Experiment studies circulation in the western South Atlantic

Eos, Vol. 77, No. 27, July 2, 1996 EOS,

T R A N S A C T I O N S ,

A M E R I C A N

G E O P H Y S I C A L

VOLUME 77 NUMBER 27

EOS

JULY 2.1996 PAGES 253-264

Experiment Studies Circulation in the Western South Atlantic PAGES 253, 259

E. J. D. Campos, Y. Ikeda, B. M. Castro, S. A. Gaeta, J. A. Lorenzzetti, and M. R. Stevenson Observations in the Brazil Current region between 20° and 30°S are revealing many in teresting oceanographic features. In the aus tral winter of 1993, what appears to be an anomalous northward penetration of waters from the Brazil/Malvinas confluence region into the South Brazil Bight was observed (Figure 1). Despite its dynamical similarity to the South Atlantic Bight (SAB) in the North Atlan tic, the South Brazil Bight (SBB) (also re ferred to as Santos Bight) is often overlooked by the physical oceanographic community. Most of the historical oceanographic cruises in this area focused on the shelf, leaving the shelf break and slope—and therefore the Bra zil Current itself—largely unstudied. For in stance, the first time a conductivitytermperature-depth (CTD) survey was made of the SBB was in January 1991 [Campos et a/., 1994,1995] when German and Brazilian oceanographers on board the German FS Vic torHensen performed a CTD hydrographic survey in the area between Santos and Rio de Janeiro. Due to this substantial lack of knowledge in a region regarded as one of the most ecologically and economically important oceanic regions along the Brazilian coast, a group of Brazilian scientists set forth a com prehensive oceanographic survey called Pro ject COROAS to study the SBB and the nearby portions of the Brazil Basin. This multi-discipli nary data collection effort, a component of the World Ocean Circulation Experiment (WOCE) Deep Basin Experiment program, is an exten

E. J. D. Campos, Y. Ikeda, B. M. Castro, and S. A. Gaeta, Instituto Oceanografico da Universidade de Sao Paulo, P.ga. do Oceanografico, 191, Cid. Universitaria, 0550&900 Sao Paulo, SP-Brazil; J. A. Lorenzzetti and M. R. Steven son, Instituto Nacional de Pesquisas Espaciais, Av. dos Astronautas 1758, CP 515, 12201, Sao Jose dos Campos, Brazil

U N I O N

sive study of part of the Brazil Basin that places particular emphasis on the SBB. COROAS focuses on different aspects of oceanography. It incorporates a high-quality

hydrographic survey, satellite remote sens ing, and Lagrangian and Eulerian current studies. Since the program began in 1993, re searchers have performed several oceano graphic cruises on board the R/V Prof. W. Besnard, conducted seasonal hydrographic surveys, and deployed and recovered an ar ray of current meter moorings along a tran sect extending from near Santos ( B r a z i l ) to the V e m a Channel. Additionally, satel lite-tracked WOCE-type drifters are col lecting data in the Brazil Current, and satellite a d v a n c e d very high resolution ra diometer ( A V H R R ) data are being continu ously recorded to map sea surface temperature.

Fig. 1. Sea surface temperature from satellite A VHRR processed at the Brazilian National Insti tute for Space Research. This image, taken on July 20, 1993, shows a northward flowing current of low-temperature and low-salinity water (blue) between the Brazil Current (red) and the coast. It is clear that this current originates to the south of 31 °S. Original color image appears at the back of this volume.

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Eos, Vol. 77, No. 27, July 2, 1996

Fig. 2. Chart of the South Brazil Bight showing the region of the COROAS ex periment. The smaller dots indicate the mesoscale array, and the bigger dots mark the position of the large-scale hy drography stations. Also shown are the site of current moor ings and drifter de ployments. The solid and dashed lines show the trajectories of two drifters launched during the austral summer of 1993.

T h e H y d r o g r a p h i c Survey The hydrographic portion of the COROAS project is being carried out by two arrays of stations. One, the mesoscale grid, is a set of transects that are more or less perpendicular to the shelf break. Each transect extends from the 50-m isobath to oceanic regions deeper than 2000 m. The purpose of this grid is to assess the water mass structure, geostrophic flow, and water transport over the conti nental shelf and shelf break regions, including a study of the dynamics of the Bra zil Current in the area surveyed. The other, usually called the large-scale grid, is a triangular array of stations. The first vertex is near Santos (Brazil), the second is in the Vema Channel, and the third is near the Brazilian coast at 30°S. These two arrays are depicted in Figure 2, along with the locations of current meter moorings and where drifters were launched. As a first step toward analysis of the COROAS data, the quasi-synoptic mesoscale hydrographic surveys conducted during sum mer and winter of 1993 were examined for water mass characteristics, geostrophic flow, and transport. As noted above, these surveys encompassed most of the continental shelf and extended through the Brazil Current into the subtropical gyre. T-S characteristics of some waters observed on the shelf indicate that South Atlantic Central Water (SACW) is upwelled at the Brazil Current front and de posited on the shelf during summer, possibly by large-scale (100-300 kilometer) mean ders. During winter, little interaction be tween shelf and deeper waters was observed. However, waters of coastal origin were found on the shelf, resulting in minimum sa linities about 3 psu lower than those ob served during summer. These low salinity waters, which are also cold, lie in an along-isobath tongue between the Brazil Current and the coast and appear to originate southwest of Santos. A satellite infrared image that was

processed at the Instituto Nacional de Pesquisas Espaciais (Figure 1) suggests that the source of this water is located south of 31 °S. The first cruise in the large-scale array was performed from March 2 to 16,1993. Esti mates for the geostrophic velocity and vol ume transports for the two deep-ocean transects of this cruise are shown in Figure 3. The Brazil Current transport intensifies south ward, as expected. The two cores of SACW are intensive anticyclonic eddies close to the Vema Channel. The imbalance in that layer may be compensated by inflow from the shelf, which is not analyzed here. Southward transports of Antarctic Intermediate Water ( A A I W ) on the continental shelf break were confirmed by direct current meter measure ments. Cores close to Vema Channel may be due to eddy activity. Direct Current Measurements The direct measurements of the surface and subsurface currents in the study area were divided into two parts: Eulerian meas urements of the surface and subsurface cur rents taken from several locations along the central transect of the mesoscale hydrographic array, and Lagrangian measurements of the surface layer currents. The former is be ing performed with a number of drifters fabri cated in Brazil according to the World Ocean Circulation Experiment/Tropical Ocean and Global Atmosphere (TOGA) experiment standards. The current meter mooring and drifter deployment locations are shown in Figure 2, which also shows part of the trajec tories of two drifters deployed during the summer of 1993.

Preliminary analysis of the recovered data shows that currents at CI00 are highly barotropic throughout the year and decou pled from the outer-shelf circulation. Midshelf currents are driven primarily by the wind, and are generally more energetic in the subtidal frequency band than in the tidal bands. C200 is inside the Brazil Current do main and flows predominantly toward the southwest, except near the bottom where cur rents tend to flow southward. Mean current of about 0.45 ms" was observed at 32 m be low the surface. CI000 is also in a region dominated by the Brazil Current flow in the upper layer (three top current meters); ob served speeds at this last station are a little larger than at C200. High correlation is observed for the flow of the Brazil Current between both stations, which are spaced 18 km apart. Variances at both C200 and C1000 are mostly subtidal. One current reversal event observed from February 12-25,1993, was noticeable for its amplitude and vertical extension. Data from the 700-m current meter at CI000 shows that the water is predominantly from the Antarc tic Intermediate Current, with northeastward mean flow of about 0.10 ms" . This contra dicts observations made recently by German oceanographers. Estimates for the Brazil Cur rent transport between C200 and C1000 are less than 2 Sv ( S v = 1 0 m / s ) . 1

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T h e Drifter Trajectories Five WOCE-type Low Cost Drifters (LCDs) were launched during each of the three mesoscale cruises; all were launched from the same location (Figure 2). The trajectories of two of the drifters launched during the January of 1993 cruise are shown in Figure 2. The solid line shows the path of a drifter that, after going south to about 29°S, returned and reached a point north of where it started. The dashed line shows the track of another drifter, which continued farther south to about 33°S before turning back and drifting northeastwardly. The meanders and loops in this trajectory suggest the presence of eddies during the southbound excursion of the drifter. Calculation for the first drifter (the one whose trajectory is indicated by the solid line) shows that the mean southward veloc ity was 17.1 cm s" toward 29°S. During that time, the mean surface water temperature was 25.40°C (±0.6°C; 1 std. dev.). As the drifter reached 29°S, it moved inshore and then proceeded parallel to the shore toward 23°S along a northward course over the shelf at -10.7 cm" . The mean surface water tem perature during the northward part of the track was 20.31°C (±2.9°C; 1 std. dev.). 1

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Current Meter Moorings Direct Eulerian current measurements were taken at three points offshore of Santos between December 1992 and May 1994. Mooring site locations corresponded to midshelf (C100), shelf break (C200), and continen tal slope (CI000), as indicated in Figure 2.


T h e A V H R R Data T o support the field activities of the COROAS experiment, a series of digital im-

Eos, Vol. 77, No. 27, July 2, 1996

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During the cruise in early 1993,51 of the hydrographic stations were used for observ ing nutrient and phytoplankton biomass and for studying the standard hydrography. In addition, usually three and sometimes four of the stations occupied in each transect were chosen to make diurnal experimental observations of in situ-simulated primary pro ductivity. Since the COROAS transects intersected every water type present on the shelf and slope regions, these observations yielded informa tion on a variety of shelf char acteristics in the region. Currently, these data, in com bination with the other COROAS data sets, are under going a series of analyses in tended to clarify the role of the Brazil Current and shelf dy namics in different biological aspects of the region.

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Concluding Remarks

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In its first 2 years, Project COROAS collected a consider Fig. 3. Estimates for the geostrophic velocity and volume transports for two sections of the large-scale hydroable amount of data, and graphic array. These results show that the Brazil Current transport intensifies to the south. The two cores of South At analysis is now under way. Pre lantic Central Water (SACW) represent an intensive anticyclonic eddy close to the Vema Channel. The imbalance in liminary results reveal a num that layer may be compensated for by inflow from the shelf. NADW, North Atlantic Deep Water; AAIW, Antarctic Inter ber of important issues that mediate Water; andAABW, Antarctic Bottom Water. Original color image appears at the back of this volume. are being studied further. point, as indicated in Figure 2. Temperature ages from the advanced very high resolution Among these are the observation of an and salinity data collected in situ during the radiometers (AVHRRs) flying on board the anomalous penetration of water south of the July 1993 mesoscale cruise also reveals the NOAA-11 and -12 satellites have been re Brazil-Malvinas confluence and a possible bifur presence of this cold water, which is very low corded since November 1992. The pictures cation of the Antarctic Intermediate Water flow in saline. were recorded in the high-resolution picture near in the region of study, where one branch The COROAS SST images confirm that transmission mode by the Brazilian National flows to the north and the other returns to the south of Cabo Frio (22.9°S and 42.7°W), flow Institute for Space Research. south along the South American continental is frequently unstable. In these images, the Analysis of the SST images obtained to slope. These issues will be addressed in forth Brazil Current meanders and cold-core cy date reveals a number of interesting oceano coming papers. clonic eddies with diameters of the order of graphic features of the region. The dominant 100 km near the western edge of the flow are feature visible throughout the year is the Acknowledgments often observed. Sometimes a mushroomlike warm water (22°-28°C) of the Brazil Current The authors gratefully acknowledge finan feature is generated at the Brazil Current, flowing south along the shelf break (200cial support from the University of Sao Paulo, forming a pair of eddies with opposite rota 2000-m isobaths). The western wall of this cur FAPESP (grant 91/0542-7), CNPq (grant tions, one at each side of the stream. While rent is easily seen by the thermal contrast 40.3007/91.7), SeCirm, and the Volkswagen the main physical mechanism responsible caused by cooler shelf waters. A strong sea Foundation. W e also thank Tom Muller (IfMfor the generation of the fluctuating compo sonal (austral spring and summer maximum) Kiel) for discussions that contributed to the nent of the flow is yet to be determined, the coastal upwelling extending from 21 °S to content of this article. satellite imagery confirms that this oceanic re 23°S, with surface temperatures in the range gion contains a very energetic eddy field. 15-16°C, is also observed in the SST images. Figure 1 shows the SST distribution on References July 20,1993. In this image a tongue of cold Campos, E. J. D., S. S. de Godoi, Y. Ikeda, T h e Biological Component water propagating northward is seen. This ob L. V. Nonato, and J. E. Goncalves, Summer time thermocline structure of the Brazil cur servation is confirmed by the LCD trajecto During each hydrographic cruise of Pro rent region between Santos (SP) and Rio ries shown in Figure 2. These LCDs were ject COROAS, several types of biological and de Janeiro (RJ), Bol. Inst. Esp. launched in January 1993, in the core of the chemical components of the water column Oceanogr., 42, 1, 1994. Brazil Current, and drifted southward with were sampled to obtain a better under Campos, E. J. D., J. E. Goncalves, and Y. the flow. At a certain point they started to Ikeda, Water mass structure and geostrophic standing of the level of biological activity, circulation in the South Brazil Bight—Sum move northward, and by July 1993 they had with special emphasis on nutrient gradients mer of 1991, J. Geophys. Res., 100, reached latitudes to the north of their starting and possible sources of supply. 18,537, 1995.


Vol. 77, No. 27, July 2, 1996

Fig. J.

Sea surface temperature from satellite A VHRR processed at the Brazilian Nationallnsti

tute for Space Research. This image, taken on July 20, J 993, shows a northward flowing current of low-temperature and low-salinity water (blue) between the Brazil Current (red) and the coast. It is clear that this current onginares to the south of 3 J oS

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Vol. 77, No. 27, July 2, 1996

22'S

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26'S • •

.II.

28'S

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50'W Fig. 3.

48'W

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•

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44'W

42'W

40'W

38'W

Estimates for the geostrophic velocity and volume transports for two sections of the large-scale hydro

graphic array. These results show that the Brazil Current transport intensifies to the south. The two cores of South Atlantic Central Water (SACHQ represent an intensive anticyclonic eddy close to the Vema Channel. The imbalance in that layer may be compensated for by inflow from the shelf NADW, North Atlantic Deep Water; MIt¥, Antarctic Inter mediate Water,' and MBW, Antarctic Bottom Water, On"ginal color image appears at the back of this volume.

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