Mechanisms of observed sea surface temperature variability in the extratropical southern hemisphere
Date
2008
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Abstract
The physical mechanisms that drive sea surface temperature (SST) variability in the extratropical Southern Hemisphere (SH) are examined using multiple ocean temperature datasets. The first part of the study provides a detailed analysis of the relationships between variability in SH SST anomalies, the Southern Annular Mode (SAM) and the El-Niño/Southern Oscillation (ENSO) during the warm (November-April) and cold (May-October) seasons. It is shown that the signatures of the SAM and ENSO in the SST field vary as a function of season, both in terms of their amplitudes and structures. SAM-related SST anomalies are primarily driven by surface turbulent heat fluxes with a smaller contribution from heat advection by Ekman currents. The role of turbulent heat fluxes in generating ENSO-related SST anomalies is less clear. Analyses of the temporal evolution of the relationships between the SAM and the SST field demonstrate that SST anomalies are largest when SSTs lag by ~1 week and persist for up to 8 weeks. In the absence of ENSO, cold season SAM-related SST anomalies persist longer than their warm season counterparts, consistent with seasonal variations in the depth of the mixed layer. The second part of the study uses observations of subsurface temperatures to examine the winter-to-winter "reemergence" of SST anomalies in the extratropical South Pacific. Reemergence is the mechanism whereby SST anomalies formed in the late winter are sequestered beneath the shallow summer mixed layer and then re-entrained into the deepening mixed layer during the following fall/winter. The results exhibit a pronounced reemergence signal in which surface temperature anomalies during the late winter season are strongly correlated with surface temperature anomalies during the subsequent early winter months, but are only significantly correlated with temperature anomalies beneath the mixed layer during the intervening summer months. The results are robust to small changes in the period of analysis and are qualitatively similar to existing evidence of reemergence in the Northern Hemisphere. The signal of reemergence evident in the subsurface data is readily apparent in SST data in the western South Pacific. Reemergence is less evident in SST data in the eastern South Pacific.
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Subject
air-sea interaction
El-Niño-Southern Oscillation
sea surface temperature
Southern Annular Mode
Southern Hemisphere
atmospheric sciences