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The western equatorial Pacific is a crossroads for thermocline and intermediate waters formed at higher latitudes. The role of the equatorward flowing, low‐latitude western boundary currents (LLWBCs) in advecting well‐ventilated (with respect to atmospheric gases), higher‐latitude waters varies with density. At densities <26.5 σθ the Mindanao Current (MC) (Wyrtki, 1961; Masuzawa, 1969) advects recently ventilated water observed as tracer maxima predominantly from the North Pacific subtropical gyre (tropical water is <3 years old and the remnant subtropical mode water is <5 years); it branches into the southern Celebes Sea feeding the Indonesian throughflow and toward the east north of the equator. Between 26.5 and 26.8 σθ the MC advects predominantly North Pacific Intermediate Water (having a component that is <20 years old) mainly into the southern Celebes Sea; there is also some indication of a tracer maximum extending eastward north of the equator. However, below 26.8 σθ, South Pacific water masses appear to be stronger, so that they are the major ventilation source for the western equatorial region, including the Celebes Sea. At 27.2 σθ the New Guinea Coastal Undercurrent advects Antarctic Intermediate Water (having a component that is <25 years) into a background of older water. The presence of subtropical mode water in the western tropical North Pacific and Celebes Sea is attributed to an equatorward LLWBC in the North Pacific (and suggests a reason for the absence of 18° water in the tropical North Atlantic). The absence of a LLWBC in the North Atlantic highlights a basic difference between the circulation of the two oceans, which may be due to the different ways they import and export water. At the western boundary in the North Atlantic, warm water is imported and cold water is exported as part of the global thermohaline circulation, whereas at the western boundary in the North Pacific, warm water (above 26.8 σθ) is mainly exported to the Indian Ocean via the Indonesian throughflow and cold water is imported.

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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....