A 6 year record of baroclinic transport variability of the Antarctic Circumpolar Current at 140°E derived from expendable bathythermograph and altimeter measurements

Abstract

Repeat hydrographic sections across the Antarctic Circumpolar Current are used to derive an empirical relationship between upper ocean temperature and the baroclinic transport stream function. Cross validation shows this relationship can be used to infer baroclinic transport (above and relative to 2500 dbar) from temperature measurements with an error of a few per cent. The mean transport distribution derived from 31 austral summer expendable bathythermograph (XBT) sections over a 6 year period consists of westward flow immediately south of Tasmania, a broad band of strong eastward flow between 50° and 55°S, and three cores of eastward flow south of 55°S. By defining a second empirical relationship between surface dynamic height and cumulative transport a continuous time series of baroclinic transport is derived from altimeter measurements of sea surface height. Transports derived from altimetry in this way agree well with simultaneous in situ estimates (root mean square error in net transport is 4 × 106 m3 s−1), suggesting sea level anomalies largely reflect baroclinic changes above 2500 dbar. The 10 day sampling of the altimeter transport time series shows the irregular XBT sampling aliases variability at unresolved timescales. The standard deviation of net transport above and relative to 2500 m is 4.3 × 106 m3 s−1. The variability in net transport is largest (2.7 × 106 m3 s−1) in the quasi‐annual band (periods of 4 months to 1.5 years), slightly smaller (2.3 × 106 m3 s−1) in the mesoscale band (<4 months), and smallest in the interannual band (>1.5 years, 1.5 × 106 m3 s−1). Changes in transport are correlated with local changes in both wind stress and wind stress curl in the quasi‐annual and interannual bands, but the transport time series is too short to draw significant conclusions.