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The South Atlantic: an Overview of Results from 1983-88 Research

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FEATURE THE SOUTH ATLANTIC: AN OVERVIEW OF RESULTS FROM 1983-88 RESEARCH By Arnold L. Gordon ~ EACH of the major ocean basins is unique, but the Initiative (SAARI) meeting repoll are available from South Atlantic is unrivaled in its number of singular Dr. Garzoli at Lamont-Doherty Geological Observa- features. In its polar region are formed intermediate tory, Palisades, NY, 10964. What follows is an and bottom water masses that cool and lower the abbreviated account of the primary results in the salinity of much of the global ocean. The subtropical SAAR1 report. Names mentioned here without at- region has an eastern boundary that doesn't quite tributive dates refer to the report, in which additional extend to the latitude of maximum westerlies, allow- information and extensive references may be ob- A curious ing thernmcline communication with its counterpart tained: an abbreviated list of references is included at feature of the in the South Indian Ocean. Within the Argentine the end of the article. Basin of the South Atlantic one can sample deep Large Scale Circulation South Atlantic is water from the North Atlantic, deep and intermediate A curious feature of the South Atlantic is the the northward heat water from the Pacific, and bottom water formed in northward heat flux across 30°S, a consequence of the Weddell Sea. These water masses spread and the global thermohaline circulation cell driven by flux across 30°S vigorously mix within tiffs cosmopolitan basin and formation of North Atlantic Deep Water (NADW). throughout much of the South Atlantic. In the classical description, southward-moving Perhaps the most startling attribute of the South NADW is replaced by an upper layer return rio,:,' Atlantic is the significant equalorward heat flux derived from Pacific water, with the introduction of within the subtropical region. Estimates range from Antarctic Intermediate and Subantarctic Mode Wa- 18 to 86 x I()~W for the northward heat flux across ter (AAIW/SAMW: the latter terrn coined by Mc- the 8°S to 32°S belt. Associated with this flux is a Cartney ). Gordon (1986) suggested that at least some strong thennohaline circulation, as upper layer water of the return flow is derived from the thermocline spreads northward to compensate the southward water of the Indian Ocean, which has access to the export of colder North Atlantic Deep Water (NADW), South Atlantic around the southern tip of Africa. The The METEOR expedition of the 1920s and the relative influence of the wann and salty Indian water work during the International Geophysical Year (IG Y) and the colder and fresher Pacific water is crucial to of the late 1950s provide a glimpse of the large scale the larger scale climate system, because the Atlantic features of the South Atlantic Ocean. In the last five meridional heat and fresh water fluxes are very years there has been a surge in our appreciation of the sensitive to the ratio of the volume fluxes of the two South Atlantic phenomena, mainly because of an water types. Gordon suggested that up to 50~ of the Office of Naval Research Accelerated Research Ini- required flow could be derived from the Indian tiative entitled "'The Southern Ocean," which was Ocean pathway. primarily directed toward improvement of the de- The SAAR1 research does indeed present evi- scription of the subtropical South Atlantic. The pro- dence of Indian Ocean water intrusion into the South gram focused on the poleward "corners" of the sub- Atlantic (Olson and Evans, 1986; Gordon et al., tropical gyre: the separation of the Brazil Current and 1987). It is still unknown what part of that inflow its confluence with the Malvinas or Falkland Current eventually contributes to cross equatorial transport in in the southwest, and the Agulhas Retroflection and the upper ocean and what part returns to the Indian Benguela Current in the southeast. Ocean along a surface route south of the Agulhas In May 1988, a meeting was organized by Silvia Retroflection. The Agulhas "'leakage" into the Atlan- Garzoli to discuss the program accomplishments. tic Ocean need not involve thermohaline forcing at Copies of the South Atlantic Accelerated Research all, because purely wind-driven models of the Agulhas Retroflection reveal transfer of Indian Ocean water A. L. Gordon, Latnont-Dohertv Geoloc, ical Ohservato#p;. Pah- into the Atlantic (cf. Fig. 3, p. 16). xade,~, N.Y 10964. :With conttibutio~t,~ fiom particq~ant,~ in the The application of inverse methods to data from ONR-sponsored Soztt/t ,.trio/trio rdsedrch l~VOk,#'dnt: S. Bennett, D. the International Geophysical Year (IGY) reveals no Boudra, O. Brown, R.Evans, R. Fine. S. Garzoh, J. Luyten, M. McCartne>. D. Olson, G. Roden. T. Whitw'orth, P. Zemba. significant Indian Ocean flow into the Atlantic 1 2 OCEANOGRAPHY,NOVEMBER.1~)88 Figure 2: The Brazil Current, the western boundary current Atlantic in a regton known as the Brazil-Malvinas Cop~/luence. associated with the subtropical gyre in the South Atlantic Ocean. Warmest smface waters (approx. 25°C) are coded in red. The flows southward along the continental margin r(South America sulfate temperature decreases are color-coded through yellow to a point off Argentina and Uruguay where, in the mean, it and green: the dark blue shows the coldest water (approv. 9 ° C). separates fi'om the coast at about 36°S. The cold.fi'esh subantarc- The image was taken at a time when the Brazil Current was in its tic water in the Malvinas or Falkland Current meets the warm. southward extension and a large anticyclonic (warm) eddy or saline Brazil Current. and both turn eastward into the open rin~ was being./brined. (Rintoul, 1988). Gordon et al. (1987), using 1983 fresher when the Drake Passage route is favored? data, found 10 Sv of Agulhas water entering the Might this have important feedback to the production Atlantic (1 Sv = 106m3 s-~). Whitworth and Nowlin rate of NADW? (1987) calculated nearly 20 Sv of inflow using the The South Atlantic subtropical gyre has an elon- 1983/84 Ajax data set, which is subject to eddy gated crest stretching across the ocean near 30 °- aliasing because of a several month hiatus between 35°S. The gyre is marked at its poleward edge by the cruise legs. In her Ph.D. thesis, Bennett evaluated the Antarctic Circumpolar Current. The northern limb of 1985 CTD data from the Agulhas Retroflection and the subtropical gyre consists of a broad sweep to- found that only 2.8 Sv of water warmer than 9°C wards the northwest. In the northeastern comer of the flowed from the Indian into the Atlantic Ocean; her South Atlantic there is a weak cyclonic (clockwise) re-evaluation of the 1983 data set lowered the esti- gyre, described by Bosley and Gordon. The gyre is mate for that year to 6.3 Sv. formed by the South Equatorial Counter Current and If the Gordon suggestion is correct, it points to a the Angola Current, which flows southward along potentially significant climate issue: Might the At- the African coast. J. Reid clarified this complex lantic become warmer and saltier during periods pattern by defining the absolute geostrophic circula- when the Agulhas route is favored, versus colder and tion of the South Atlantic. OCEANOGRAPHY-NOVEMBER, 1988 13 The large scale circulation and the distribution of was in the process of separation, whereas the south- eddy' variability in the near surface layers of the ern one had been separated for at least a month and South Atlantic are another focus of research, pursued had been modified by the late winter atmosphere. by Olson. The dynamic height field relative to the These modified warm core eddies may not re-coa- 2000db level and surface velocities flom the SAARI lesce with the main thennocline, and hence they and FGGE drifter data sets show thai the 2000db represent a potentially significant salt flux into the reference level is best lk)r the eastern portion of the subantarctic zone of the South Atlantic. basin: however, that reference level clearly is inade- At depth, the most saline NADW is associated quate along the Brazilian coast, where the actual with the western meander of the Brazil Current. The level of no motion probably is higher in the water NADW continues tlowing south, beneath the Malv- column (cf. Fig. 1, p. 15). The drifter tracks also inas Current. which indicates that the separation of reveal the closed anticyclonic flows near the mid- the deep western boundary current need not be coher- Atlantic ridge that appear in most descriptions of the ent with the shallower thermocline separation lati- South Atlantic circulation. tude: this further complicates the definition of the BraziI-Malvinas Connuenee Brazil Current. Contrasting water masses are brought together in Piola and Gordon presented a detailed picture of the Argentine Basin by' a layer cake of boundary the AAIW/SAMW in the Argentine Basin. The ~, currents: The eddy-rich BraziI-Malvinas (Falkland) density interval 27.05 to 27.20 kg in 3 in the suban- Confluence dominates the upper layer, with deeper tarctic zone of the northern Drake Passage is charac- meridional flow involving NADW, Circumpolar terized by two water types with potential tempera- Deep Water and Antarctic Bottom Water. At the tures of 3.7 ° and 4.8~C, respectively: both have meeting, there was not complete agreement as to the salinity of approximately 34.2 ppt. Thermohaline Contrasting exact definition of the Brazil Current, but in general, characteristics are modified by, winter sea-air inter- water masses are reference to the Brazil Current is confined to its action near Burdwood Bank and by' mixing with the thermocline expression.
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