Thermocline and intermediate water communication between the south Atlantic and Indian oceans

Abstract

A conductivity-temperature-depth and tracer chemistry section in the southeast South Atlantic in December 1989 and January 1990 presents strong evidence that there is a significant interocean exchange of thermocline and intermediate water between the South Atlantic and Indian oceans. Eastward flowing water at 10øW composed of South Atlantic Central (thermocline) Water is too enriched with chlorofluoromethanes 11 and 12 and oxygen to be the sole source of similar O-S water within the northward flowing Benguela Current. About two thirds of the Benguela Current thermocline transport is drawn from the Indian Ocean; the rest is South Atlantic water that has folded into the Benguela Current in association with the Agulhas eddy-shedding process. South Atlantic Central water passes in the Indian Ocean by a route to the south of the Agulhas Return Current. The South Atlantic water loops back to the Atlantic within the Indian Ocean, perhaps mostly within the Agulhas recirculation cell of the southwest Indian Ocean. Linkage of Atlantic and Indian Ocean water diminishes with increasing depth; it extends through the lower thermocline into the Antarctic Intermediate Water (AAIW) (about 50% is derived from the Indian Ocean) but not into the deep water. While much of the interocean exchange remains on an approximate horizontal "isopycnal" plane, as much as 10 x i0 6 m 3 s -! ofindian Ocean water within the 25 x 10 6 m 3 s -I Bengueia Current, mostly derived from the lower thermocline and AAIW, may balance deeper Atlantic export of North Atlantic Deep Water (NADW). The addition of salt water from the evaporative Indian Ocean into the South Atlantic Ocean thermocline and AAIW levels may precondition the Atlantic for NADW formation. While AAIW seems to be the chief feed for NADW, the bulk of it enters the subtropical South Atlantic, spiked with Indian Ocean salt, within the Benguela Current rather than along the western boundary of the South Atlantic. Paper number 92JC00485. 0148-0227/92/92J C-00485 $05.00 overall heat and salinity budgets of the South Atlantic and may play a role in the global thermohaline circulation [Gordon, 1985, 1986, 1988]. In effect, the Atlantic's salinity is increased by drawing salty water from the evaporative Indian Ocean (which north of 30øS loses fresh water to the atmosphere at a rate of 5.1 x 105 m 3 s -1 only slightly less than the Atlantic Ocean [Baumgartner and Reichel, 1975]). Estimates of the leakage of Indian Ocean water into the South Atlantic by eddies and plumes range from 3 to 20 Sv (1 Sv = 106 m 3 s-i). It is not an easy matter of calculating this value, since the Indian Ocean Central Water and the South Atlantic Central Water (SACW) have very similar potential temperature-salinity (0-S) structure [Gordon, 1985]. Gordon et al. [1987] calculate 10 Sv of IOCW and Antarctic Intermediate Water (AAIW) entering the Atlantic in late 1983' Whirworth and Nowlin [1987] show inflow of nearly 20 Sv in early 1984. Bennett's [1988] evaluation of the 1983 and 1984 data arrives at lower values, 6.3 and 9....

Impact of Interbasin Exchange on the Atlantic Overturning Circulation

“…Does the Indian-Atlantic interbasin exchange of heat and salt through Agulhas Leakage significantly influence the overturning strength, as was speculated earlier (Gordon 1986, Gordon et al 1992)? What will happen if Agulhas leakage is shut off, and the South Atlantic surface salinities drop by about 0.5 psu?…”

“…Freshening of Atlantic surface waters might reduce the convective activity in the formation region of NADW, as suggested by Gordon et al (1992), and so reduce the production of deep water. On the other hand, the resulting reduction of the salinity stratification will enhance the downward diffusion of buoyancy, which turned out to be a (relatively small) source of potential energy in our model.…”

“…Paleoceanographic data indicate large shifts in the latitudinal positions of the major fronts in the Indian Ocean (Howard and Prell 1992), and suggest that the present-day position of the Subtropical Convergence, which might be thought of as the southern edge of the wind-driven subtropical gyres, is several degrees south of the average position over the last 500 kyr. Cai and Greatbatch (1995) have considered the suggestion of Gordon et al (1992) by comparing the overturning strength with and without Agulhas Leakage in a lowresolution ocean model. In their model the overturning strength did not change in response to shutting off the Agulhas Leakage.…”

Modification and pathways of Southern Ocean Deep Waters in the Scotia Sea

“…As postulated by Stommel and Arons (1960a,b), the oceanic heat balance is maintained through deep western boundary currents carrying cold, dense waters from the polar regions to lower latitudes, where they upwell (Rintoul, 1991;Gordon et al, 1992). Because the deep western boundary currents of the southern hemisphere are composed primarily of Southern Ocean waters (Mantyla and Reid, 1983), the properties and circulation of these waters are important factors in the climate system.…”

Opal burial in the equatorial Atlantic Ocean over the last 30 ka: Implications for glacial‐interglacial changes in the ocean silicon cycle

“…Furthermore, as the Agulhas system and the Drake Passage are the two primary avenues for return flow to the North Atlantic [Gordon et al, 1992], the glacial reduction in supply of Agulhas water may have been compensated for by increased supply via the Drake Passage. If the Agulhas slowdown indeed resulted in increased input of Drake Passage water to the equatorial Atlantic, and if this water had greater dissolved Si concentrations than today, then the supply of Si to the equatorial Atlantic would have increased (again coupled with decreased upwelling suppressing dissolved Si in the equatorial Pacific).…”