The Reemergence of SST Anomalies in the North Pacific Ocean

Abstract

Sea surface temperature (SST) data and two different upper-ocean temperature analyses are used to study the winter-to-winter recurrence of SST anomalies in the North Pacific Ocean. The SSTs recur when temperature anomalies that form in the deep ocean mixed layer in late winter/early spring are isolated from the atmosphere in the summer seasonal thermocline and then reemerge at the surface when the mixed layer deepens during the following fall/winter. This ?reemergence mechanism? is evaluated over the basin by correlating the time series of the leading pattern of ocean temperature anomalies in the summer seasonal thermocline (?60?85 m in August?September) with SST anomalies over the course of the year. The results indicate that the dominant large-scale SST anomaly pattern that forms in the North Pacific during late winter, with anomalies of one sign in the central Pacific and the opposite sign along the coast of North America, is sequestered in the seasonal thermocline in summer and returns to the surface in the following fall, with little persistence at the surface in summer. Regions in the east, central, and west Pacific all show signs of the reemergence process but indicate that it is influenced by the timing and amplitude of the mean seasonal cycle in mixed layer depth. The maximum mixed layer depth increases from east to west across the basin: as a result, the thermal anomalies are shallower and return to the surface sooner in the east compared with the west Pacific. At some locations, the reemerging signal is also influenced by when the SST anomalies are created. In the east Pacific, SST anomalies that are initiated in February?March extend through a deeper mixed layer, persist at greater depths in summer, and are then reentrained later in the year compared with those initiated in April?May.