W, i.e. the centre of the

PIRATA-SWE array, where both observed and simulated curves take very

similar shapes. The two largest differences (~ 4 x 10

J.m

-2

), both indicating

a greater observed value of heat content vs. the climatological simulation,

are noted from February to June for the southern site, and from April to

June for the northern site.

Chapter 5 – Near surface transport and heat budget in the Southwestern …

5.3.3 Oceanic dynamics vs. atmospheric forcing

After checking that ROMS approach is capable of reproducing the

ocean thermodynamics in the study region, ROMS results was used to test

the relative influence of the ocean dynamics vs. the atmospheric forcing for

settling the seasonal variation of the mixing layer temperature. For that, the

three PIRATA sites are now focused, adding three other locations with the

same latitudes as those of PIRATA sites, but along the Brazilian coast (see

Figure 5.1).

Considering the local heat budget inside the mixing layer, the different

components of the temperature equation can be written as:

{

(

)

{

'T'u

FLUXHEAT

)DIFFUSIONADVECTION(OCEAN

CHANGELOCAL

=Φ−

∂

444344421

(5.1)

where U

is the mean velocity, T (or SST) is the temperature of MLD, and

is the total heat flux used to force the ROMS (Marchesiello et al., 2003).

Temporal changes of the three terms on the left of Eq. 1 (in °C/Month),

as well as T (in °C), have been monthly integrated through MLD, and are

presented in Figure 5.7 for the three sites of PIRATA-SWE and the three

coastal locations. A first overview indicates that the main seasonal behaviors

of the dynamics are very similar in the whole domain, though we note

somewhat larger amplitudes in higher latitudes than in equatorial areas, and

along the coastline rather than in the open ocean. Highest T (dotted black

curves) values are centered around March (from 28.50°C in the northern

locations, to 27.75°C in the southern locations), and coldest T values are

centered around August-September (from 26.25°C in the north, to 24.00°C

in the south). The seasonal change of temperature is roughly of sinusoidal

type, except in the southern coastal site (Figure 5.7e), located at the eastern

limit of the coldest waters (see Figures 5.2a,b), where T undergoes a rapid

reduction of half degree in November-December, thus slowing down the

Chapter 5 – Near surface transport and heat budget in the Southwestern …

thermal increase between minimum (~ 24.8°C) in July-August and maximum

(~ 28.0°C) in February-March. As related on Eq. 5.1 the local changes in T

(continuous black curves on Figure 5.7) results from the combination of net

atmospheric forcing (“FLUX” in red) and net oceanic balance (“ADV+DIFF” in

blue). In agreement with previous works (e.g. Carton & Zhou, 1997), and

except for the location 19°S-38°W, which shall be discussed hereafter, local

variations of T are mainly driven by the atmospheric forcing in the sense that

positive (negative) values of

∂

generally occur during positive (negative)

values of “FLUX”. It is however interesting to note that, as opposed to what

was generally thought until now for this region, the net oceanic contribution

evolves according to an amplitude of the same order of magnitude (until

3°C/Month, peak-to-peak) rather than the seasonal variations of

atmospheric forcing.

The time duration of positive values of “FLUX”, occurring mainly from

August-September to March-April, (i.e. when the South Hemisphere is

warmer), is significantly longer than the duration of negative occurrences in

May-June-July, i.e. when the sun is at its northern position. These time

durations are slightly modulated according to latitude: close to equator the

“FLUX” is positive during a longer period (~ 8-9 months), while in the south

of the study domain, positive and negative periods tend to be of equal

duration (~ 6 months). Another characteristic is that the positive values of

“FLUX” are generally of larger amplitude than the negative values.

Consequently, in order to maintain the local equilibrium of

∂

(a longer

duration associated with larger amplitudes) positive “FLUX” must thus be

compensated by an inverse net oceanic contribution during the same

periods. In fact, “ADV+DIFF” opposes to atmospheric forcing “FLUX” most of

the time, with larger and longer negative values during the period August-

September to March-April, and shorter and weaker positive values during

the remainder of the year, centered around June. Additional analyses carried

out from individual oceanic components (not shown here), and confirmed by

results from another recent study (Servain & Lazar, personal

Chapter 5 – Near surface transport and heat budget in the Southwestern …

communication), brings some additional information: the cooling of the

mixing layer by oceanic effect noted here in the whole region comes primarily

from a mixing by vertical diffusion between MLD water and deeper colder

waters, while the warming by oceanic effect mainly comes from horizontal

advection and lateral diffusion.

Let us take a closer look at the 19°S-38°W site, in the southern

studied region. It is positioned in a complex region, where the interaction

between southward Brazil Current and Vitoria-Trindade ridge, during the

austral summer, may induce the formation of cyclonic thermocline eddies,

which trap cold waters from an extending upwelling regime north of Cabo

Frio (Castelão et al., 2004; Campos, 2006).

Chapter 5 – Near surface transport and heat budget in the Southwestern …

(a)

(b)

(e)

(f)

Figure 5.7. Seasonal evolution of SST (°C) (black dashed line), atmospheric (red line)

and oceanic (blue line) contributions to the local change of SST (°C/Month) (black

continuous line) provided by ROMS at 8°S-34°W (a), 8°S-30°W (b), 14°S-38°W (c), 14°S-

32°W (d), 19°S-38°W (e) and 19°S-34°W (f) locations.

Chapter 5 – Near surface transport and heat budget in the Southwestern …

5.3.4 Mass transports across sections

Mean (yearly averaged) meridional current and transport values

obtained from the ROMS simulation along zonal sections (0-1500m) at 8°S,

14°S and 19°S (i.e. the latitudes of the three PIRATA-SWE buoys) are

presented in Figure 5.8a,b,c. A fourth section (Figure 5.8d) provides current

and transport information within the 0-600m across this mooring track.

Three density levels are indicated on the plots. The

σ = 24.5 kg.m

-3

separates the upper Tropical Surface Water (TSW) from the upper

thermocline waters. The

σ = 26.8 kg.m

-3

(at about 150 m depth) is the lower

level of the water supplying the EUC (Schott et al., 1998). The

σ = 32.15

kg.m

-3

(at about 1100 m depth) indicates the lower boundary of the upper

warmer waters as well as the lower boundary of the Antarctic Intermediate

Water (AAIW). Below 1100 m depth we found the North Atlantic Deep Water

(NADW) extending to about 4000 m depth (

= 45.9 kg.m

-3

). This is the

layer where the Deep Western Boundary Current (DWBC) transports

southward cold waters from the North Hemisphere.

The zonal sections (Figures 5.8a,b,c) show a clear representation of the

NBUC-NBC skirting the coastline from 100 to 1000m, with a northward

transport increasing from 5.8 Sv at 19°S to 17.4 Sv at 8°S. The site 8°S-

34°W is located above the core of the NBUC which is situated at about 50

km from the coast, at 180-to-500 m depth approximately (Silveira et al.,

1994; Schott et al., 2002; Stramma et al., 2003; Schott et al., 2005). These

results have been detected from field measurements. Stramma & England

(1999) showed from hydrographic data that the SEC bifurcation latitude is

16°S in near surface layer (top 100m), 20°S in the layer (100-500m) and

26°S in the AAIW (500-1200m). Wienders et al. (2000) also used

hydrographic data to indicate that SEC bifurcation latitude is 14°S at the

surface, 24°S in the (400-500m) layer and around 26°S-28°S in the AAIW.

Chapter 5 – Near surface transport and heat budget in the Southwestern …

(a) (b)

(c)

(d)

Figure 5.8. Mean annual volume transport averaged across three zonal sections at

8°S (a), 14°S (b) and 19°S (c), and across the section along the PIRATA-SWE array (d).

Positive (negative) values indicated by solid (dashed) white lines correspond to

northward (southward) currents (panels 8a, 8b, and 8c), while positive (negative)

values indicated by solid (dashed) white lines for the section along the PIRATA array

correspond to eastward (westward) currents (panel 8d). The three horizontal solid

black lines indicate the 24.5, 26.8 and 32.15 sigma-t values (in kg.m-3).

The ROMS results also agree with the numerical findings of Harper

(2000), Malanotte-Rizzoli et al. (2000) and Rodrigues et al. (2007), indicating

a poleward depth increasing of the SEC bifurcation along the Brazilian edge,

as well as its seasonal variability: the SEC bifurcation reaches its

southernmost position in July and its northernmost position in November

(not shown here).

The flowing southward BC, confined to the shallow and near shore

part of the Brazilian continental slope, is especially recognizable on the 19°S

section (Figure 5.8c). These numerical results were observed in previous

Chapter 5 – Near surface transport and heat budget in the Southwestern …

studies. Miranda & Castro (1981) identified the BC at 19°S as a surface

narrow current (~ 75 km) limited to upper 500 m depth. Evans et al. (1983)

indicate that BC is still confined and organized above the continental shelf at

20.5°S. The World Ocean Circulation Experiment (WOCE) current-moorings

measurements obtained at 19°S pointed out a BC confined to the upper 200

m depth, with a mean southward velocity of approximately 15 cm.s

-1

(Müller

et al., 1998).

The current and transport section along the PIRATA-SWE array (Figure

5.8d) allows us to have an idea of how the southern part of the SEC extends

before reaching the western continental boundary. Indeed, we may note a

succession of more powerful westward systems (one core with 4.8 Sv, from

50 to 200 m depth, and with a 300-400 km width) and less powerful

eastward systems (each one with a core less than 1.0 Sv, from 0 to 100 m

depth, and with a width lower than 100 km). Let us note however that the

most important westward transport of 7.8 Sv shallower 600 m between 19°S-

34°W and 14°S-32°W PIRATA sites has large extension with small velocity

intensity values across the section. Elsewhere in the deeper ocean the E-W

transport given by the model results is relatively weak.

According to ROMS simulation, the annual-mean westward transport

between 8°S-30°W and 19°S-34°W for the SEC is 15.6 Sv for the upper 600

m, which is in good agreement with observations and previous numerical

studies. For instance, Wienders et al. (2000) estimated a westward transport

of 21.5 Sv for the SEC within the SACW layer between 7.5°S to 20°S, while

the hydrographic data of Stramma (1991) showed a westward SEC transport

of about 20 Sv for this same region. Furthermore the recent numerical

simulations of Rodrigues et al. (2007) indicated an annual-mean westward

SEC transport between 6°S and 22°S (along the meridian 30°W) of 18 Sv.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

Chapter 6

6. Interannual high-resolution

regional ocean dynamics simulation in

the Southwestern Tropical Atlantic

Inside this area there is a complex current system of great importance from

an oceanographic point of view, being a key driver for regional climate. A

high resolution ocean model has been adapted for the simulation of this

system, and it is now forced with interannual data sets to be compared with

PIRATA observations. It should be helpful in obtaining a better knowledge of

this oceanic system, its dynamic equilibriums and evolutions. The goal here

is to analyse the accuracy of a versatile new generation state-of-the-art

Regional Ocean Modeling System (ROMS) to reproduce some aspects of

intra-seasonal to interannual ocean dynamics and heat contents in the

region of the southernmost extension of the westward SEC.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

6.1. The high-resolved interannual ROMS approach

The case study presented here involves the open ocean area near the

NE Brazilian coast. The integration domain, bathymetry and boundary

conditions are imposed as presented in the previous chapter. Changes were

made in assimilation of geostrophic currents induced on lateral boundaries,

based on a daily-mean (2005-2007) hydrography scale and currents derived

from OGCM–ECCO (http://ecco.jpl.nasa.gov/

). At sea surface these changes

were based on daily-mean (2005-2007) wind stress by QuickSCAT (Liu,

2002), combined with monthly-averaged heat and fresh water fluxes derived

from COADS (da Silva et al., 1994).

6.2 Thermal structure in Southwestern tropical Atlantic

6.2.1. Instantaneous SST evaluation of interannual simulation

After establishing kinetic energy, in order to validate the numerical

results, the model accuracy was evaluated by comparing daily SST surface

maps derived from the ROMS model to daily infrared SSTs derived from the

GODAE High Resolution SST Pilot Project (GHRSST-PP, 2007).

This comparison was done for two different scenarios: September 15

2005, which corresponds to the austral winter time, and March 15

2006,

which corresponds to the austral summer time (Figure 6.1). The satellite-

derived SST maps have a horizontal resolution of 9.25 km. A full description

of the GHRSST-PP can be found on the GHRSST-PP web site at

http://www.ghrsst-pp.org.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

(a)

(b)

(c)

(d)

Figure 6.1. Comparison between ROMS-derived SST (a and c) and GHRSST-PP-

derived SST (b and d) for September 15

2005 and March 15

2006.

In the region of interest, the model and daily SST satellite patterns are

consistent, showing a distinct meridional seasonal change along the South

Atlantic western boundary. The so-called Southwestern Atlantic Warm Pool

(SAWP) (Huang et al., 1995; Nobre et al., 2005), which is marked by high

SST values (> 27°C), extends off the South American coastline from the

equator to about 12

S on September 15

, 2005, including the northern part

of the PIRATA-SWE (Figures 6.1a, b). Six months later, on March 15

, 2006,

these high SST values invade the whole area of study, and warmer waters

are observed at the three PIRATA buoy sites (Figures. 6.1c, d). Besides its

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

meridional seasonal migration, the SAWP pattern also records seasonal

zonal changes with a more eastward extension during the summer,

especially in the northern part. The seasonal spreading out of cold waters (<

22°C) in the open ocean in the southern limit of the study domain follows the

same meridional progression as that observed for the SAWP. These cold

waters are advected off the southern limit of the study area in March.

Besides the fact that the model accurately reproduces the satellite-

derived SST overall, it also resolves the mesoscale dynamical processes quite

well, including frontal structures, meanders and local upwelling regions. For

instance, the cold water filaments observed in the model outputs for March

, 2006 in the vicinity of the Abrolhos Bank (Lat. 17-18

S – Long. 38.5

’-

39.5

W) and the Vitória-Trindade Ridge (Lat. 20°S – Long. 34-38°W) are good

indicators of observed and modeled upwelled waters and mesoscale cyclonic

structures previously documented in this area (Schmid et al., 1995; Campos,

2006). Indeed, the modeled SST is especially efficient compared to the

satellite-derived SST estimates along the coastline where the infrared

retrievals may be cloud-contaminated. Although the horizontal resolution of

the model and the GHRSST-PP SST product are almost the same (~10 km),

the fine mesoscale surface structures evident in the model results seem to be

smoothed in the infrared SST maps. This is probably due to the fact that

several individual images and infrared products are combined and used to

estimate the SST. For example, it does not seem random that higher

differences between satellite and modeled SST are present in late austral

summer close to the coastal region between 16°S and 22°S (Figures 6.1c, d);

this is a well-known cloudy area under the influence of the atmospheric

South Atlantic Convergence Zone – SACZ (Chaves and Nobre, 2004; De

Almeida et al., 2007).

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

6.2.2. Seasonal (2005-2007) evaluation of the subsurface temperature

and mixed layer depth

In this second set of model evaluations, samples of local time series of

the vertical distribution of temperature given by ROMS are directly compared

to the first available two-year period dataset provided by PIRATA-SWE.

Figure 6.2a,b,c primarily shows the simulated and observed variations of the

0-500 m temperature profile (Sep 2005-Jun 2007) for the three PIRATA-SWE

locations (08°S-30°W, 14°S-32°W and 19°S-34°W). The modeled temperature

is represented by black contours (for a two-day period), while the PIRATA

temperature appears as white contours (also for a two-day period), and

shaded colors (daily). Vertical interpolations were processed on the PIRATA

data when only one out of three consecutive temperature sensors along the

moored line was dysfunctional. That was the case for the first 50 m level at

the 08°S-30°W mooring site from the 10

of November 2006 to the end of

June 2007, as well as for the levels below 180 m at the 19°S-34°W mooring

from the 3rd of November 2006 to the end of June 2007. Missing values are

represented by blank areas. ROMS reproduces well the tightening of the

thermocline for most of the northernmost PIRATA-SWE site (08°S-30°W,

Figure 6.2a), and the relaxation of the vertical gradient for the two

southernmost sites (Figure 6.2b, c). In terms of the seasonal cycle within the

mixed layer, the simulated and observed cores of the warmest (>28°C for the

near equatorial site, >26°C for the two other sites) shallow waters (0-75 m)

occur within the same period of the year. There are, however, a few episodic

discrepancies between ROMS and the PIRATA-SWE data in the levels

including the thermocline and the layer depths from ~75 to ~200 m for the

northernmost site, and from ~100 to ~250 m or even more for the other two

sites. For most of these differences, the 26°-to-16°C isotherm depths given

by the model are shallower than the PIRATA-SWE observations, extending

from a few meters (e.g., the northern site) to a few tens of meters (e.g., the

20°C isotherm depth for a few episodes in central and southern sites). This

indicates a relatively permanent cold bias in the model for these depths,

which can vary from 0.8°C at 08°S-30°W to more than 2°C at 19°S-34°W at

the most. These discrepancies are probably due to the model’s difficulty in

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

accurately reproducing the ventilation of subducted lower-thermocline

waters coming from the subtropical South Atlantic Ocean. For the lower

levels (e.g., ~16°-to-12°C isotherms) the model again becomes generally

consistent with the observations.

The PIRATA-SWE data clearly show intraseasonal variations of the

thermocline depth within a 3-4 month periodicity, especially for the central

and southernmost locations (Figure. 6.2b, c). This observed variability can

be partially explained in terms of an ocean adjustment to disturbances in

the buoyancy field due to the propagation of Tropical/Kelvin-Helmholtz

instability waves (Proehl, 1996; Polito and Cornillon, 1997; Jochum et al.,

2004). These disturbances cause vertical displacement of the isotherms and

propagate equatorward keeping the coastal boundary to the left in the

southern hemisphere. Of great interest here is the apparent capacity of

ROMS to generate such variability in the temperature profile (see the black

lines on Figure 6.2b,c around the 20°C isotherm depth), even if simulated

and observed fluctuations are not always in phase.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

(a)

(b)

(c)

Figure 6.2. Comparison between simulated and observed temperature (

C) variations

for the upper 500m from September 2005 to June 2007 for the three PIRATA-SWE

locations at (a) 08°S-30°W, (b) 14°S-32°W and (c) 19°S-34°W. Model-derived

temperature is represented by black contours and the PIRATA-SWE observed

temperature appears as white contours and shaded colors.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

In order to gain better insight into the preceding discussion, four

selected monthly mean temperature profiles (0-500 m), computed from

ROMS outputs, are compared to the observational data derived from the

three PIRATA-SWE moorings for the same months (Figure 6.3), i.e.

September 2005, December 2005, March 2006 and June 2006. For most of

the 12 temperature profiles (also marked by vertical black lines on Figure

6.2), the model and observations are in good agreement. This is especially

the case during the first two months (September and December 2005), where

a cold bias, such as those previously discussed, occurs in September 2005

at 14°S-32°W within 100-250 m, and in December 2005 at 19°S-34°W within

50-300 m. Other examples of limited negative biases are observed in March

and June 2006 at these same depths.

We conclude this subsection with an evaluation of the temporal

evolution of the mixed layer depth (MLD) for both the model runs and the

observations. Following Sprintall and Tomczak (1992), we computed the

MLD in terms of temperature and density steps (see the Appendix for

details). Note that for these calculations, the number of available levels for

the modelled temperature and salinity is 20 for the first 300 meters, i.e. two

(five) times the temperature (salinity) number of levels of the PIRATA-SWE

data for the same depth. Figure 6.4 shows the simulated and observed

temporal evolutions of the MLD from September 2005 to July 2007

according to the ROMS outputs (solid lines) and the in-situ PIRATA-SWE

measurements (dashed lines). The ROMS-derived MLD vs. the PIRATA-SWE-

derived MLD shows a similar pattern between the model and the

observations with a few discrepancies that are more pronounced at the 8

S-

W mooring site.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

Figure 6.3. Comparison of monthly averaged vertical profiles of temperature for the

first 500m between the PIRATA-SWE in-situ observations (dashed line) and the ROMS

simulation (solid line) for: (a) September 2005, (b) December 2005, (c) March 2006 and

(d) June 2006. Colours are associated to each PIRATA-SWE buoy: 08°S-30°W (blue),

14°S-32°W (black) and 19°S-34°W (red).

The calculated cross-correlation coefficients between observed and

modelled MLD signals are 0.65 (8°S-30°W), 0.45 (14°S-32°W) and 0.71 (19°S-

34°W) (95% confidence level and 321 degrees of freedom). The cross-

correlation coefficient is a measurement of similarity between the signals.

Quite good agreement is evident and the observed and simulated MLD

curves have similar shapes, ranging from small values (~30-60 m) during the

austral summer, to large values that reach up to 100 m at the end of the

austral winter. Note that the PIRATA-SWE-derived MLD was not plotted at

19°S-34°W from 10 May to 11 November 2006 due to missing salinity

observations in the upper level. Perhaps the largest difference for the MLD

estimation between ROMS and in-situ data is noticed here at the 08°S-30°W

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

site (Figure 6.4a). Indeed, even if the seasonal evolutions are in phase, the

simulated MLD is generally systematically shallower (from 0 to about 30 m)

than the observational data. This feature is already visible in Figure 6.2,

where the ROMS 26°C isotherm depth at 08°S-30°W is always significantly

shallower. The systematic underestimation of the MLD at 08°S-30°W seems

to be related to the difficulty in estimating observational MLD. The vertical

measurements of salinity on the ATLAS moorings are presently limited to

only four levels (1, 20, 40, and 120 m) within the upper layer (see section 2.2

on Chapter 2). Another reason for this discrepancy can be related to the use

of climatological heat flux to forcing surface layer. One way to investigate

this last hypothesis is through a series of case studies to check the model’s

sensitivity to different forcings.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

(a)

(b)

(c)

Figure 6.4. Comparison of temporal evolution of seasonal mixed layer depth (MLD)

at: (a) 8°S-30°W, (b) 14°S-32°W and (c) 19°S-34°W, provided by PIRATA-SWE in-situ

observations (dashed line) with ROMS (solid line) for the period of September 2005 to

June 2007.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

6.3. Transport and pathways in the Southwestern tropical Atlantic

6.3.1. Meso-scale activity in the Southwestern tropical Atlantic

The averaged (2005-2007) simulated Sea Surface Height (SSH) and the

derived surface Eddy Kinetic Energy (EKE) of the study area are presented in

Figs. 6a-d. In these figures, the numerical results are compared to the

AVISO Rio05 product for the same periods. Mean SSH was obtained from

AVISO Rio05, combining hydrographic data, surface drifters velocities,

altimetry and a geoid model (Rio and Hernandez, 2004). Simulated and

measured surface Eddy Kinetic Energy (EKE) were calculated from SSH

gradients with a similar temporal sampling.

The ROMS-simulated boundary systems (Figures 6.5a and 6.5b,

respectively) bear good resemblance to those of AVISO Rio05 data (Figs. 6.5c

and 6.5d). The similarity of the isocontours as well as the eddy-induced

structures along the BC is particularly strong. The highest values are greater

than 50 cm

-2

, found in cSEC and NBUC/BC areas; this holds for both

AVISO Rio05 and ROMS. In general, observations show stronger meso-scale

activities in the Southwestern tropical Atlantic boundary as compared to the

ROMS simulation, but the geographic patterns are similar. It is possible to

identify three common characteristic areas of high surface variability that

are present in ROMS and AVISO Rio05 EKE charts. First, the near-shore BC

patch south of 16

S, where meso-scale cyclonic structures are documented

(Schmid et al., 1995; Campos, 2006). Second, along the Brazilian edge, north

of Salvador, following the NBUC jet, and finally the zonal band close to the

northernmost boundary of the integration domain (5-6

S), where the central

branch of the SEC develops (Lumpkin and Garzoli, 2005). Concerning the

coastal region, SSH (and consequently EKE) maps indicate that the

NBUC/BC proper is not well represented in AVISO Rio05 dataset. This is

probably associated with the partial inability of the altimetry data to

accurately capture part of the strong boundary current inertia near the

coastline.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

Figure 6.5. Averaged (2005-2007) SSH and EKE comparisons between ROMS

simulations and AVISO Rio05 data: (a) ROMS - SSH (cm), (b) ROMS - EKE (cm

-2

), (c)

RIO05+AVISO - SSH (cm) and (d) AVISO - EKE (cm

-2

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

Figure 6.6. Transport function (Sv) for the modelled seasonal averaged (three

months, 2005-2007) currents integrated from 600 m to the sea surface for a) MJJ and

b) OND. The 1000 m isobath is represented by a red line.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

6.3.2. Seasonal (2005-2007) variability of the sSEC/NBUC/BC system

The simulated volume transport in Sv (1 Sv = 10

-1

) of the

sSEC/NBUC/BC system between the surface and 600 m is shown in Figure

6.6a, b for the averaged period of MJJ and OND (2005-2007), respectively.

The first characteristic of ocean circulation expressed in Figure 6.6a, b is the

inclination of the transport isolines during MJJ compared to the OND

period. Indeed, during OND, when less intense SW trade winds are present,

the transport lines west of 24

W approach the continent at more or less

constant latitudes. In this case, the southern branch of the SEC, which

carries 5 Sv, reaches the coast at about 16

S, while the transport isoline of

10 Sv approaches the land near 10

S. Even during MJJ, characterized by

more intense action of the SW trades, the transport isolines of 5 and 10 Sv

are clearly tilted in relation to the parallels, approaching the edge of the

continental shelf at around 18

S and 14

S, respectively. Through these

figures, we can also see a strengthening of the alongshore NBUC transport

during the period in which the isolines reach the coast further south, with

simultaneous weakening of the transport of BC to the south (MJJ, Figure

6.7a). The inverse situation is observed in Figure 6.6b (OND), when the

bifurcation of sSEC occurs further north. In this case, the NBUC is

weakened and the BC is intensified (see also Figure 6.10a-b).

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

Figure 6.7. Model simulation of meridional current and transport values averaged

from September 2005 to July 2007 along zonal sections (0-1500 m) at: (a) 8°S, (b)

14°S and (c) 19°S. Positive (negative) values indicated by solid (dashed) lines

correspond to northward (southward) currents.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

Meridional current and transport estimates averaged for the period of

September 2005 to July 2007 are presented here along zonal sections (0-

1500 m) at 8°S, 14°S and 19°S, which correspond to the latitudes of the

three PIRATA-SWE buoys (Figure 6.7a-c). Three

levels are selected

according to the local dynamics. The first sigma level (

σ = 24.5 kg m

-3

)

separates the upper Tropical Surface Water (TSW) from the upper

thermocline waters. The second sigma level (

= 26.8 kg m

-3

), which is

located at about 150 m, is the lower level of the water supplying the EUC

(Schott et al., 1998). The third one (

= 32.15 kg m

-3

), which corresponds to

~1000 m, indicates the lower boundary of the upper warmer waters as well

as the lower boundary of the Antarctic Intermediate Water (AAIW). Below a

depth of 1100 m, we found the North Atlantic Deep Water (NADW) extending

to about 4000 m. This is the layer where the Deep Western Boundary

Current (DWBC) transports southward cold waters from the northern

hemisphere.

The zonal mean sections in Figure 6.7a-c show a clear representation of

the NBUC skirting the coastline from 100 to 1000 m, with a northward

transport increasing from 6.7 Sv at 19°S to 20.3 Sv at 8°S. Along the latitude

of 8°S, the core of the NBUC is located about 50 km from the coast, at

approximately 50-to-600 m depth (Fig. 8a), and between 250-to-650 m depth

at 14°S (Fig. 8b) and at 19°S (Figure 6.7c). Still, in the first 1500 m, the

presence of a mean southward flow east of the lower NBUC is observed from

the model transects at 8

S and 14

S (i.e. 150 and 350 km from the coast,

respectively), suggesting a continuous offshore recirculation branch at

depths between 200-1400 m. The southward transports of 2.3 Sv at 8°S and

4.3 Sv at 14°S (Figure 6.7a, b) were also found in the LADCP measurements

and EOF analysis performed by Schott et al. (2005) and Schuckmann (2006),

who found mean representative values of -5.2±4.9 and -4.1±3.7 Sv at 5°S

(ship sections) and 11°S (ship sections + mooring array), respectively. There

are three possible explanations for the origin of this flow. One could be the

deflection of zonal currents. However, no evidence of a southward deflection

has been reported in the literature for these currents, because the South

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

Equatorial Undercurrent and the Southern Intermediate Countercurrent

transport stays practically constant between 35

W and 28

W (Schott et al.,

2005). A second explanation for this southward flow located east of the

NBUC is an inflow from the east as part of the deep cSEC, which deflects

southward before reaching the Brazilian coast, and thus forms an offshore

counterflow (hereinafter referred as CFLOW) to the deep NBUC.

Furthermore, the flow could be explained by a retroflection of the deep NBUC

just a little bit north of the northeastern tip of Brazil to supply the

southward offshore flow across 5

S, identified in the high resolution (1/12

)

MICOM model simulation (Schott et al., 2005). In order to investigate these

hypotheses, we plotted in Fig. 6.8 the annual averaged (2005-2007)

meridional current (cm s

-1

) and transport (in Sv) obtained by ROMS for: (a)

the along zonal section (0-1500 m) at 7°S, and (b) the depth range of 200-

1000 m, where southward CFLOW is stronger (see Fig. 6.7a and b). Despite

the proximity of the northern boundary to the integration domain (and as a

consequence, of the influence of the numerically-imposed sponge layer

condition between 5-6

S), our modeling results plead in favor of the second

case mentioned above, i.e., that the offshore counterflow to the deep NBUC is

fed by the southernmost limb of the deep cSEC.

Further east of the CFLOW (-2.3 Sv) shown in Figure 6.7a, we can

distinguish two main current systems, one to the north with a corresponding

transport of 2.1 Sv from 200 to 300 km away from the coast, and another to

the south (-3.5 Sv) located between 300 and 500 km from the shoreline.

These meridional offshore cores were also found by Schott et al. (2005)

between the 9

S and 11

S transects.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

(a)

Figure. 6.8. Annual averaged (2005-2007) meridional current (cm s-1) and transport

(in Sv) obtained by ROMS for: (a) the along zonal section (0-1500 m) at 7°S. Positive

(negative) values indicated by solid (dashed) lines correspond to northward

(southward) currents, (b) the depth averaged range 200-1000 m, where CFLOW is

stronger

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

The southward-flowing BC, confined to the shallow and near-shore

part of the Brazilian continental slope, is especially recognizable at the 19°S

section with a mean transport of -2.8 Sv and meridional velocities ranging

from 10 to 20 cm s

-1

(Figure 6.7c). Miranda and Castro (1981) identified the

BC at 19°S as a surface narrow current (~ 75 km) limited to the upper 500

m. Evans et al. (1983) indicate that the BC remains confined and organized

as a coherent flow above the continental shelf at 20.5°S. The World Ocean

Circulation Experiment (WOCE) current-mooring measurements obtained at

19°S show a BC confined to the upper 200 m depth, with a mean southward

velocity of approximately 15 cm s

-1

(Müller et al., 1998). The mean cross-

shore section at 19°S obtained here from the 2005-2007 simulations (Figure

6.7c) confirms the presence of a southward BC tight flow (less than 100 km)

limited to the top 200 m. At 19°S east of the NBUC flow (6.7 Sv), one can find

the upper part of the DWBC transporting cold waters southward (1.8 Sv)

above

σ = 32.15 kg m

-3

(Fig. 6.7c).

In our simulation, the sSEC bifurcation reaches its southernmost

position in MJJ and its northernmost in OND, which corresponds

respectively to the months of July (southernmost) and November

(northernmost) found in the climatological runs of Rodrigues et al. (2007).

Furthermore, a time variation of the model outputs indicates (Figure 6.9)

that the NBUC system strengthens around May 2006 and May 2007, i.e.

when the sSEC bifurcation reaches its southernmost position, while the BC

transport is decreasing at that time until May 2006, when it is practically

null. On the other hand, maximum southward BC transports are verified

during January 2006 as well as in January 2007 and March 2007, just after

the period when sSEC bifurcation reaches its lowest latitudes, with a

minimum northward NBUC flow in December 2005 and October/December

2006 (Figure 6.9).

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

Figure. 6.9. Interannual variability of the North Brazil Undercurrent (NBUC) transport

along 8oS (from the surface to 400 m), and Brazil Current (BC) transport along 19oS

(from the surface to 400 m), obtained from ROMS simulations for the period of

September 2005 to July 2007. Positive (negative) values indicated by the black (gray)

line correspond to northward (southward) transports.

The depth dependence of the sSEC bifurcation latitude is abridged in

Figure 6.10a, b. In these figures, the sSEC bifurcation is represented by a

white line, where the averaged (2005-2007) MJJ and OND meridional

velocity (spatially averaged within a 1° longitude band off the Brazilian coast)

is zero. The white areas in Figure 6.10a-b represent the Vitória-Trindade

Ridge and Abrolhos Bank. ROMS also agrees well with the numerical

findings of Harper (2000), Malanotte-Rizzoli et al. (2000) and Rodrigues et al.

(2007), indicating a poleward depth increase of the sSEC bifurcation along

the Brazilian coastline. Averaged MJJ results in Figure 6.10a show that the

bifurcation varies from near the surface at 13°S to 500 m depth at the model

boundary at 24°S. During this period, the NBUC is strengthened and the BC

weakened. In contrast, the OND averaged results indicate that the

bifurcation shifts southward from 8°S at the surface layers to 20°S at 500 m

depth, when a weaker NBUC and stronger BC are observed (Figure 6.10b).

This southward deepening of sSEC bifurcation along the shoreline results in

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

a similar poleward depth increase of the NBUC cores, as verified in the 8°S,

14°S and 19°S transects (Figures 6.7a-c). These numerical results have also

been detected in field measurements. Stramma and England (1999) and

Rodrigues et al. (2007) showed from hydrographic data that the SEC

bifurcation takes place at 14-16°S in a near-surface layer (top 100 m), then

at 14-20°S in the layer between 100-500 m, and at 21-26°S in the AAIW

(500-1200 m). Wienders et al. (2000) also used hydrographic data to indicate

that the SEC bifurcation latitude is 14°S at the surface, 24°S in the 400-500

m layer and around 26°S-28°S in the AAIW. More recently, the annual mean

dynamic height and geostrophic flow charts generated by Rodrigues et al.

(2007) from CTD and bottle data (Curry, 1996) confirmed the poleward shift

of the sSEC with increasing depth.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

Figure. 6.10. Seasonal averaged (three months, 2005-2007) meridional velocity (m s-

1) obtained from the ROMS simulation for a) MJJ and b) OND. The velocities are

averaged within a 1° longitude band off of the Brazilian coast. The white line is the

contour of zero velocity that represents the bifurcation of the sSEC. The white areas

represent the Vitoria-Trindade Ridge and Abrolhos Bank.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

6.3.3 Coupling westward sSEC and wind stress

The zonal current and transport averaged from September 2005 to

July 2007 along the PIRATA-SWE array indicate how the southern part of

the SEC extends before reaching the western continental boundary (Figure

6.11a). Indeed, we note a complex succession of more powerful westward

systems. From south to north, we find two near-surface cores of -8.5 and -

2.9 Sv, corresponding to the broad and relatively weak sSEC westward flow

between 10°S and 19.5°S (Stramma, 1991; Stramma and Schott, 1999;

Lumpkin and Garzoli, 2005). More to the north, in the vicinity of the

northern PIRATA mooring, a third westward core of 2.8 Sv surfaces as the

southernmost part of the cSEC (Lumpkin and Garzoli, 2005). ROMS results

also point out a less powerful near-surface eastward core of 1.25 Sv located

between sSEC and cSEC. Other subsurface narrow eastward circulations

may reach the whole water column between 200 and 1200 m depth (not fully

shown in Fig. 6.11a).

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

Figure 6.11. (a) Mean annual volume transport averaged across the section along the

PIRATA-SWE array between 8°S-30°W and 19°S-34°W. Positive (negative) values indicated by

the solid (dashed) lines correspond to eastward (westward) currents. The two horizontal solid

black lines indicate the 24.5 and 26.8 sigma-t values (in kg m-3), respectively. (b) Time

evolution (Sep. 2005-June 2007) of the westward SEC transport (0-400 m depth) obtained

from ROMS simulations along the PIRATA-SWE array (solid line), and the QuickSCAT zonal

wind speed anomalies over the model domain (dashed line). The variance spectra for: (c) the

zonal wind, and (d) westward sSEC transport. The confidence levels are indicated by shaded

areas in both spectra.

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

According to this ROMS simulation, the mean westward transport

between 8°S-30°W and 19°S-34°W, corresponding to the sum of the sSEC

and cSEC, is -14.9 Sv for the upper 400 m, which agrees well with previous

observations and numerical studies. For instance, the hydrographic data

used by Stramma (1991) showed a westward SEC transport of about -20 Sv

along 30°W for the upper 500 m comprised between 3°S and 19°S.

Furthermore, the recent climatological simulations of Rodrigues et al. (2007)

indicate an annual mean westward SEC transport between 6°S and 22°S of -

15 Sv along the 30°W meridian and down to 400 m. These authors found

maximum SEC transports during JFM of about -15.6 Sv, and minimum

flows of -14.0 Sv during MJJ. From September 2005 to July 2007, monthly-

averaged westward transport (0-400 m depth) obtained from the ROMS

simulation across the PIRATA-SWE moorings (Fig. 6.11b), shows large

interannual and intraseasonal variability. The weakest westward transport (~

10-11 Sv) is noted in mid-2006, while the strongest westward SEC transport

(~ 18-19 Sv) occurs in February 2006, and three times from September 2006

to June 2007. The time evolution (Sep. 2005-Jun. 2007) of the QuickSCAT

zonal wind speed anomalies over the model domain is also plotted in Figure

6.11b. Since the mean zonal wind for this region is negative, a positive

difference indicates weaker westward zonal winds than the mean, while

negative deviation values indicate monthly averaged winds greater than the

mean for the simulation period. Positive zonal wind anomalies are associated

with a decrease in the upper transport of SEC, with time lags between zero

and two months. These values occur in April 2006, July 2006, November

2006, February 2007 and April 2007 (Figure 6.11b). Minimal SEC transports

are observed in May 2006, August 2006, January 2007 and April 2007. On

the other hand, the negative zonal wind anomalies in Figure 6.11b

(December 2006, January 2006, August 2006, September 2006, January

2007 and March 2007), are followed by an increase in the SEC transport,

which is observed in February 2006, September 2006, February 2007 and

April 2007. In Figure 6.11c-d the zonal wind speed anomalies over the model

domain are correlated with westward sSEC transport (0-400 m depth)

obtained from the ROMS simulation across the PIRATA-SWE (Sep. 2005 to

Chapter 6 – Interannual high-resolution regional ocean dynamics simulation …

Jun. 2007). Since the interest here is in the seasonal signals, a 30-180 day

band-pass filter was applied to both the atmospheric and the modeling data

to be compared. For the cross-correlation field, the variance-preserving

spectra are constructed for the area with higher cross-correlation. The

confidence levels are indicated by shaded areas in both spectra. The spectral

analysis points out the signal periodicity of both datasets. The calculated

maximum cross-correlation between zonal wind and westward transport is

0.51. The spectral analyses show two dominant signals at approximately 90

and 50 day periods for both zonal wind speed and sSEC transport (Figure

6.11c and d, respectively). In addition to seasonal effects, these ocean

signals may also represent the westward propagation of Rossby waves, as

evidenced in the mooring measurements and analyses of Schott et al. (2005)

and Schuckmann (2006).

Chapter 7 – Regional biogeochemical modelling in Southwestern tropical …

Chapter 7

7. Regional biogeochemical modelling

in the Southwestern tropical Atlantic

In the oligotrophic regions of the ocean, the supply of inorganic nutrients in

the euphotic layer may limit the concentration of microalgal biomass, the

rate of phytoplankton growth, or both (Marañón et al., 2000). Several

mechanisms have been proposed to explain the formation and maintenance

of the deep chlorophyll maximum (DCM), which is an oceanographic feature

of tropical and subtropical oceans. Higher in-situ growth at the nutricline

rather than in the upper mixed layer, and acclimation of phytoplankton to

low irradiance, has been suggested (Cullen,1982) as the cause of DCM

formation on subtropical gyres.

Early reports of low productivity in the subtropical gyres (Steeman-

Nielsen and Jansen, 1957; Thomas, 1970; Eppley et al., 1973) have been

considered to be in error due to methodological deficiencies (Fitzwater et al.,

1982). Nevertheless, recent work carried out in oligotrophic regions, using

new techniques (Malone et al. 1993; Letelier et al. 1996), presents a large

range in the rates of phytoplankton production and growth inside these

areas. In fact, this controversy will be maintained, while the lack of

Chapter 7 – Regional biogeochemical modelling in Southwestern tropical …

knowledge on temporal and spatial variability in the biology of open ocean

still exists (Marañón et al., 2000). Consequently, any uncertainties in the

productivity estimates in these areas will have significant effects on the

prediction of global biogeochemical models (Marañón et al., 2000). On the

other hand, there are few studies on the physical processes related with

DCM formation inside the Southwestern tropical Atlantic. Montes (2003)

suggests that the support of nutrient inside the euphotic zone, in the

Southwestern tropical Atlantic, is maintained by diffusion process in

nutricline basis. In this chapter, we use a coupled physical-biogeochemical

modelling approach to increase the knowledge of physical processes that

contribute to the formation of DCM, and investigate their seasonal variability

related with SEC/NBUC/BC system current.

7.1 The coupled ROMS approach

The ROMS version used in this chapter was previously presented in

Chapter 3. This modelling was based on physical approach of climatological

forcings at the Southern tropical Atlantic grid (Figure 7.1) adopted in

Chapter 5. The initial condition to start the biogeochemical model was

obtained from WOA2005 (Conkright et al., 2002) for nutrient and from

seasonal and annual SeaWifs data (Penven et al., 2008; O’ Reilly et al., 1998)

for Chlorophyll a, phytoplankton and zooplankton concentrations inside the

euphotic layer.

Chapter 7 – Regional biogeochemical modelling in Southwestern tropical …

Figure 7.1. Model domain (dashed gray lines) with the PIRATA-SWE locations (filled

triangles). Section along three PIRATA-SWE sites (solid lines). Thin dashed lines are

100m and 1000m isobaths, respectively.

7.2 Evaluation of simulated and satellite Chlorophill a data

As a preliminary test to validate the numerical results, the model

accuracy was evaluated by comparing monthly mean of chlorophyll a surface

maps derived from the ROMS model to monthly averaged 2001-2006 daily

SeaWifs Chlorophyll a data. This comparison was done for two different

scenarios: November, which corresponds to the austral spring (Figure

7.2a,c), and July, which corresponds to the austral winter (Figure 7.2b,d).

In the region of subtropical gyre, the model and SeaWifs results are

consistent, showing a low concentration of chlorophyll a in these two months

that correspond to oligotrophic areas (Figures. 7.2a, c and 7.2b,d). However

the bloom caused by upwelling on the south Brazilian coast is not in phase

with SeaWifs maps. The higher concentrations along the coast occur in July

to SeaWifs (Figure 7.2b) as the ROMS result indicates in November. It

suggest that constant primary production and growth rates does not

Chapter 7 – Regional biogeochemical modelling in Southwestern tropical …

represent locally the near coast upwelling. Furthermore, along the north

boundary (5ºS) ROMS indicates concentrations of chlorophyll a that does not

agree with satellite images. These vortices with blooms discrepancies

indicates that boundary conditions applied on NCPZD model can take more

influence inside domain that the thickness of relaxation layer of open

boundaries, mainly at north boundary.

(a)

(b)

(c)

(d)

Figure 7.2. Chlorophyll a distribution on surface to SeaWifs on: a) November b) July

and ROMS results on: c) November d) July

Chapter 7 – Regional biogeochemical modelling in Southwestern tropical …

7.3 Salinity intrusion causes and implications

Figure 7.3a presents monthly averaged results of simulated salinity to

Maximum Salinity Waters (MSW, hereinafter) values of salinity (salt

concentration > 37.5) occur south of 14ºS, a region inside the subtropical

gyre, with high surface salty waters. At mid-latitude the negative fresh water

balance is the main process that contributes to the increase in salinity by

high evaporation rates and low rain quantity. Furthermore, the surface

forced wind that transports surface waters masses to north are weaker in

November than in July. This holds the spreading towards the north of these

waters. Figure 7.3b shows an intrusion of MSW between 50 and110 meters

until it reaches near 10ºS. The equatorward transport of MSW is due to wind

driven circulation (Rodrigues et al., 2007). It transfers surface MSW of

subtropical gyre to lower latitudes. This presented result also suggests that

the subduction of MSW (finger) and northward spreading, in July, is

associated with the presence of lower density tropical water ( σ

< 24.8) at

low-latitudes in the SAWP region. The seasonal variability of MSW along this

section causes the occurrence of isohaline layer shallower (deeper) than the

isothermal layer in November (July). This has been studied, in the tropical

ocean, since the Meteor research cruises in 1936 (Defant, 1961). The barrier

layer (BL), which is located between the halocline and the thermocline

(Lukas and Lindström, 1991), may isolate the upper isohaline layer from the

cold and rich nutrient waters.

Chapter 7 – Regional biogeochemical modelling in Southwestern tropical …

(b)

(a)

Figure 7.3. Vertical salinity distribution of ROMS along PIRATA-SWE sites on: a)

November b) July.

Chapter 7 – Regional biogeochemical modelling in Southwestern tropical …

7.4 Seasonal Nitrate concentration

The monthly averaged nitrate results of the biogeochemical model in

November and July (Figures 7.4a,b respectively) shows that there doesn’t

exist great differences between nitrate distributions along this section.

Although, small differences that appears in the inclination of nutricline

between 14ºS and 8ºS, following the 36.5 isoline of salinity, shows best

agreement with Marañón et al. (2000) results of south subtropical gyre. This

small nutricline displacement to a deeper level between the two northern

PIRATA-SWE sites, in July, showing more nutricline establishment, may be

caused by intrusion of surface and poor nutrient MSW, which comes from

mid-latitudes and subductes until it reaches near 10ºS (Figure 7.3b). The

presence of a null concentration line along the section around σ

=26.8, July

(Figure 7.4b), indicates that the subduction of poor nutrient MSW, may

decrease the nutrient support on upper layers. Besides, in July (austral

winter) intensified trade winds, in this region, causes enlargement MLD. On

the other hand, in November (Figure 7.4a) at late austral spring, a small and

shallow thermocline appears between 19ºS and 14ºS, due to more intensities

of heat flux and weaker trade winds, the null line of nitrate concentration

almost disappears. However, low concentrations still are achieved along the

MLD. This agrees with Montes (2003) results, suggesting that the major

source of nutrients comes from deeper rich nutrient water by diffusion

processes.

Chapter 7 – Regional biogeochemical modelling in Southwestern tropical …

Figure 7.4. Vertical nitrate distribution of ROMS along PIRATA-SWE sites on: a)

November b) July.

Chapter 8 - Summary and perspectives

Chapter 8

8. Conclusions and perspectives

The tropical South Atlantic is a region of complex links between climatic

variability of the sea surface temperature (SST) and its heat content in upper

layers to an atmospheric convective system as precipitation on the adjacent

continent. The main goal of this thesis was to investigate the seasonal and

intraseasonal variability of mass and heat transport in two different regions

of the South Tropical Atlantic: the equatorial band and the Southwestern

tropical Atlantic. In order to achieve the proposed goal, a new generation

state-of-the-art ROMS was applied to reproduce the dynamic of these

100

Chapter 8 - Summary and perspectives

systems.

Numerical transports and currents results were compared to ocean

measurements obtained from oceanography cruises. These comparisons

suggest that the climatological modeling approach employed here reproduces

the main features of the zonal circulation observed from field measurements.

For example, numerical currents and transports distributions showed, in

Chapter 4, a very neat distinction between eastward and westward flows

along the equator. The model stressed the presence of a EUC core with

velocities reaching 0.8 m.s

-1

, occupying a deeper position in August (80-110

m depth) and shallower in March (50-80 m depth). The transport of EUC

ranged from 7.9 Sv (March, at the 15°W cross-equatorial section) to 15 Sv

(August, at 25°W). These numerical findings are confirmed by the sea

measurements and analysis of Stramma et al (2003), Giarolla et al. (2005)

and Brandt et al. (2006). The westward flows of the SECc and the SECn were

also well represented by this approach. The transport of SECc ranged from -

2.8 Sv (August, at 15°W) to -12 Sv (March, at 25°W), while the SECn

intensity ranged from -1.9 Sv (August, at 25°W) to -11.9 Sv (March, at 15°W).

Closer to the African shore, the seasonal variation of the equatorial currents

transport along the 5°W section shows highest values of the EUC and GC in

the period April–August, and lowest values occurring in the period

September-March. On the other hand, the SEC seasonality presents inverse

standards, i.e., weaker SECc and SECn transports in April–August and

stronger westward flows during the period September–March. The thermal

structure of the upper 500 m obtained in simulations was compared with

PIRATA Program dataset measured at the equatorial moorings. Special

101

Chapter 8 - Summary and perspectives

interest can be seen in the very well reproduced MLD for the equatorial

PIRATA sites. In particular, the upwelling induced shallower MLD verified by

the eastern buoys during austral winter and late spring. The Hovmüller

diagrams indicated that the vertical distribution of the simulated

temperature agrees with field measurements. Best agreement between model

and sea measurements is achieved close the surface on the western part of

the Atlantic basin (PIRATA sites 35°W and 23°W). Larger differences (~ 1 to

3°C) are noted between 100 and 300 m for eastern PIRATA sites (10°W and

0°W) in austral winter. The reduced flow in zonal currents observed (and

numerically reproduced) in August is associated to the intensification of the

equatorial upwelling mechanism in the eastern basin (Stramma and Schott,

1999). These authors also remind us that an equatorial upwelling and

downwelling exist on the equatorial currents system but where it takes place

exactly is an unsolved question. It seems to be a next interesting research

step to be explored with the numerical simulation approach. The seasonal

evolution of the Southwestern Atlantic Warm Pool (SAWP) – Cold Tongue

system was accurately represented on simulation. This is a very important

modelling issue since the eastern part of the Brazilian Northeastern Region

is subject to the effects of equatorial easterly atmospheric waves emanating

from convection over Africa. The generalized flooding conditions over the

coast of this region are observed when pulses of easterly waves encounter

warmer SST in the SAWP area (Nobre et al., 2004). These results are very

encouraging. The ROMS seems to be capable of reproducing the main

features of the mean EUC/NECC/GC and SECc/SECn at different sections

along the equator. The verified model adjustment to offshore field data is

102

Chapter 8 - Summary and perspectives

promising. For example, these results can be extensively used in the future

as boundary conditions for smaller-scale sea modelling, for evaluating the

remote influences of the open ocean forcing over the dynamics of the still

little studied oceanic islands in the northeastern Brazilian waters.

In Chapter 5, a climatological modelling brings new insights to Oceanic

terms (advection + diffusion) responsible for the seasonal evolution of the

heat budget inside the upper ocean mixed layer (MLD), which was calculated

from numerical outputs and compared with the atmospheric forcing balance.

Local illustrations of these results are given for the PIRATA-SWE sites and

along the western Atlantic Ocean boundary. It is confirmed that the

atmospheric surface fluxes are the main forcing mechanism acting on the

heat budget in upper ocean mixing layers. Nevertheless the oceanic

contribution is also very important, and can compensate the atmospheric

forcing effect in the southern part of the study region. During practically all

year, the oceanic component acts as an inversed contribution of the

atmospheric forcing. During the warm season, that corresponds for instance

(i.e. positive effect of the atmosphere) to a relatively strong cooling of the

mixing layer mainly by vertical diffusion through the thermocline from colder

deeper waters, and, during the cold season (i.e. negative effect of the

atmosphere) to a weaker warming of the surface waters, mainly by horizontal

advection.

These ROMS outputs, in this region, confirm the extreme complexity of

the oceanic circulation in this area. Along the virtual track represented by

the three PIRATA moorings (about 1400 km), we highlighted no less than six

westward currents and four eastward currents on a yearly average. Although

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Chapter 8 - Summary and perspectives

the annual net mass transport across this PIRATA track is normally directed

towards the west (18.8 Sv for the upper 600 m, in agreement with early

estimations), that indicates that it will not be possible to perform any

geostrophic computations using only the density profiles measured by the

PIRATA sites. This implies that a continuous observation of the ocean

dynamics in this area will be necessary to implement the PIRATA network

with other better adapted systems.

Chapter 6 presents the use of ROMS forced by interannual boundary

conditions in order to investigate the intraseasonal and interannual

variability in the southwestern Atlantic for the 2005-2007 period. After

checking that ROMS makes it possible to capture correctly the meso-scale

phenomena illustrated by instant SST patterns, numerical temperature

values, issued from ROMS simulation, were compared to vertical profiles

obtained by the first two years of available data from the recent deployed

PIRATA-SWE buoys. Comparison between simulated temperature and in-situ

temperature data are in good agreement for the first 500 m ocean layer.

These ROMS outputs confirm the extreme complexity of the oceanic

circulation in this area. This is obviously the case along the western

continental boundary where many alongshore currents coexist and can

interact. The interannual numerical results indicate a close relationship

between SEC and the western boundary currents flowing along the Brazilian

edge. When the sSEC bifurcation reaches its southernmost position (MJJ),

the northward NBUC transport is stronger (May 2006, May 2007) and the

BC transport decreases. Otherwise maximum southward BC flows are

verified during January 2006 and January/March 2007, with minimum

104

Chapter 8 - Summary and perspectives

northward NBUC flows in December 2005 and October/December 2006,

during the period when sSEC bifurcation reaches its lowest latitudes (OND).

Along the section delineated by the PIRATA-SWE moorings, we

highlighted the existence of three strong near surface westward currents

alternated by two weak eastward currents on a yearly average. Although the

annual net mass transport across this PIRATA track is usually westward

(14.9 Sv for the upper 400 m), this indicates that geostrophic computations

using only the density profiles measured by the three PIRATA-SWE sites are

somewhat misfit. This situation is worsened because the vertical

measurements of salinity on the ATLAS moorings are presently limited to

only four levels within the 0-120 m upper layer. This implies that for a

continuously observation of the ocean dynamics in this area it will be

necessary, not only to upgrade the vertical sampling of the PIRATA-SWE

network, but, to also certainly implement other adapted systems.

The evaluation of this high resolution regional ocean simulation shows

the capability of the model in reproducing the known ocean dynamics in the

region and their variability. One can note that it has been achieved on a

relatively long period and as a free run (i.e. without the need of artificially

restoring the model solution towards observations). Hence, we can analyze

now this model solution to rigorously diagnose the dynamical balances and

the eddy dynamics.

Chapter 7 presents results of an ecological model that runs coupled

with the Climatological ROMS, as set in Chapter 5. Evaluation of monthly

mean chlorophyll a between simulated and SeaWifs satellite data on two

105

Chapter 8 - Summary and perspectives

different months shows that the model can reproduce main features of

primary production in an oligotrophic area limited by nutrients support,

where small oscillation in this support can produce high difference in

results. The main contribution of this modelling to knowledge of

biogeochemical processes around Southwestern tropical Atlantic occurs

when this simulation is capable of representing a seasonal wind driven

transport. That shows increases in subduction of MSW in July. This comes

poor nutrient shallow waters to the thermocline basis. On other hand, small

differences between nutrient distribution along nutricline in November and

the maintenance of low primary production inside the euphotic zone along

the year at the subtropical gyres, confirmed by Marañón et al. (2000),

suggest that the vertical diffusion of nutrients acts to balance a deeper

mixed layer and decreasing in light availability in July. The use of ROMS

coupled with NCPZD could be used in the future as a useful tool to estimate

the dispersion of the tuna eggs and larvae in the divergence of SEC (Mullon

et al., 2002; Hugget et al., 2003).

A potential outcome of this thesis is the possibility of coupling the

ROMS with a dynamically active regional atmospheric model, when SST,

heat and freshwater fluxes are calculated from processes which couple both

marine boundary layers. These studies are underway. The ROMS can be

used with a higher resolution to resolve smaller scales, using improved

turbulence closure schemes, or using better surface forcing or lateral

boundary conditions. Using the nesting capability of ROMS (Penven et al.,

2006), we could also explicitly resolve fast propagating signals across the

whole Tropical Atlantic while addressing the coastal response in the

106

Chapter 8 - Summary and perspectives

107

Southwestern region.

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