Download Langmuir circulations and enhanced turbulence beneath wind

A SIMULATION OF THE DIVERGENCE OF THE SOUTH EQUATORIAL CURRENT
NEAR BRAZILIAN EDGE
Moacyr ARAUJO1,§, Dóris VELEDA1, Marcus SILVA1, Jacques SERVAIN2 and
Pierrick PENVEN2
Abstract : The seasonal variability of the South Equatorial Current (SEC) divergence off Brazil has
an important influence on the surface ocean heat transport along the western Atlantic boundary, and
weather changes of the eastern South America. This work aimed at exploring the thermohaline
structure and circulation at the tropical Atlantic (5-25S and 20-47W) where SEC diverges. The
Regional Ocean Model System (ROMS) was used to simulate the circulation, with an isotropic 1/3o
horizontal grid resolution and 20 terrain-following layers. Numerical results are compared to T/S
profiles collected during the Brazilian REVIZEE/SCORE-NE Program, and to current
measurements obtained from German partners of CLIVAR Program. These comparisons show
thermodynamic fields, as well as the northward and southward flows along Brazilian coast that form
after SEC bifurcation, in agreement with observations: The northward current reaches up to 70 cm.s1
near 5oS and is less intense southward, about 15 cm.s-1 at 21oS. Simulations indicate that SEC
divergence is shifted southward as depth increases (from 6S at surface to 20S at the 500 m), as
well as during June and July.
Keywords : South Equatorial Current divergence; Lat 5S-25S; Long 20W-47W; Ocean
modelling; ROMS.
Resumo : A variabilidade sazonal da divergência da Corrente Sul Equatorial (SEC), que ocorre nas
proximidades da costa brasileira, influi significativamente no transporte oceânico de calor ao longo
da fronteira oeste do Atlântico, assim como nas condições climáticas da porção leste da América do
Sul. Este trabalho analisa a estrutura termohalina e a circulação no Atlântico tropical onde a SEC se
bifurca (5S-25S e 20W-47W). O Sistema Modelo Regional Oceânico (ROMS) é utilizado para
simular a circulação, considerando-se uma malha horizontal isotrópica de 1/3o e 20 camadas
verticais. Os resultados numéricos são comparados com os perfis de T/S do Programa
1
Laboratório de Oceanografia Física Estuarina e Costeira, Departamento de Oceanografia da Universidade Federal de
Pernambuco -LOFEC/DOCEAN/UFPE. Av. Arquitetura s/n, 50740-550, Cidade Universitária, Recife, PE, Brasil.
§ Corresponding author ([email protected]); CNPq fellow.
2
Laboratoire de Physique des Océans, Institut de Recherche pour le Développement – IRD. Boite Postale 809, 6,
Avenue Le Gorgeu, 29285 Brest CEDEX, France.
REVIZEE/SCORE-NE, e com as medidas de corrente do obtidas pelos parceiros alemães do
Programa CLIVAR. As comparações sugerem que os campos termodinâmicos, bem com as
estruturas dos fluxos para norte, e para sul, originados com a bifurcação da SEC, estão de acordo
com as observações de campo: correntes alcançam 70 cm.s-1 para norte (5oS), e valores inferiores de
cerca de 15 cm.s-1 para o sul (21oS). As simulações indicam que a divergência da SEC é deslocada
para sul à medida que a profundidade aumenta (de 6S na superfície até cerca 20S a 500 m), assim
como nos meses de Junho e Julho.
Palavras-chave : Divergência da Corrente Sul Equatorial; Lat 5S-25S; Long 20W-47W;
Modelagem oceânica; ROMS
INTRODUCTION
The oceanic circulation in equatorial regions of the Atlantic Ocean is a particularly interesting
due to the complex system of currents oriented mainly in the zonal direction. The South Equatorial
Current (SEC), is formed by at least three easterly branches separated by regions of less evident
counter-currents (Stramma, 1991). SEC flows westwards until it encounters the South American
continental boundary. There is divides in two western boundary currents flowing north and south:
the North Brazil Current (NBC) and the Brazil Current (BC).
The latitude where SEC bifurcation occurs is still unknown, although it is generally accepted
that the NBC begins generally about 10°S, when a branch of the SEC flows north and merges with
the North Brazil Under Current (NBUC). The southern branch becomes the BC, which flows
southwest, merging into the South Atlantic gyre system (Silveira et al, 1994; Stramma et al., 1990,
1995; Schott et al., 1998). This work has several applications as studying the seasonal, intraseasonal and interannual dynamics of this oceanic area, which is poorly understood. The historical
observations of subsurface are almost non-existent. However, this area is the cradle of multiple
oceanic-weather forcings of great importance. There are (i) transfer of heat and mass between
different sectors of the tropical Atlantic subsurface. This is the region of oceanic divergence of SEC,
in a northern and a southern branches along the Brazilian coastline, and in a subsurface eastern
branch along the equator; (ii) exchanges of heat and fresh water between the ocean and the
atmosphere in the tropical Atlantic surface; (iii) complex links between climatic variability of the
sea surface temperature (SST) and heat content of the upper layers of the tropical Atlantic and
related atmospheric systems which causes precipitation on the continents, with emphasis to the
Brazilian Northeast. This work intends to contribute to better defining the arguments of the project
for extending PIRATA (Pilot Research moored Array in the Tropical Atlantic) network (Servain et
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al., 1998), i.e. the South-West Extension (PIRATA-SWE) which will run along the edge of Brazil's
coastline south of the equator.
The Regional Ocean Modeling System (ROMS) was used for evaluating the seasonal
variability of the SEC divergence off Brazil. The ROMS is a versatile new generation state-of-theart model that is rapidly growing in the ocean modeling community. It was used to model the
circulation in different regions of the world ocean (Haidvogel et al., 2000; Malanotte-Rizzoli et al.,
2000; She and Klinck, 2000; Penven et al., 2000, 2001a,b; MacCready and Geyer, 2001; Lutjeharms
et al., 2003).
The next section presents the experimental data set, which will be compared to model results.
These data are from a high vertical resolution CTD, obtained during the cruises of the Brazilian
REVIZEE (Assessment Living Resources in the Exclusive Economic Zone) Program, as part of
REVIZEE/SCORE-NE Program (Programa Nacional de Avaliação do Potencial Sustentável de
Recursos Vivos da Zona Econômica Exclusiva/Sub-Comitê Região Nordeste). A Brazilian Program
for Assessing the Sustainable Potential of the Live Resources of the Exclusive Economic Zone
(EEZ), within the ambit of the Inter-ministerial Commission for Sea Resources-CIRM that resulted
from the commitment undertaken by Brazil when ratifying, in l988, the UN Convention on the Law
of the Sea, in force since November 1994. Another used field data is from cinematic measurements
obtained in German CLIVAR (Climate Variability and Predictability Programme) Program.
Finally, a model description is followed by a discussion of the results. The last Section presents a
summary and conclusions.
EXPERIMENTAL DATASET
Two different sources of field data were used for evaluating the model’s performance. In the
first one, T/S data were collected during the NOc. Antares cruises at Northeastern Brazilian waters
(1995-2000). This T/S data set comprises 618 CTD continuous profiles taken with a Sea Bird
Electronics SBE911plus CTD with C (resolution=0.00004 S.m-1), T (resolution=0.0003 °C) and P
(resolution=0.068m) sensors and a centrifugal pump. The CTD operated connected to a SBE 11plus
deck unit, allowing real time control of the data. A descent rate of 1m.s-1 and a sampling rate of 24
Hz were used. During measurements, the ship was positioned bow to wind and typical ship drift was
less than 1 knot. Station coordinates refers to the position of the beginning of the CTD profiling.
Figure 1 shows the study area, indicating the CTD stations and the positions of the Transects T11
(11oS) and T21 (21oS).
3
Figure 1. Study area showing model integration domains and CTD stations from
REVIZEE/SCORE-NE Program, and the positions of Transects T11 (11oS) and T21 (21oS).
The second field data set was obtained during the period March to November 2000 at 5oS
section (Stramma et al., 2003) and at the current-array maintained by the Institut fur Meereskunde
(IfM) as part of the German CLIVAR tropical Atlantic project. This is an array of five current-meter
moorings along 225 km across the NBC (10-11oS). It is located just off the shelf edge at 2320 m
water depth and is equipped with an upward-looking ADCP measuring the currents profile between
300 m and 30 m below the surface (Schott et al., 2002). The position of the German current-array is
indicated by transect T11 in Figure 1.
NUMERICAL SIMULATIONS
The ROMS solves the free surface, primitive equations in a Earth-centered rotating
environment, based on the classical Boussinesq approximation and hydrostatic vertical momentum
balance. The model considers coastline- and terrain-following curvilinear coordinates, which allows
minimizing the number of dead points in computing the solution. The boundary conditions for the
model are then appropriate for an irregular solid bottom and coastline, free upper surfaces and openocean sides away from the coastline. These include the forcing influences of surface wind stress,
4
heat and water fluxes, coastal river inflows, bottom drag, and open-ocean outgoing wave radiation
and nudging towards the specified basin-scale circulation.
Upstream advection in ROMS is treated with a third-order scheme that enhances the solution
through the generation of steep gradients as a function of a given grid size (Shchepetkin and
McWilliams, 1998). Unresolved vertical sugbrid-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 may be found in Haidvogel et al. (2000), and Shchepetkin and McWilliams
(2003, 2004).
The study case presented here concerns to the ocean area near the Brazilian coast where the
southern branch of the South Equatorial Current bifurcates. Integration domains are comprised
within 5oS and 25oS, and 20oW and 47oW. An isotropic 1/3o horizontal grid was used for
simulations, resulting in 62 x 80 horizontal mesh cells.
Vertical discretization considers 20 levels. Bottom topography was derived from a 2’
resolution database ETOPO2 (Smith and Sandwell, 1997), and a “slope parameter” r  h h  0.20
has been used to prevent errors in the computation of pressure gradient (Haidvogel et al., 2000).
At the three lateral open boundaries (North, East and South) an active, implicit, upstream
biased, radiation condition connects the model solution to the ocean surroundings. Horizontal
Laplacian diffusivity inside the integration domain is 1.3 x 103 m2.s-1 (Ferreira, 2001; Moore et al.,
2004), and a 4-points smooth increasing is imposed (up to 104 m2.s-1) in sponge layers near open
ocean boundaries. The model equations were subject to no-slip boundary conditions at solid
boundaries. A basin scale seasonal hydrology derived from WOA2001 database (monthly
climatology at 1o resolution) was used to infer thermodynamics (temperature and salinity) and
geostrophy induced currents at the open boundaries. The circulation was forced at the surface by
winds, heat fluxes and fresh water fluxes derived from the COADS ocean monthly fluxes data at
0.5o resolution (Da Silva et al., 1994).
RESULTS AND DISCUSSION
Using the forcings presented in previous sections, the model run from a state of rest during
10 years. Figure 2 shows a time series of the integration domain kinetic energy (KE) from the
model. It indicates that after a spin-up period of about one year, the model has achieved a
statistically steady state. All the numerical results examined correspond to the last year of
simulation (year 10).
5
Figure 2. Time series of integrated kinetic energy from the model during 10 year spin-up.
The annual cycles of the winds and sensible heat stored in the upper layers of the tropical
Atlantic are influenced by meteorological systems over and off the continents, especially the
Intertropical Convergence Zone (ITCZ), Cold Fronts, Easterly Waves, Upper Air Cyclonic Vortices
(Cold Lows) and Land/See Breezes. Nevertheless, the interannual variability of winds and SST in
the tropical Atlantic are modulated by the solar heating annual cycle. As a consequence, the
predominant spatial standards of the SST annual cycle in the Atlantic presents a North-South pattern
more pronounced than an East-West.
Seasonal evolution of SST obtained from simulations is presented in Figure 3. Plotted SST
surfaces correspond to mid-month snap-shots for January, March, May, July, September and
December. These figures indicate that the waters off the South America southeastern coast present
strong gradients of SST and a displacement of cold water to near the confluence of the Brazil
Current and the Malvinas Current. The cold water reaches low latitudes during the southern winter.
Vertical profiles of temperature and salinity (T/S) issued from numerical results are compared
to field CTD measurements in Figure 4. These examples correspond to REVIZEE/SCORE-NE
cruise performed during March 1997. Figure 4 suggest that the modeling approach used here is able
to describe the main thermohaline structures (and water masses) observed at the study area.
Figures 5, 6 and 7 present model results for the seasonal evolution of the horizontal current vector

v  u 2  v2

1/ 2
at surface, 200 m and 500 m depth, respectively. Comparisons among these results
6
indicate that, for the same period of analysis, the SEC bifurcation shifts southward as ocean depth
increase. It varies from 6S at surface waters to 20S at the 500 meters depth layer. By the same
time, the seasonal variability of the SEC dynamics observed in Figures 5 to 7 shows, for all depths,
that the divergence takes place at southward during the austral winter (June-July). SEC divergence
occurs at lower latitudes at spring-time (November).
Figure 3. Seasonal evolution of Sea Surface Temperature (SST, oC) obtained from the model
7
simulations. Dashed lines correspond to 100 m and 500 m isobaths.
5
REVIZEE STATION
LATITUDE
0
-5
-10
Recife
BRAZIL
Salvador
-15
-20
Rio de Janeiro
-25
-50
-45
-40
-35
-30
-25
-20
LONGITUDE
Figure 4. Comparison between numerical (ROMS) and experimental (REVIZEE/SCORE-NE) T/S
profiles.
8

Figure 5. Seasonal evolution of horizontal current vector v  u 2  v2

1/ 2
at surface obtained from
simulations.
9

Figure 6. Seasonal evolution of horizontal current vector v  u 2  v2

1/ 2
at 200 m depth obtained
from simulations.
10

Figure 7. Seasonal evolution of horizontal current vector v  u 2  v2

1/ 2
at 500 m depth obtained
from simulations.
Maximum speed values of about 70 cm.s-1 are verified near shore at 5-6oS (Figure 6, 200 m
depth). These numerical intensities are in good agreement with the field velocity-vector
11
measurements from the 5oS ADCP section (175 m to 225 m depth) in March and April 2000
(Stramma et al., 2003).
Snap-shots of the meridional velocity (v) from modeling along transects T11 and T21 are
presented in Figures 8 and 10. These transects are situated near 11oS and 21oS, respectively. Positive
values indicate northward currents in both situations. Figure 8 shows the intense near shore
northward transport induced by the NBUC pointed out by the horizontal velocity fields (Figures 5 to
7). This transport is stronger between 200 m and 300 m depth, as already identified from field
measurements (Schott et al., 2002; Stramma et al., 2003). Toward the surface the velocities
decrease considerably, confirming the undercurrent characteristic of the NBUC. Model results
suggest that NBUC at 11oS presents an almost steady flow, with a slightly higher northward
transport in austral summer (January-March) and slower meridional velocities during spring
(August to November).
Below the warmer and northward NBUC, Figure 8 confirms the presence of the North
Atlantic Deep Water (NADW) flowing between 1400 and 1900 m depth. Much more instable than
the NBUC, the deeper and colder flow transporting NADW presents more or less alternate periods
with northward and southward (predominant) transports along the continental slope.
Examples of speed time series obtained in simulations are plotted in Figure 9. These results
correspond to current vectors at 11.6oS - 35oW grid point located just off the shelf edge in 2500 m
water depth, and for different model vertical layers. The stick diagrams in Figure 10 indicate that the
northward transport is weak near surface (50 m layer), even with registered periods of southward
transport. Under the upper well-mixed surface layer we met the core of the NBUC, ranging from
100 to 1000 m depth (Figure 8). Maximum northward speeds are verified at these depths. The 1100
m depth diagram represents the current evolution at the reference level that divides the northward
warm water flow from the southward flowing NADW. The results in Figure 9 are in agreement with
the vertical structure of NCB/NBUC observed by Schott et al. (2002), although more intense values
of northward currents were obtained from (L)ADCP and mooring-array data.
12
Figure 8. Snap-shots of the model meridional velocity (v, m.s-1) along transects T11 - NBC/NBUC
11oS. Positive values indicate northward currents.
13
50 m
270 m
550 m
780 m
1100 m

Figure 9. Stick diagram of current vector v  u 2  v2

1/ 2
obtained in simulation at 11.6oS - 35oW.
14
Positive values indicate northward currents.
Figure 10. Snap-shots of the model meridional velocity (v, m.s-1) along transects T21 – BC 21oS.
Negative values indicate southward currents.
The seasonal changes in BC flow near 21oS may be seen from model results in Figure 10. In
opposition to NBC/NBUC (Figure 8), these transects indicate a dominant southward (negative
velocities) flow, which is more confined to the shallow and near shore part of the continental slope
15
when compared to the NBUC at 11oS. These numerical results seem also to be observed in previous
studies. Miranda and Castro (1981) identified the BC at 19oS as a surface narrow current (~75 km)
limited to upper 500 m depth. Evans et al. (1983) indicate that BC keeps confined and organized
above the continental shelf at 20.5oS. The recent WOCE current-moorings measurements obtained
at 19oS pointed out a BC confined to the upper 200 m depth, with mean southward velocity about
15 cm.s-1 (Müller et al., 1998). All these evidences are well reproduced in Figure 9.
CONCLUSION
The primary motivation behind this study was to investigate the thermohaline and circulation
structures in the western Atlantic ocean boundary comprised within 5S-25S and 20W-47W.
These coordinates limits the area where the South Equatorial Current (SEC) encounters the South
American continent and bifurcates, with the water flowing north and south as two western boundary
currents (North Brazil Current - NBC and the Brazil Current - BC, respectively). This is a region of
complex links between climatic variability of the sea surface temperature (SST) and heat content of
the upper layers of the tropical Atlantic, atmospheric convective systems and precipitation on the
adjacent continent, especially on the Brazilian Northeast region. We are particularly interested on
evaluating the capacity of a specific numerical modeling technique on reproducing seasonal transfer
of heat and oceanic mass between different sectors of the subsurface tropical Atlantic. This was
accomplished by using the regional ocean circulation model ROMS (Regional Ocean Model
System).
Numerical temperature/salinity (T/S) results were compared to vertical profiles obtained
during the Brazilian REVIZEE/SCORE-NE cruises, and simulated currents were evaluated from the
ocean measurements obtained by the German part of CLIVAR program and by the WOCE
experiment. These comparisons suggest that the modeling approach employed herein reproduces the
main features of the thermodynamics and circulation observed from field measurements. For
example, numerical T/S distributions along water depth are similar to REVIZEE measurements.
Model results show a strong northward under surface speeds up to 70 cm.s-1 near 5oS, corresponding
to the North Brazilian Under Current – NBUC field data (Stramma et al., 2003). Also surface, less
intense, southward speeds of about 15 cm.s-1 at 21oS, are in good agreement to measurements
obtained by Müller et al., (1998) for the Brazilian Current – BC. Concerning the location of SEC
divergence near Brazilian edge, simulations indicate that SEC divergence shifts southward as ocean
depth increases (it varies from 6S at surface to 20S at the 500 m), as well as during austral winter
(June-July).
16
The results are very encouraging. This is the first time (to our knowledge) that ROMS is used
for examining SEC divergence area off Brazil, hence the verified model adjustment to field data
seems to be promising. Concerning the modeling approach, future exercises with more refined
isotropic grids (1/6o, 1/12o), as well as parameterizations for the horizontal Laplacian diffusivity
(e.g. Smagorinsky formulae) should be performed. From the oceanographic point of view, the
biogeochemical/ecological routines could be linked to the physical circulation part, especially for
investigating near shore transfer of plankton biomass and nutrients. In this case, the use of nested
high-resolution grids for coastal area will be a very useful technique.
One approach to enhancing climate prediction skill is to try to improve the coupled models as
a system (Murtugudde et al., 1996, among others). A potential outcome of this work is the
possibility of coupling ROMS with a dynamically active atmospheric model, when SST, and heat
and freshwater fluxes can be calculated from properly modeling the atmospheric processes that
couple the marine boundary layer.
This study is also defined as an oceanic contribution to develop a research and academic
network dedicated to promote research initiatives, operational and monitoring activities, and
training at several levels in the subjects of climate and climate variability, focusing on the tropical
Atlantic and the Semi-Arid region of Northeast Brazil (NEB). The emphasis will be on their impacts
on the environment and socio-economic issues of this region. This Project, named CAMISA - Clima
do Nordeste e do Atlântico Tropical e o Monitoramento dos Impactos no Semi-Árido (Climate of
the Northeast Brazil and Tropical Atlantic and Monitoring their Impacts in the Semi Arid Region)
accounts for the participation of Universities and Research Centers from the Northeast of Brazil,
and worldwide (e.g. IRD, IRI).
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