Transport

Until recently, the best available estimates of the transport of the ACC were based on the density field alone. Such relative transport estimates assume that the current speed approaches zero at great depth. The most consistent transport figures come from regions where the ACC is geographically constrained: between Australia-New Zealand and Antarctica and in Drake Passage. The relative transport through Drake Passage is about 115 Sv (1 Sverdrup = 106 m3 sec."1) according to Reid and Nowlin (1971) and that south of Australia and New Zealand is about 145 Sv (Gordon, 1975). The lesser value at Drake Passage is largely attributable to the shallower bottom depth there.

The introduction of reliable internally-recording current meters made it possible to measure deep currents rather than assume that they vanished. This technology is especially important in the ACC, which is known to be deep-reaching and have non-negligible bottom speeds. Speed estimates from current meters provide a reference with which to adjust the relative speeds obtained from the density field and permit estimates of the net transport. Using this technique, Reid and Nowlin (1971) measured a transport of 237 Sv at Drake Passage, and Callahan (1971) estimated the transport south of Australia at 233 Sv. It is now believed that both of these estimates are too high since both relied on a few widely-spaced current measurements of only a few days duration. Because of the banded nature of the flow in the ACC, many direct measurements are needed to characterize the deep flow accurately, and the agreement of these two estimates is doubtless fortuitous.

During ISOS, transport calculations were made by adjusting density measurements to more extensive current meter and pressure gauge data. In 1979, the net transport through Drake Passage averaged 130 Sv (Whitworth et al., 1982). Over a three-year period, the net transport ranged between 95 and 158 Sv (Whitworth and Peterson, 1985). Excluding Drake Passage, transport out of the Pacific is limited to the Bering Strait and the East Indian Archipelago. The outflow through the Bering Strait has been estimated at 0.8 Sv by Coachman and Aagaard (1981) and 0.6 Sv by Aagaard et al. (1985). Various estimates of the transfer of Pacific waters to the Indian Ocean through the Indonesian Sea have been summarized by Gordon (1986). The estimates range from 1.7 to 14 Sv, and the average transport from all estimates is 9.2 Sv. The mean transport of the ACC as it enters the South Pacific is therefore about 140 Sv (130 + 0.7 + 9.2 Sv). In the open ocean, it is more difficult to establish northern and southern bounds of the ACC, and there are therefore no reliable transport estimates in the central part of the Pacific Sector.

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