Wind-driven transport fluctuations through drake passage: A southern mode

It is proposed that, for periods between about 10 and 220 days, the variability in Antarctic circumpolar transport is dominated by a barotropic mode that follows flH contours almost everywhere. Theoretical arguments are given that suggest the possible importance of this mode and show that bottom pressure to the south of the current should be a good monitor of its transport. The relevance of these arguments to eddy-resolving models is confirmed by data from the Fine Resolution Antarctic Model and the Parallel Ocean Climate Model. The models also show that it may be impossible to distinguish the large-scale barotropic variability from local baroclinic processes, given only local measurements, although this is not generally a problem to the south of the Antarctic Circumpolar Current. Comparison of bottom pressures measured in Drake Passage and subsurface pressure on the Antarctic coast, with wind stresses derived from meteorological analyses, gives results consistent with the models, showing that wind stress to the south of Drake Passage can explain most of the observed coherence between wind stresses and circumpolar transport. There is an exception to this in a narrow band of periods near 20 days for which winds farther north seem important. It is suggested that this may be due to a sensitivity of the "almost free" mode to winds at particular locations, where the current crosses flH contours.