On the wind‐forcing of bottom pressure variability at Amsterdam and Kerguelen Islands, southern Indian Ocean

Abstract

Bottom pressure records from Amsterdam and Kerguelen Islands (southern Indian Ocean) were collected between 1986 and 1989, and interpreted previously in terms of volume transport fluctuations between the islands [Vassie et al., 1994]. Since a substantial portion of the mean transport of the Antarctic Circumpolar Current (ACC) flows between the islands, it was assumed that ACC transport variability was the predominant signal being measured, and comparisons with ACC variability at other longitudes were made. Here we examine the relationship between atmospheric forcing and observed bottom pressure variability. We find that Kerguelen bottom pressure is forced predominantly by wind stress curl over the Kerguelen Plateau for periods between around 5 days and 1 year. A weaker correlation with circumpolar eastward winds is observed, but is seen not to be causal. Amsterdam pressure shows a similar local response to curl for high frequencies (5–10 days), but at longer periods is sensitive to curl over the sub‐basin between Kerguelen Plateau, the Southeast Indian Ridge, and Antarctica. This is the region of long‐wavelength sea level variability observed by Chao and Fu [1995], and interpreted in terms of a geostrophic resonance by Webb and de Cuevas [2002a]. Altimetric studies confirm the usefulness of the Amsterdam location in monitoring this mode. We conclude that the Amsterdam‐Kerguelen pressure difference is not a useful measure of ACC transport variability, and that previous agreement found between variability observed here and other locations is largely fortuitous. Instead, the series separately reflect different important dynamical modes in the southern Indian Ocean.