On the temporal variability of the transport through Drake Passage

A recently obtained series of bottom pressure measurements from various positions in the Drake Passage (DP) has been used to estimate the transport variability of the Antarctic Circumpolar Current (ACC) under the assumption that the variability is predominantly barotropic and thus proportional to the north-minus-south pressure difference, The standard deviations in derived transport range from 5.3 Sv in 1993 to 8.9 Sv in 1990 (10-day filter; 1 Sv = 10(6) m(3) s(-1)). All values are less than the 10 Sv (IO-day filter) obtained during the International Southern Ocean Studies (ISOS) program at DP between the mid 1970s and early 1980s [Whitworth and Peterson, 1985]. Although some of this discrepancy is due to differences in procedure in dealing with gaps in the data, our data suggest that the ACC transport can be persistently less variable over several years than was previously thought. In particular, the ISOS data set contains two large-amplitude changes in pressure difference over timescales less than 2 weeks, while no similar events were observed in the more recent data. Although the more recent pressure gauges were deployed at different positions to those used in ISOS, their reliability as indicators of ACC transport changes has been established by examination of simultaneous pressure measurements within DP.