Relative Importance of the Austral Summer and Autumn SAM in Modulating Southern Hemisphere Extratropical Autumn SST*

Abstract Sea surface temperature (SST) variability in the extratropical Southern Hemisphere is mainly determined by physical processes at the air–sea interface associated with the Southern Hemisphere annular mode (SAM). Both the austral summer [December–February (DJF)] and autumn [March–May (MAM)] SAM can imprint their signals on southern extratropical MAM SST. Here three approaches are employed to determine the relative importance of the DJF and MAM SAM in modulating southern extratropical MAM SST: a simple lead–lag correlation without SST decomposition, a decomposition method based on linear regression, and a new approach named the persistent signal decomposition (PSD). The results show that the DJF SAM plays a more important role than the MAM SAM in driving MAM large-scale southern extratropical SST anomalies, implying that MAM SST anomalies caused by the preceding DJF SAM would not be largely perturbed by the MAM SAM, and thus the DJF SAM can be regarded as an effective predictor for the following season’s climate. The PSD also provides an estimation of the contribution of atmospheric persistence and SST persistence toward cross-seasonal influence of the DJF SAM on MAM southern extratropical SST. The results show that this cross-seasonal influence is mainly caused by the SST persistence. The detection of the relative importance of the preceding and contemporaneous atmospheric signal in driving SSTA contributes to the understanding of air–sea interactions and helps to obtain better SST-based statistical predictions. The PSD has the potential to be employed in the North Atlantic and other extratropical oceans.