An extension of the simple stochastic climate model of Frankignoul and Hasselman that includes the effects of seasonal variations in upper-ocean mixed layer depth upon the persistence of winter sea surface temperature (SST) anomalies is proposed. Seasonal variations in mixed layer depth allow for the "reemergence mechanism,'' whereby thermal anomalies stored in the deep winter mixed layer persist at depth through summer and become partially reentrained into the mixed layer during the following winter. In this way, SST anomalies can recur from winter to winter without persisting through the intervening summer. Reformulating the simple stochastic climate model in terms of an effective ocean thermal capacity given by the depth of the winter mixed layer, thereby implicitly taking into account reemergence, is shown to provide a favorable fit to the observed winter-to-winter SST autocorrelations in the North Atlantic and Pacific, and represents a considerable improvement over the original model. The extended model also compares favorably with results from an entraining bulk ocean mixed layer model coupled to an atmospheric general circulation model. The authors propose that the extended model be adopted as the new "null hypothesis'' for interannual SST variability in middle and high latitudes.
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