Observations of large-scale ocean-atmosphere interaction in the Southern Hemisphere

In this study, the authors provide a detailed examination of observed ocean-atmosphere interaction in the Southern Hemisphere (SH). Focus is placed on the observed relationships between variability in monthly and weekly SH extratropical sea surface temperature (SST) anomalies, the Southern Annular Mode (SAM) and El Niño-Southern Oscillation (ENSO). The results are divided into austral summer (November-April) and winter (May-October) to investigate the seasonal variations in these relationships. It is shown that the signature of the SAM and ENSO in the SH SST field varies as a function of season, both in terms of amplitude and structure. Because variability in the SAM and ENSO are only significantly correlated during the warm season, strong similarities between the associated patterns of SST anomalies are observed in the warm season but not in the cold season. The relative roles of surface turbulent and Ekman heat fluxes in driving the seasonally varying SST anomalies are investigated. Sensible and latent heat fluxes play a substantial role in driving SST variability throughout the SH ocean basins. Anomalous Ekman heat fluxes contribute to the observed SST anomalies in the vicinity of the Antarctic Circumpolar Current but are of secondary importance over other regions of the Southern Hemisphere. The analysis of weekly data reveals that variability in the SAM tends to precedes anomalies in the SST field by ~1 week, but no distinct pattern of SST anomalies appears to lead variability in the SAM. Comparison of the lagged autocorrelations of the SH SST pattern associated with the SAM with the North Atlantic SST pattern associated with the Northern Annular Mode (NAM) reveals an increased persistence in the SH ocean basins not observed in its North Atlantic counterpart. Possible mechanisms that explain this increased persistence in the SH SSTs associated with the SAM are discussed.