On the temporal variability of the transport between Amsterdam and Kerguelen islands

Abstract

Data from over 3 years of bottom pressure recording together with Geosat altimetry and the Fine Resolution Antarctic Model numerical ocean model have been used to investigate the temporal variability of the Antarctic Circumpolar Current between Amsterdam and Kerguelen islands in the southern Indian Ocean. An upper limit to the standard deviation of the variability in transport derived from pressure differences has been calculated to be 10–15 Sv, subject to an assumption of barotropic flow dominating the variability and after the application of a 31‐day low‐pass filter. This is slightly larger than the variability obtained in a similar experiment at the Drake Passage a decade ago. Although the seasonal cycles in pressure difference and in sea surface height difference measured by altimetry are different each year, their means are consistent with the presence of a semiannual component with an amplitude in terms of (assumed barotropic) transport of the order of 5 Sv, which is comparable to the Drake Passage findings. In addition, the pressure difference data contain a large, quasistationary annual component of amplitude of approximately 5 mbar, a possible source of which is different seasonal steric fluctuations at the two sites. Insofar as the bottom recorder data can be considered ground truth for the altimetry, results from the different techniques show some features in common, although significant differences between data sets remain, for which possible explanations are offered. It is concluded that when they are used for monitoring transport fluctuations, bottom pressure recorders should be accompanied by inverted echo sounders and comprehensive hydrographic measurements.