A pulsation mode in the Antarctic Circumpolar Current south of Australia

Abstract

A streamfunction EOF method is developed to identify long-term thermohaline variations within a strong baroclinic current. The temporal variability associated with meandering fronts and mesoscale eddies is removed by projecting hydrographic data into a baroclinic streamfunction space. The residual field, after removing the streamfunction mean field, is analyzed to find empirical orthogonal functions. The method is applied to a time series of hydrographic sections across the Antarctic Circumpolar Current (ACC) south of Australia. The temperature variation in the surface layer (0-300 dbar) is dominated by a seasonal signal. In the subsurface water (300-3000 dbar), a separately calculated first EOF mode dominates the thermohaline variation and exhibits two phases. In the strengthening phase both salinity and temperature in the Subantarctic Mode Water increase and the ACC section is characterized by less Antarctic Intermediate Water and higher salinity at the core of the Circumpolar Deep Water. The water masses vary conversely in the relaxing phase. The authors call this mode the ACC pulsation mode and hypothesize that it is related to the ACC barotropic transport and is a response to the large-scale wind stress variation. Observations of westerly winds and ACC transport appear to support the hypothesis as they all display semiannual periods nearly in phase with higher coherence to the south.