Subsurface structure of interannual temperature anomalies in the Australian sector of the Southern Ocean

Abstract

A 7 year time series of austral summer expendable bathythermograph (XBT) sections between Tasmania and Antarctica is used to describe the subsurface structure of Southern Ocean interannual temperature anomalies. Comparison of the discontinuous XBT record with a continuous (weekly average) satellite-based SST data set confirms that the XBT sampling is adequate to resolve interannual variability. Significant correlations are found between changes in surface and subsurface temperature along much of the section. In the Subantarctic zone, surface and subsurface changes are coherent because deep convection in winter homogenizes the upper 600 m, and the summer mixed layer temperature reflects the preconditioning of the previous winter. At other latitudes the regions of high correlation generally extend well beyond the depth of direct atmospheric influence (i.e., the depth of the winter mixed layer). Negative (positive) temperature anomalies at interannual timescales are coherent with poleward (equatorward) displacements of fronts. The simplest explanation for the correspondence between deep, vertically coherent temperature anomalies and the location of ACC fronts is that the observed features reflect meridional shifts of temperature gradients associated with the fronts. The link between temperature anomalies and front displacements suggests Southern Ocean sea surface temperature (SST) anomalies primarily reflect a dynamical response of the ocean to wind or other forcing rather than surface-trapped features produced by anomalous air-sea heat flux. The XBT record contains both interannual and longer (unresolved) period signals, which cannot be isolated in the short discontinuous XBT time series. Spectral analysis of the 17 year SST data set reveals roughly equal variability in 2-7 year and >7 year bands.