A Modelling Study of Coastal Upwelling Driven by Wind and
Meanders of the Brazil Current
Renato M. Castelãoa, , Edmo J. D. Camposa and Jerry L. Millerb,
a
Instituto Oceanográfico, University of São Paulo, Praça do Oceanográfico, 191, Cid.
Universitária, 05508-900 São Paulo, SP, Brazil.
bOceanography Division, Naval Research Laboratory, Stennis Space Center, MS, USA.
ABSTRACT
A numerical model is used to investigate coastal upwelling in the South Brazil
Bight. The wind in the area is predominantly from the northeast, especially in
summer, which is upwelling favorable. Reversals of the wind direction are
frequent and intense during the winter, due to the passage of frontal systems. The
offshore circulation is dominated by the Brazil Current, which flows southward
meandering around the 200 m isobath. Significant shelf-break upwelling has
being associated with Brazil Current cyclonic meanders. To assess the relative
importance of the two processes in the pumping of South Atlantic Central Water
(SACW) onto the continental shelf, three cases are analyzed: (1) wind-driven
upwelling; (2) upwelling induced by Brazil Current meanders and (3) both effects
acting together. The results show that in the coastal area upwelling/downwelling
is mainly caused by the wind, whereas the cyclonic meanders of the Brazil
Current are the dominant mechanism in the generation of vertical velocities over
the shelf break and slope. This meander-induced upward motion brings the
SACW to shallower depths, where it is influenced by the wind. In this situation,
when both effects act together, the SACW penetrates all the way to the coast.
INDEX WORDS: Coastal oceanography; Coastal upwelling; Wind-driven
circulation; Shelf dynamics; Brazil Current; Cyclonic meander; Brazil; South
Brazil Bight
INTRODUCTION
The Study Area
The area of interest of this study is
usually referred in the literature as the
South Brazil Bight (SBB), with Cabo Frio
as its northern limit, and Cabo de Santa
Marta as its southern limit (Figure 1).
CASTRO and MIRANDA (1998), in their
review of coastal oceanography off Brazil,
consider that the water on the upper slope
and shelf of the SBB are the result of
mixing of three water masses: Tropical
Water - TW (T>200C, S>36.40), South
Atlantic Central Water - SACW (T<200C,
S<36.40) and Coastal Water - CW, a low
salinity water mass resulting from dilution
of oceanic water by fresh water input from
estuaries along the SBB coast.
The wind is predominantly from the
northeast, especially in summer, which
implies in offshore Ekman transport near
the surface. Underneath the Ekman layer,
cross shelf circulation brings SACW to near
the
coast,
increasing
productivity
(MATSUURA, 1996). In summertime, the
1
SACW has been detected in the SBB as
close as 50 km near coast (CAMPOS et al.,
1999). CASTRO and MIRANDA (1998)
report the detection of SACW even closer
to the coast. During the winter, reversals of
the wind direction are frequent and intense,
due to the passage of frontal systems that
propagate northward. This promotes
downwelling and the SACW retreats
toward the shelf break (CASTRO et al.,
1987), being detected only in the middle
and outer shelves (depths greater than 100
m – CAMPOS et al., 1999). In this
situation, the stratification is very small.
Oceanographic Conditions in the South
Brazil Bight
Figure 1 - Study area with bottom topography. The
light shaded area shows the model domain. The dotted
thick line shows the location of Line A, and the dotted
thin lines mark the area shown in plots of model results.
Topographic contours are (in meters) 50, 100, 200, 500,
1000, 1500, 2000, and 2500. In the lower right hand
corner, an inserted satellite-derived image illustrated the
meandering of the Brazil Current in the region.
The circulation in the SBB is
dominated by the Brazil Current, which
flows southward meandering around the
200 m isobath, as illustrated by the satellite
image inserted in Figure 1 (CAMPOS et al.,
1999). The change in the coastline
orientation at Cabo Frio induces a
meandering pattern, which frequently
becomes unstable forming strong cyclonic
and anticyclonic frontal eddies (CAMPOS
et al., 1995; CAMPOS et al., 1996). The
role of the meanders and eddies in the shelf
break upwelling system, according to
OSGOOD et al. (1987), can be understood
as follows. A meander trough consists of a
cyclonically rotating dome of cold upwelled
water, which lies between the western
boundary current and the continental slope.
The meander water is continually
exchanged due to divergence on the leading
edge of the dome and convergence on its
trailing edge. As the leading portion of the
meander flows southward, upwelled water
is advected toward the continental shelf.
Such meanders and eddies appear to be a
ubiquitous feature of western boundary
current systems (MILLER and LEE,
1995a). The meanders, occurring in the area
between the shelf and the deep ocean,
provide an important communication
between these two regions, since most of
the nutrients that support the high
biological productivity of continental
shelves come from the deeper adjacent
ocean. LEMING (1981) observed large
amounts of cold water pumped onto the
continental shelf close to Cape Canaveral,
associated with a shelf break meander of the
Gulf Stream. Gulf Stream cold cores and
warm filaments transport heat, salt, and
other substances between the continental
shelf and the adjacent deep ocean to such an
extent that the hydrography (ATKINSON et
al., 1983) and primary productivity (LEE et
al., 1991) of the shelf can be substantially
altered by meander behavior (MILLER and
LEE, 1995b).
Significant shelf break upwelling
has been associated with Brazil Current
cyclonic meanders. BRANDINI (1990)
states that, at some places along the SBB
shelf break, productivity can be higher than
in the coastal zone, especially during
summertime. CAMPOS et al. (1995)
suggest that the combination of near-shore
wind-driven upwelling and deeper shelf
2
break upwelling may be responsible for the
transport of the nutrient-rich SACW from
regions deeper than 200 m to the shallower
parts of the continental shelf near the coast.
Objective and Structure of this Article
The present study was based on
numerical simulations with the objective of
investigating the importance of the two
processes in pumping SACW onto the
continental shelf.
Section 2 presents the model
configuration and the initial and boundary
conditions used. The wind driven
upwelling, the upwelling induced by
cyclonic meanders of the Brazil Current and
the interactions of the two processes are
discussed in section 3. Section 4 presents
the summary and conclusions.
METHODOLOGY
Model Configuration and Initial
Conditions
The model used is the Princeton
Ocean Model (POM) (BLUMBERG and
MELLOR, 1987). It is a finite difference,
three-dimensional model, containing a
second-order turbulence closure submodel
providing the vertical mixing coefficients
(MELLOR and YAMADA, 1982). The
domain used (Figure 1) extends about 1380
km alongshore from Cabo de Santa Marta
to Cabo Frio, and about 850 km offshore
from Santos, covering the entire South
Brazil Bight. The horizontal grid spacing
was 10 km in the alongshore direction, and
5 km in the offshore direction. The first
baroclinic Rossby radius at the SBB is
approximately 20 km (HOURY et al.,
1987). The grid is rectangular and the axes
have been rotated to 500 in order to better
align with the coastline. In the vertical, 21
sigma levels were used, more compressed
near the surface and the bottom in order to
resolve the respective boundary layers. The
maximum depth is set to 2500 m in order to
reduce the constraint on the time step.
The temperature and salinity fields
used as initial conditions for all the
simulations were obtained during the
austral summer cruise (Jan/14-Feb/2 1993)
of the COROAS Experiment, a Brazilian
contribution to WOCE. The temperature
and salinity measured along a cross-shelf
section from the central SBB were repeated
for the whole domain, according to the local
depth, so there were no initial along-isobath
temperature or salinity gradients. A cross
section of the initial temperature and
salinity structure along Line A (see Figure 1
for location) is shown in Figure 2a,b. Note
that there is already a core of the SACW
over the shelf, but it is disconnected from
the same water mass where it is present on
the continental slope.
The
geostrophic
velocities
associated with the density field were
calculated and used to initialize the model
(Exp. 3-5, see Table 1). The reference level
used was 900 dbar, the same as that used by
CAMPOS et al. (1995), and the f-plane
approximation was used. The imposed
transport of the Brazil Current was 12.4 Sv,
which is probably overestimated (CAMPOS
et al. (1995) estimated the transport of the
Brazil Current around 250S to be 8.8 Sv).
This bigger transport was an artifact for
getting a more unstable flow in Exp. 3-5.
At the open boundaries, three sets
of boundaries conditions were used. For the
wind-forced simulation, an ORLANSKI
(1976) radiation condition was adopted for
all dynamical variables. This treatment of
the open boundaries was sufficient to
maintain mass conservation over the model
domain (LI and WEISBERG, 1999). For
the Brazil Current simulations, a relaxation
scheme, based on Martinsen and
ENGEDAHL (1987), was used for the
internal velocities normal to the boundary.
The velocity calculated by the model was
relaxed to the geostrophically balanced
velocity used as the initial condition. When
both forcing effects were considered
together, a partially-clamped condition was
used for the velocities normal to the
3
boundary, following BLUMBERG and
KANTHA (1985).
Numerical Experiments
In order to understand the relative
importance of wind forcing and cyclonic
meanders of the Brazil Current in upwelling
the SACW onto the shelf, a set of numerical
experiments were pursued, which are
summarized in Table 1. On Experiments 1
and 2, a spatially uniform time-independent
wind stress of 0.1 Pa was considered as
forcing. The wind was oriented 500 relative
to true north (Exp. 1 – NE, Exp. 2 – SW).
No initial velocities were imposed. The
model was run for 16 days, so there was not
enough time for the western boundary
current to develop and become unstable,
and changes on the mass field are mostly
wind driven. Although 16 days of constant
wind is unrealistic, a relatively long
simulation is needed to represent an
integrated effect of the predominance in the
wind direction in each season (northeasterly
on summer, southwesterly on winter). On
Experiment 3, both the density field and the
associated velocity field were used as initial
conditions. No wind forcing was applied, so
the upwelling induced by cyclonic
meanders of the Brazil Current could be
isolated. Experiments 4 and 5 are similar to
Experiment 3, except that the wind was
turned on after 30 days (with the same
characteristics as on Experiments 1 and 2)
in order to simulate the combined effect of
the wind and the meanders. The mass field
computed on Experiments 1 and 2 after 16
days of wind forcing is then compared with
the mass field computed on Experiments 35 after 46 days at Line A. This represents
the moment of maximum meander induced
upwelling at Line A (Exp. 3) and, for
Experiments 4 and 5, 16 days of wind
forcing (from day 30 to 46).
Wind forcing experiments
The ocean response to northeasterly
winds (Exp. 1) is similar to the classical
upwelling picture described by ALLEN
(1980). The wind stress drives an offshore
transport in a surface Ekman layer that
generates a depression of the sea surface. A
geostrophic jet is established, flowing
southward. The coastal jet is stronger where
the shelf is narrow (close to Cabo Frio).
While the surface velocities are deflected
offshore (relative to the geostrophic flow),
an onshore flow develops below. The
onshore flow offsets the offshore surface
Ekman layer flows, allowing the sea surface
slope to remain in a quasi-steady state while
feeding an upwelling circulation (LI and
WEISBERG, 1999). Near the coast, then,
the
ocean
response
comprises
a
superposition of geostrophic and Ekman
circulation. Offshore of this region, surface
velocities are deflected nearly 450 to the left
of the wind direction, while the bottom
velocities are zero or very small. The
temperature and salinity sections along Line
A after 16 days are shown on Figure 3. The
wind effect can be clearly seen, especially
close to the coast, where the most intense
vertical velocities are found. The
northeasterly wind advects surface water
offshore (e.g. 260C isotherm, 36.4
isohaline) and upwells water from the
bottom (Figure 3 a,b). After 4 days (not
shown), the 200C isotherm reaches the
surface. After 16 days, it is possible to
observe some upwelling at the shelf break
(seen as a slight elevation of the isotherms
and isohalines), but the upwelling there is
not as evident as in the coastal zone. The
upwelling
circulation
forced
by
northeasterly wind alone was not enough to
connect the SACW core present on the shelf
with the same water mass in the deep ocean.
The shelf response to southwesterly
wind (Exp. 2) is opposite to the previous
case. The surface Ekman transport is
onshore, creating a surface convergence
which results in a sloping sea surface. Since
the alongshore component of flow is
essentially in geostrophic balance, a coastal
jet is established, flowing northward nearly
4
parallel to isobaths. The surface velocities
are deflected toward the coast, while below
the velocities are deflected in the offshore
direction. This offset of the velocities feeds
the downwelling circulation. After 16 days,
isotherms slope downward toward the coast
(Figure 4a). The region closer to the coast
(inshore of 40-50 m) is essentially
barotropic (Fig 4 a,b), a result similar to
what was found by ALLEN and
NEWBERGER (1996) for the Oregon coast
under downwelling wind conditions.
The computed volume of SACW in
a one meter wide slice over the shelf along
Line A is given on Table 2. There is a
general tendency for decreasing the volume
of SACW with time. Comparing
Experiments 1 and 2, we see that the
upwelling winds tend to retard this process,
while the downwelling winds accelerate the
decrease of the volume.
Brazil Current experiment
On Experiment 3 (no wind forcing,
initial velocities imposed), the model
started to generate meanders after 15 days
(not shown). These meanders propagated to
the south with a velocity of approximately
7-10 km per day. As each moved
southward, other meanders were generated
and also propagated in the same direction.
Figure 5 shows the surface velocities for
days 30, 46 and 60, and Figure 6 shows the
temperature and salinity cross-section along
Line A for the same times. The discussion
is centered on the cyclonic meander
(inshore of the Brazil Current) and on the
anticyclonic eddy that crosses Line A
during this time.
On day 30, neither the cyclonic
meander nor the anticyclonic eddy had
reached Line A, so the temperature and
salinity structure (Figure 6 a,b) had not
been disturbed by them. On day 46, the
cyclonic meander is just crossing Line A,
and the anticyclonic eddy had reached Line
A. Upwelling could be seen over the
continental slope, with the elevation of
isotherms and isohalines (by ~15-20 m
compared to day 30). CAMPOS et al.
(1999), using observational data and
numerical modeling, also observed shelf
break upwelling induced by cyclonic
meanders of the Brazil Current. Although
the upwelling compared to day 30 is
relatively small, it should be noted that the
volume of SACW over the shelf increases
by ~25% relatively to day 38 (Table 2).
This increase must be caused by upwelling
northward of Line A, since there is no
connection between the SACW in the deep
ocean with the core over the shelf along that
section. A section 10 km to the north indeed
shows a continuous presence of SACW
over the shelf and the slope (Figure 7a,b).
This increased upwelling seems to be
localized, since the presence of SACW over
the shelf and the slope is not continuos 70
km to the north of Line A (Figure 7c,d). On
day 60, both the cyclonic meander and the
anticyclonic eddy had already crossed Line
A, and the thermohaline structure was
similar to day 30 (Figure 6e,f).
Wind forcing and Brazil Current
experiment
In order to investigate the
importance of the northeasterly wind and
the cyclonic meanders of the BC in
upwelling the SACW onto the shelf, both
processes are considered on Experiment 4.
The results in the region close to the coast
(inshore of the 50 m isobath) at day 46 are
similar to the simulation where only the
wind forcing was considered (Exp. 1),
showing that the meander effect is confined
to the shelf break and slope (Figure 8). On
the shelf break, however, both effects act
together and the SACW in deep water is
connected to the core over the shelf which
was present in the initial conditions. A core
of TW is present over the shelf, presumably
being advected southward. A temperature
and salinity section 120 km to the north of
Line A (Figure 9) shows a core of SACW
over the shelf disconnected from the same
5
water mass present on the slope, suggesting
that the increase on the volume of SACW
along Line A is caused by local upwelling
in the vicinities of that section, and not by
southward advection from farther north.
Indeed, a plot of the minimum depth
reached by the SACW (Figure 10) shows
that the upwelling along Line A is
localized. It can also be seen that upwelling
is enhanced in the vicinities of Cabo Frio.
Figure 11 shows a contour plot of vertical
velocity at Line A, day 46. Two local
maximums of vertical velocity are found,
one over the mid-shelf (associated with the
wind forcing) and another over the slope
(caused by the cyclonic meander). Values
of computed upward velocity over the slope
were similar to values found by CAMPOS
et al. (1999). The cyclonic meander brings
the SACW to shallower depths, exposing it
to northeasterly wind effect. Both effects
acting together increase the amount of
upwelled water (Table 2). When the wind is
southwesterly (downwelling favorable,
Exp. 5), the two mechanisms opposed one
another. There are still some upwelling at
the shelf break, which is caused by the
cyclonic meander by itself (Figure 12 –
compare to Figure 6, parts a-d), but it is not
enough to connect the deep ocean SACW
and the shelf core. Close to the coast,
isotherms slope downward, similar to the
results using the southwesterly wind as the
only forcing (Exp. 2, Figure 4), and the
volume of SACW over the shelf is greatly
decreased (Table 2).
SUMMARY AND CONCLUSIONS
A primitive equation model has
been used to simulate Brazil Current
meanders and the wind driven circulation of
the South Brazil Bight. The numerical
experiments performed show that the
coastal ocean response to wind forcing in
the SBB comprises a superposition of
geostrophic and Ekman circulation.
Alongshore wind generates a surface cross-
shelf transport. When the wind is from the
northeast, this surface cross-shelf transport
is offshore, causing coastal divergence and
establishing a coastal jet. The flow below
the surface is deflected toward the coast,
feeding upwelling circulation. When the
wind is from the southwest, the surface
cross-shelf transport is onshore. Northward
geostrophic flow is established. In this
situation, the flow is deflected offshore
close to the bottom. The offset of the
surface and bottom velocities maintains a
downwelling circulation.
At the shelf break, the cyclonic
meanders of the Brazil Current represent an
efficient mechanism for generation of
vertical velocities, acting decisively in the
upwelling of SACW. Combined with
upwelling favorable winds, there is a
positive interaction: the meander-induced
upwelling brings the SACW to shallower
depths, exposing it to the northeasterly
wind effect. In this situation, the SACW in
deep water is connected to the core over the
shelf which was present in the initial
conditions, reaching all the way to the coast
along the continental shelf. This is similar
to the findings of CAMPOS et al. (1999)
data results, which show that during the
winter, when coastal upwelling is
diminished and practically only the
meander-induced upwelling occurs, the
SACW was confined to the shelf break.
There is a general tendency for
decreasing the volume of SACW with time
(Table 2). A comparison between
Experiments 1 and 2 shows that upwelling
winds tend to retard this process, while the
downwelling winds accelerate the decrease
of the volume. When no wind forcing is
imposed (Exp. 3), the volume of SACW
over the shelf after 30 days is ~73% of the
volume present on the initial conditions.
After continuing decreasing (day 38), the
cyclonic meander causes an increase in its
volume (day 46 – Exp. 3). This increase can
be greatly amplified by the northeasterly
wind forcing (Exp. 4). Experiment 5 is
6
similar to Experiment 2, with the
southwesterly winds accelerating the
decrease of the volume of SACW over the
shelf. Both the wind and the cyclonic
meanders of the Brazil Current seem to be
important in the upwelling process,
pumping the SACW from deep water onto
the continental shelf, which may have
important
chemical/biological
consequences. Indeed, BRANDINI (1990)
found concentrations of phosphate and
nitrate within the euphotic zone at a cold
meander of the SBB comparatively higher
than
in
adjacent
oceanic
waters.
ATKINSON et al. (1996) found that
northward wind stress, together with frontal
eddies of the Gulf Stream, causes cold,
nutrient rich water to rise to the shelf edge
and penetrate shoreward to the coast.
Although the wind-forced onshore
transport of nutrient-rich water certainly has
a high seasonal variability, being higher in
summer, the cyclonic meander-induced
upwelling can occur even in winter.
DEIBEL (1985), in a study off South
Carolina (USA), report high levels of
biological activity during wintertime,
implying nutrient input from the Gulf
Stream. This suggests that, at least on the
outer continental shelf of the South Brazil
Bight, onshore nutrient transport may occur
during all seasons.
ACKNOWLEDGEMENTS
We gratefully acknowledge support
from the Fundação de Amparo à Pesquisa
do Estado de São Paulo (FAPESP – grant
98/14648-0) and from the Inter American
Institute for Global Change Research (IAI),
through projects SAMC (IAI-ISP1) and
SACC (CRN-061). Project COROAS was
funded by FAPESP (grant 91/0542-7) and
CNPq
(Conselho
Nacional
de
Desenvolvimento Científico e Tecnológico
– grant 40.3007/91.7). J. Miller was funded
by U.S. Grants.
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8
Figure captions
Figure 1 - Study area with bottom topography. The light shaded area shows the model
domain. The dotted thick line shows the location of Line A, and the dotted thin lines
mark the area shown in plots of model results. Topographic contours are (in meters) 50,
100, 200, 500, 1000, 1500, 2000, and 2500. In the lower right hand corner, an inserted
satellite-derived image illustrated the meandering of the Brazil Current in the region.
Figure 2 - Initial (a) temperature and (b) salinity cross section. The thicker line
represents the interface between Tropical Water and South Atlantic Central Water
(T=200C, S=36.4). Contour interval is 20C (temperature) and 0.2 (salinity)
Figure 3 - Temperature and salinity cross-section on Line A at day 16 for the simulation
forced by the northeasterly wind (Exp. 1). Contour intervals as on Figure 2.
Figure 4 - Temperature and salinity cross-section on Line A at day 16 for the simulation
forced by the southwesterly wind (Exp. 2). Contour intervals as on Figure 2.
Figure 5 - Surface velocities for the Brazil Current simulation (Exp. 3) at days (a) 30,
(b) 46 and (c) 60. The across-shore line shows the location of Line A. The 100 m
isobath is shown. CF: Cabo Frio, CSM: Cabo de Santa Marta.
Figure 6 - Temperature and salinity cross-section on Line A for the Brazil Current
simulation (Exp. 3) at days (a, b) 30, (c,d) 46 and (e,f) 60. Contour intervals as on
Figure 2.
Figure 7 - Temperature and salinity cross-section at day 46 for the Brazil Current
simulation (Exp. 3) (a,b) 10 km and (c,d) 70 km northward of Line A. Contour intervals
as on Figure 2.
Figure 8 - Temperature and salinity cross-section on Line A at day 46 for the simulation
forced by the northeasterly wind and the Brazil Current (Exp. 4). Contour intervals as on
Figure 2.
Figure 9 - Temperature and salinity cross section at day 46 for the simulation forced by
the northeasterly wind and the Brazil Current (Exp. 4) 120 km northward of Line A.
Contour intervals as on Figure 2.
Figure 10 – Minimum depth reached by the SACW at day 46 for the simulation forced
by the northeasterly wind and the Brazil Current (Exp. 4). Contours are (in meters) 70,
80, 90, 100, 120 and 140. The thicker contour is the 100 m isobath. The across-shore
line shows the location of Line A. CF: Cabo Frio, CSM: Cabo de Santa Marta.
Figure 11 – Vertical velocity cross-section at day 46 for the simulation forced by the
9
northeasterly wind and the Brazil Current (Exp. 4). Contour interval is 2e-5 ms-1, except
in the shaded area (1e-4, 2e-4 and 3e-4 ms-1 contours). The thicker line is the zero
velocity contour. Negative values are dashed.
Figure 12 - Temperature and salinity cross-section on Line A at day 46 for the
simulation forced by the southwesterly wind and the Brazil Current (Exp. 5). Contour
intervals as on Figure 2.
Table 1 – Summary of numerical experiments
Experiment
Forcing mechanism
Imposed
velocities
Exp. 1
NE wind
zero
Exp. 2
SW wind
zero
Exp. 3
BC meanders
geostr. vel.
Exp. 4
BC meanders + NE wind
geostr. vel.
Exp. 5
BC meanders + SW wind
geostr. vel.
Begin wind
forcing (day)
zero
zero
_______
30
30
Duration (days)
of experiment
16
16
60
60
60
10
Table 2 - Volume of SACW (km3) in one meter wide slice over the shelf along Line A
for all experiments
Day 0
Day 8
Day 16
Exp. 1
40
38
36
Exp. 2
40
34
31
Day 30
Day 38
Day 46
Exp. 3
29
24
30
Exp. 4
29
28
49
Exp. 5
29
23
20
11