D. R. Munday, H. L. Johnson and D. P. Marshall, Palaeoceanography, submitted.
The date of inception of the Antarctic Circumpolar Current is debated due to uncertainty in the relative opening times of Drake Passage and the Tasman Seaway. Using an idealised eddy-resolving numerical ocean model, we investigate whether both ocean gateways have to be open to allow for a substantial circumpolar current. We find that overlapping continental barriers do not impede a circumpolar transport in excess of 50Sv, as long as a circumpolar path can be traced around the barriers. However, the presence of overlapping barriers does lead to an increased sensitivity of the current's volume transport to changes in wind stress. When the prevalent momentum balance is that of the modern Antarctic Circumpolar Current, i.e. surface wind stress balanced by bottom form stress, the transport of the circumpolar current is relatively insensitive to wind stress changes. This is due to the mean flow independent nature of the balance, which relies upon eddy form stresses. However, when the eddy form stress is low, the input of momentum by the surface wind stress is balanced by a mean flow dependent sink. As a result, the mean circumpolar transport must change with wind stress.
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