Geophys. Res. Letters (accepted in Dec/1999) - Preprint
Shelf break upwelling driven by Brazil Current cyclonic meanders
Edmo J. D. Campos , Denise Velhote and Ilson C. A. da Silveira
Instituto Oceanografico, Universidade de Sao Paulo, Brazil
Abstract
Analysis of hydrographic data and results of numerical
modeling show the occurrence of upwelling associated
with the cyclonic meanders of the Brazil Current, in the region known as the Southeast Brazil Bight or Santos Bight.
The temperature and salinity distributions collected during
summer and winter confirm the idea that meander induced
shelf break upwelling plays an important role in the pumping of South Atlantic Central Water from the slope region
onto the continental shelf.
-50˚
-22˚
-48˚
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BRAZIL
-42˚
-40˚
-22˚
Cabo
Frio
50
-24˚
Santos
-24˚
0
20
00
20
50
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T2
T3
T4
T5
T6
T7
T1
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Introduction
-28˚
Campos et al., [1995], based on analyses of hydrographic
data and on Osgood et al.’s [1987] ideas, proposed a
conceptual model explaining the shelf break upwelling induced by a Brazil Current (BC) meander in the following
way. A meander trough consists of a cyclonicaly rotating
dome of cold upwelled water which lies between the western boundary current and the continental slope (Fig. 1).
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 towards the
continental shelf. The opposite occurs on the trailing part.
Since the water advected back on the trailing edge is not
necessarily the same water pushed inshore in the leading
part, this might result in a net deposition of upwelled water
on the shelf.
N
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x
x
x
x
x
x
x
.
.
.
.
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.
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Leading edge
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-40˚
Map of the region surveyed by the COROAS hydro-
The Southeast Brazilian Bight (SBB) or the Santos Bight
(Fig. 2) is a region along the Southeastern Brazilian coast
between Cabo Frio (23 S) and Cabo de Santa Marta
(28 S). At the latitude of Cabo Frio, the SBB is characterized by a relatively narrow shelf, with the coastline changing abruptly from north-south to east-west direction. The
water mass structure in the SBB is mainly composed of
three water masses: the Tropical Water (TW, T 20 C and
S 36.40); the South Atlantic Central Water (SACW, T 20 C and S 36.40); and the Coastal Water (CW), a low
salinity water mass resulting from dilution of oceanic water
by fresh water input from small to medium-sized estuaries
along the SBB coast [Campos et al., 1995].
17 18
16
15
z
(upwelling)
S
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The Study Aread
20
zone of
upwelling
-46˚
In this paper, we present some additional evidences of
shelf break upwelling driven by Brazil Current cyclonic meanders along the Brazilian Southeastern coast. We base
these evidences on the analysis of hydrographic data collected during three different cruises, and on results of numerical experiments with a regional implementation of the
Miami Isopycnic Coordinate Ocean Model (MICOM).
19
zone of
downwelling
-48˚
predominant wind direction.
S
Cold
dome
-28˚
graphic cruises. The thick arrow over the transects represent the
Trailing edge
(downwelling)
0
Sta. Marta
N
0
300
Cabo de
Shelf break upwelling driven by a cyclonic meander.
The wind regime over the SBB is dominated by the presence of the South Atlantic Convergence Zone (SACZ), with
winds blowing predominantly from the northeast most of
the time (the thick arrow in Fig. 2 represents the predominant wind direction). However, migrations of the SACZ,
especially during the winter, might result even in complete
reversal of the wind direction . Castro and Miranda [1998],
Upwelled water is pushed inshore in the leading part of the cold
dome.
Corresponding author address: LABMON - Inst.
Oceanografico, Univ. Sao Paulo, 05508-900 Sao Paulo,
SP, Brazil; e-mail: [email protected]
report that strong coastal upwelling, with SACW intrusion
onto the shelf, is observed in the vicinity of Cabo Frio (2224 S) year round due to the predominant northeasterly
winds.
30
28
26
24
Indications of Shelf-break Upwelling from Hydrographic Data
The hydrographic data used in this work were collected
during oceanographic cruises in the central part of the
SBB, as indicated in Fig. 2 (the COROAS Experiment;
Campos et al., [1996]). Three quasi-synoptic hydrographic
surveys were conducted during the austral Summer (HM1,
Jan/14-Feb/2) and Winter (HM2, Jul/15-31) of 1993, and
the austral Summer (HM3, Jan/15-31) of 1994, in the central part of the SBB (Fig. 2). The entire water column of
100 stations was sampled during each cruise.
The T-S characteristics of the stations in regions shallower
than 200 m, based on the T-S diagram of Fig. 3 and on results of unpublished volumetric analysis, indicate the presence of pure SACW near the bottom over a large area of
the continental shelf, during all three cruises. In the summer, the SACW was detected in the SBB as close as 50 km
to the coast (Castro and Miranda, [1998] report the detection of SACW even closer). In the wintertime, the SACW
retreats to near the shelf break, being detected only in the
middle and outer shelves (depths greater than 100 m).
This seasonal behavior is very likely to be controlled by
the wind regime [Castro and Miranda, 1998]. However,
the reason why the SACW is found in the outer regions of
the shelf year round is not yet known, since the SACW’s
core is usually found at depths greater than 200 m. In the
slope region, there might be a mechanism responsible for
pumping that water onto the shelf. Campos et al. [1995]
proposed that during the summer this mechanism could
be a combination of shelf-break upwelling, induced by the
BC cyclonic meanders, and coastal wind-driven upwelling.
The COROAS data set corroborates Campos et al.’s
[1995] ideas. Horizontal distributions of temperature at
100 m depth (Fig. 4) show that the three cruises captured
the leading part of cyclonic meanders crossing the sixth
transect (T6, Fig. 2). The T6 vertical sections of temperature and salinity for cruises HM1 and HM2 (Fig. 5) indicate
that the SACW (temperatures lower than 20 C in Fig. 5)
was found “climbing” the shelf (follow the 16 C isotherm
temperature
22
The circulation offshore is dominated by the BC, flowing
southward and meandering around the 200 m isobath. Although much weaker than the Gulf Stream, in terms of
volume transport, the Brazil Current presents a relatively
strong eddy activity in the SBB. 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].
22
23
24
20
25
18
16
27
26
14
28
12
10
8
6
HM1
4
HM2
2
HM3
0
32.0
32.8
33.6
34.4
35.2
36.0
36.8
37.6
salinity
!
Scatter T-S diagram for the three COROAS cruises.
Only stations over the continental shelf (depths less than 200 m)
are shown. It is clear he presence of the SACW over the shelf, in
both summer (HM1, HM3) winter (HM2). TW:Tropical Water; CW:
Coastal Water; SACW: South Atlantic Central Water).
shape). However, note that only in the summer (Fig. 5,
upper panel), the SACW reached the shallower regions,
probably due to the additional action of the wind driven
Ekman pumping. In the winter (Fig. 5, lower panel), the
SACW was confined to the shelf break.
The combined effect of coastal wind-driven and meanderinduced upwelling can be represented schematically as in
Fig. 6. During the summer (upper panel), when the wind
is predominantly from the northeast, coastal upwelling occurs in response to the offshore Ekman transport near the
surface. The resulting divergence pumps water inshore,
in the bottom layers. In the presence of meander-induced
upwelling near the shelf break, the combination of the two
effects results in a strong mechanism capable of bringing
the SACW from the slope regions to near the coast. During the winter (lower panel), when coastal upwelling is diminished, practically only the meander-induced upwelling
occurs.
Numerical Simulations of the Brazil Current Meanders in the SBB
Velhote [1998] used a regional, open-boundary implementation of Miami Isopycnic Coordinate Ocean Model (MICOM) of the SBB area to simulate a meandering western boundary current that closely resembled the observed
Brazil Current. We center our analysis of Velhote’s [1998]
results in a situation when the modeled BC had a well developed cyclonic meander in the vicinity of Cabo Frio. The
meander grew in amplitude as it migrated downstream.
The computed cross-isopycnal (“vertical”) velocity field between the layers " = 26.18 and 26.52 #%$%&('*) (approximately 100 m depth near the shelf break), when the meander was in the COROAS region, is shown in Fig. 7 (upper
+
,-./017684
Vertical temperature profiles along transect T6 for the
summer (upper panel) and winter (lower panel) of 1993.
panel) superimposed upon the horizontal velocity field in
layer 9 = 26.18. The upwelling and downwelling zones associated with the divergence and convergence of the horizontal flow, respectively, are evident. Values of upward
velocity in the leading part of the meander are about of
2.5 : 10 ;=< cm/s. These modeling results support Campos
et al.’s [1995] ideas: the advection of cooler water from
the slope region towards the continental shelf. This is confirmed by the lower panel of Fig. 7, which shows the velocity field superimposed upon the temperature field.
Summary and Conclusions
,-./013254
Horizontal distributions of temperature at 100 m
depths during the three COROAS cruises. Notice the signature
of a cyclonic meander in the southwestern quadrant of the surveyed area.
The COROAS data set showed the presence of the SACW
over a large portion of the continental shelf in the area
surveyed. Vertical profiles of temperature and salinity
strengthen Campos et al.’s [1995] speculations about the
causal relationship between the presence of the SACW on
the outer regions of the continental shelf with the Brazil
Current cyclonic meandering inside the SBB. Thus, the
mechanism responsible for pumping the SACW onto the
shelf was mainly due to meander induced shelf break upwelling, during the winter, and a combination of shelf-break
and wind-driven upwelling in the summertime, as illustrated by Fig. 6. In this process, SACW is initially pumped
onto the shelf by divergence in the leading part of the
meander. During the summer, when the wind is, on av-
>
?@ABCDFEHG
Schematic of the combined effect of coastal wind-
driven and meander-induced upwelling. During the summer (upper panel), coastal upwelling occurs in response to the offshore
Ekman transport near the surface (CW: Coastal Water; SACW:
South Atlantic Central Water; TW: Tropical Water).
erage, upwelling-favorable, the offshore Ekman transport
accounted for the coastal upwelling. During the winter this
wind-driven process is weakened and, although SACW
can still be found in the outer shelf, much less of it reaches
the surface layers.
Results of the eddy-resolving simulations with the MICOM
are in good agreement with the general picture indicated
by the hydrographic data. The upward motion computed
by the model, associated with the divergence near the
leading edge of the cyclonic meanders are, at least qualitatively, comparable with the results obtained in the literature.
Acknowledgements: This work was supported by
the S~
ao Paulo State funding agency, FAPESP (Grants
91/0542-7, 94/3091-4 and 96/4060-0) and by the InterAmerican Institute for Global Change Research (IAI). The
second author acknowledges the support given by CNPq
through a M.Sc. fellowship. Special thanks to R. Bleck, for
his help in dealing with MICOM.
References
Campos, E.J.D., J.E. Goncalves & Y. Ikeda, Water Mass
Characteristics and Geostrophic Circulation in the
South Brazil Bight - Summer of 1991. J. Geophys.
?@ABCDJIKG
Numerical simulation of the circulation in the SBB
with the Miami Isopycnic Coordinate Model (MICOM). The velocity vectors are superimposed upon the cross-isopycnal velocity
(upper panel) and temperature (lower panel) distributions at about
100 m depth.
Res., 100(9), 18537-18550, 1995.
Campos, E.J.D., Y. Ikeda, B.M. Castro Fo., S.A. Gaeta,
J.A. Lorenzzetti & M.R. Stevenson (1996), Experiment Studies Circulation in the Western South Atlantic. EOS, Transactions, Am. Geophys. Union, 77
(27) 253,259, 1996.
Castro, B.M., and L.B. de Miranda, Physical Oceanography of the Western Atlantic Continental Shelf Located
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encia
de Quebra de Plataforma Induzida por Vortices
Cicl^
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