On the intermediate and deep water flows in the South Atlantic Ocean (original) (raw)

Dynamics of Intermediate Water Circulation in the Subtropical South Atlantic

Journal of Physical Oceanography, 2000

The circulation of the low-salinity Antarctic Intermediate Water in the South Atlantic and the associated dynamical processes are studied, using recent and historical hydrographic profiles, Lagrangian and Eulerian current measurements as well as wind stress observations. The circulation pattern inferred for the Antarctic Intermediate Water supports the hypothesis of an anticyclonic basinwide recirculation of the intermediate water in the subtropics. The eastward current of the intermediate anticyclone is fed mainly by water recirculated in the Brazil Current and by the Malvinas Current. An additional source region is the Polar Frontal zone of the South Atlantic. The transport in the meandering eastward current ranges from 6 to 26 Sv (Sv ϵ 10 6 m 3 s Ϫ1 ). The transport of the comparably uniform westward flow of the gyre varies between 10 and 30 Sv. Both transports vary with longitude. At the western boundary near 28ЊS, in the Santos Bifurcation, the westward current splits into two branches. About three-quarters of the 19 Sv at 40ЊW go south as an intermediate western boundary current. The remaining quarter flows northward along the western boundary. Simulations with a simple model of the ventilated thermocline reveal that the wind-driven subtropical gyre has a vertical extent of over 1200 m. The transports derived from the simulations suggest that about 90% of the transport in the westward branch of the intermediate gyre and about 50% of the transport in the eastward branch can be attributed to the wind-driven circulation. The structure of the simulated gyre deviates from observations to some extent. The discrepancies between the simulations and the observations are most likely caused by the interoceanic exchange south of Africa, the dynamics of the boundary currents, the nonlinearity, and the seasonal variability of the wind field. A simulation with an inflow/outflow condition for the eastern boundary reduces the transport deviations in the eastward current to about 20%. The results support the hypothesis that the wind field is of major importance for the subtropical circulation of Antarctic Intermediate Water followed by the interoceanic exchange. The simulations suggest that the westward transport in the subtropical gyre undergoes seasonal variations. The transports and the structure of the intermediate subtropical gyre from the Parallel Ocean Climate Model (Semtner-Chervin model) agree better with observations.

The intermediate depth circulation of the western South Atlantic

Geophysical Research Letters, 1999

The subsurface oceanic circulation is an important part of the Earth climate system. Subsurface currents traditionally are inferred indirectly from distributions of temperature and dissolved substances, occasionally supplemented by current meter measurements. Neutrally-buoyant floats however, now enable us to obtain for the first time directly measured intermediate depth velocity fields over large areas such as the western South Atlantic. Here, our combined data set provides unprecedented observations and quantification of key flow patterns, such as the Subtropical Gyre return flow (12 Sv; 1 Sverdrup = 106m3s4), its bifurcation near the Santos Plateau and the resulting continuous narrow and swift northward intermediate western boundary current (4 Sv). This northward flowing water passes through complex equatorial flows and finally enters into the North Atlantic.

Intermediate layer water masses in the western tropical Atlantic Ocean

Journal of Geophysical Research, 1996

Intermediate layer water masses are defined according to temperaturesalinity relationships derived from conductivity-temperature-depth (CTD) observations measured during four 1990-1991 Western Tropical Atlantic Experiment (WESTRAX) hydrographic surveys. The intermediate layer, bounded by density surfaces of sigma theta 26.00 and 27.65 (approximately 150 and 1300 m deep, respectively), is conveniently divided into upper and lower layers by the relatively low salinity Antarctic Intermediate Water (AAIW) which is centered at sigma theta 27.25 (approximately 700 m deep). Approximately 604-5% of the region's waters are traced to a southern hemisphere origin, indicating the importance of AAIW in the western tropical Atlantic's water mass structure. The southern source water masses, South Atlantic Central Water and AAIW, enter the WESTRAX region (west of 44øW and between the equator and 9øN) as part of the subthermocline North Brazil Current. Depending on the season, all or part of these southern waters retrofiect anticyclonicMly through the region and flow eastward into the North Equatorial Undercurrent. The primary northern source water mass, North Atlantic Central Water, enters the northeastern corner of the WESTRAX region as part of a cyclonic branch of the North Equatorial Current (NEC) and converges with the southern water. This meeting produces mixture water masses which make up 454-4% of the region in volume and are predominantly of a southern nature. The patterns of the mixture water masses which fill the areas between the source water masses suggest the importance of lateral mixing in this ocean region. Further, some of the mixture water in the upper layer appears to be part of the NEC, suggesting southern water recirculation in the tropical Atlantic gyre. A time dependent water mass box model of advective and mixing transports is used to suggest that lateral mixing do•ninates vertical mixing by a ratio of approximately 10 to 1. Typical box model results for the fall-winter 1990-1991 period indicate that 13 Sv of mixture water masses are produced through mixing (a sum of 9 Sv and 4 Sv from southern and northern source water masses, respectively), while a net 17 Sv of mixture water masses are exported from the region. Atlantic Experiment (WESTRAX) was conducted during 1990 and 1991 to address these issues [Brown et al., 1992]. As part of this program, five major hydrographic and velocity surveys were carried out in the region west Copyright 1996 by the American Geophysical Union. Paper number 95JC03372. 0148-0227/96/95JC-03372509.00 of 44øW and bounded by the equator and 15øN. In this paper we describe some of these observations in terms of the seasonal evolution of the structure of intermediate layer water masses. In a companion paper we will describe and discuss the transports of these water masses through the WESTRAX region (F. L. Bub and W. S. Brown, manuscript in preparation, 1996)o Historical potential temperature-salinity (O-S) relationships for water types and water masses of the western tropical Atlantic Ocean have been proposed by Sverdrup e! al. [1942], Mamayew [1975], Emery and Dewar [1982], and Emery and Meincke [1986]. The water masses of the western tropical Atlantic are bracketed, both geographically and in O-S space (Figure 1), by waters which Emery and Dewar [1982] associate with the

Water Mass Transformation and Subduction in the South Atlantic

Journal of Physical Oceanography, 2005

The transformation of water masses induced by air-sea fluxes in the South Atlantic Ocean is calculated with a global ocean model, Ocean Circulation and Climate Advanced Modeling (OCCAM), and has been compared with several observational datasets. Air-sea interaction supplies buoyancy to the ocean at almost all density levels. The uncertainty of the estimates of water mass transformations is at least 10 Sv (Sv ϵ 10 6 m 3 s Ϫ1 ), largely caused by the uncertainties in heat fluxes. Further analysis of the buoyancy budget of the mixed layer in the OCCAM model shows that diffusion extracts buoyancy from the water column at all densities. In agreement with observations, water mass formation of surface water by air-sea interaction is completely balanced by consumption from diffusion. There is a large interocean exchange with the Indian and Pacific Oceans. Intermediate water is imported from the Pacific, and light surface water is imported from the Indian Ocean. South Atlantic Central Water and denser water masses are exported to the Indian Ocean. The air-sea formation rate is only a qualitative estimate of the sum of subduction and interocean exchange. Subduction generates teleconnections between the South Atlantic and remote areas where these water masses reemerge in the mixed layer. Therefore, the subduction is analyzed with a Lagrangian trajectory analysis. Surface water obducts in the South Atlantic, while all other water masses experience net subduction. The subducted Antarctic Intermediate Water and Subantarctic Mode Water reemerge mainly in the Antarctic Circumpolar Current farther downstream. Lighter waters reemerge in the eastern tropical Atlantic. As a result, the extratropical South Atlantic has a strong link with the tropical Atlantic basin and only a weak direct link with the extratropical North Atlantic. The impact of the South Atlantic on the upper branch of the thermohaline circulation is indirect: water is significantly transformed by air-sea fluxes and mixing in the South Atlantic, but most of it reemerges and subducts again farther downstream.

Northern and southern water masses in the equatorial Atlantic

Deep Sea Research Part I: Oceanographic Research Papers, 1998

In the framework of the WOCE Hydrographic Program, two trans-Atlantic CTDO/tracer sections with closely-spaced stations, along 7'30" and 4'30's (WHP Lines A6 and A7), and two meridional sections, along 3"50W and 35"W joining the two zonal sections, were occupied in January-March 1993 (CITHER 1 cruise on board the N I 0 L'ATALANTE). CTD profiles and nutrient (silicate, phosphate and nitrate) data at 32 depths between surface and bottom were obtained at each station. The distributions on vertical sections, and on isopycnal surfaces, of these three chemical tracers are presented and discussed in the context of large-scale circulation in the equatorial Atlantic Ocean.

South Atlantic mass transports obtained from subsurface float and hydrographic data

Journal of Marine Research, 2010

Mean total (barotropic ϩ baroclinic) mass transports of the oceanic top 1000 dbar are estimated for two regions of the South Atlantic between 18°S and 47°S. These transports are obtained by using Gravest Empirical Mode (GEM) fields calculated from historical hydrography with temperature and position data from quasi-isobaric subsurface floats deployed from 1992 through 2001. The float-GEMestimated total mass transports reveal a Brazil Current with a southward flow of 20.9 Sv at 30°S and 46 Sv at 35°S (1 Sverdrup, Sv ϭ 10 6 m 3 s Ϫ1 ). Two recirculation cells are identified in the southwest corner of the subtropical gyre north of 40°S, one centered at 48°W, 37°S recirculating 28.5 Sv and another centered at 40°W, 38°S recirculating 13.9 Sv. The South Atlantic Current (SAC) flows eastward with 50 Sv at 30°W and splits into two branches in the east, one north of 38°S transporting 19 Sv and one south of 41°S transporting 31 Sv. Of the 39.7 Sv of SAC transport that comes from the Malvinas Current/Antarctic Circumpolar Current (ACC) system in the western basin, only 8.7 Sv flow with the northern branch and the remaining 31 Sv flow as the southern branch out of the South Atlantic rejoining the ACC directly (20.6 Sv) or interacting with the Agulhas Current Retroflection (10.4 Sv). From the northern branch, only 4.7 Sv of Malvinas Current/ACC origin and 10.3 Sv of Brazil Current origin (a total of 15 Sv) stays in the South Atlantic forming the Benguela Current, recirculating within the subtropical gyre. The Agulhas Current Retroflection reaches westward as far as 10°E with a transport of 48 Sv. In terms of mean total transport, the cold-water route carries 4.7 Sv in the upper 1000 dbar whereas the warm-water route carries 8.5 Sv. However, considering the interaction between waters from both origins, there is a total of 19.1 Sv of waters entering the Cape Basin from the Pacific Ocean and 18.5 Sv from the Indian Ocean.

Introduction to special section: World Ocean Circulation Experiment: South Atlantic Results

Journal of Geophysical Research, 1999

Oceanographers are increasingly urged to think globally. The World Ocean Circulation Experiment (WOCE) organized and implemented a strategy for a global view of the ocean. Yet the global ocean and its influence on the climate system consist of an interlocked array of regions. Heat and freshwater pass between component oceanic regions which, on coupling by sea-air fluxes, act in concert with atmosphere fluxes, the essence of climate. Each ocean basin plays its role, differing by virtue of its unique geometry and atmospheric coupling. Deep convection of the saline North Atlantic must balance the freshwater accumulation within the North Pacific, instilling global thermohaline circulation. Similarly, deep convection in the Southern Ocean must somehow be compensated by largerscale thermohaline circulation. What is the role of the South Atlantic in the global reference frame? WOCE observational (Figures 1 and 2)and model studies have done much to address this topic. The subtropical South Atlantic forms varied types of Subtropical Mode Water; shallow upwelling occurs in the eastern boundary regions and within the equator belt, as with other oceans. However, what it does uniquely is that it exposes the North Atlantic water mass products to the Antarctic Circumpolar Current, giving it global access. Equally important is the associated climate issue of the nature of the upper ocean flow that balances the export of deep water. The South Atlantic thermocline exchange with the Indian Ocean thermocline and the injection of Pacific Ocean derived Antarctic intermediate and mode water masses in Drake Passage are all part of the hotly debated "warm route, cold route" subject, echoed in this collection of WOCE based articles. At the crux of the debate is that the South Atlantic displays the most curious feature of the global ocean: oceanic heat flux toward the equator! Warm upper waters move northward, compensating southward moving, cooler deep water. Ultimately, this heat warms the northern climes of the North Atlantic. Estimates range from 0.2 to 0.9 PW for the northward heat flux across the subtropical belt, with the error estimated in excess of 0.2 PW. There appears to be significant northsouth divergence of the meridional heat flux with seasonal (large) and interannual (weaker) variability, hence a large range in heat flux estimates is to be expected. Recent attention to this issue by models and observations has not yet

Intermediate waters in the southwest South Atlantic

1989

The density (sigraa-theta, ao) interval 27. 05-27.20 in the Subantarctic Zone of the northern Drake Passage is characterized by two water types with potential temperatures of 3.7 and 4.8°C, respectively, both with salinity of approximately 34.2. These major contributors to the low salinity intermediate water mass are advected northward along the continental slope of South America. The lower density water type enters the Argentine Basin both east and west of Burdwood Bank. Its thermohaline characteristics are modified by winter sea-air interaction near Burdwood Bank and mixing with the surrounding waters further north. The denser water type flows east of Burdwood Bank, undergoing salinity decrease, primarily by isopycnal processes. Low salinity water, derived from the Polar Front, is introduced into a still denser horizon (27.25 ~0), from along the axis of the cyclonic circulation feature described by the Malvinas Current and its return to the south. The thermohaline structure across the Malvinas Current is similar to the water mass zonation observed in the northern Drake Passage.

Deep circulation in the tropical North Atlantic

Journal of Marine Research, 1993

Deep circulation in the tropical North Atlantic by Marjorie A. M. Friedrichs' and Melinda M. Hall' ABSTRACT A transatlantic CTD/ADCP (Conductivity, Temperature, Depth/Acoustic Doppler Current Profiler) section along llN, taken in March 1989, has been used to compute geostrophic velocities; geostrophic transport is required to balance in situ values of the Ekman and shallow boundary current transports. The horizontal flow structure is described for eight layers, with particular emphasis on deep and bottom waters (four layers below 8 = 4.7"C). In the shallow layers, total North Brazil Current (NBC) transport agrees with other observations previously made in the month of March, while net northward flow of these layers across the western basin is also consistent with recent observations to the north. For each of the four deep layers, circulation patterns are illustrated by means of schematic cartoons. Each of these layers flows southward in the Deep Western Boundary Current, which has a magnitude of 26.5 Sv. Roughly half of this flow returns northward to the west of the Mid-Atlantic Ridge, confirming the existence of a hypothesized cyclonic recirculation gyre in the western basin of the tropical Atlantic. To varying degrees the deep and bottom waters also circulate cyclonically in the eastern basin, with net northward flow across this basin. Partly as a result of the unusual appearance of the North Equatorial Countercurrent in March 1989, the in situ values of the meridional overturning cell (5.2 Sv), heat flux (3.0 x 1014 W), and freshwater flux (-0.65 Sv) computed from the 11N section depart significantly from estimates of these quantities in the literature. By forcing the 11N geostrophic velocities to balance annual average Ekman and NBC transports, annual average values of these fluxes (12 Sv; 11 x lOi W;-0.6 Sv) are obtained, and are shown to agree well with historical estimates.