Summertime Evolution of Decadal Sea Surface Temperature Anomalies in the Midlatitude North Pacific

Abstract

Abstract To clarify the summertime evolution of decadal sea surface temperature (SST) anomalies and related physical processes in the midlatitudes of the North Pacific, numerical solutions of a three-dimensional bulk mixed layer model are analyzed, focusing on the contribution of the net shortwave radiative forcing at the sea surface. A quantitative heat budget analysis for the ocean mixed layer relating to late-1980s decadal SST change reveals that the decadal SST anomalies decay from late spring to early summer over the entire midlatitudes of the North Pacific. This quasi-seasonal decay of the decadal SST anomalies is controlled by an anomalous local thermal damping (i.e., anomalous surface heat fluxes). From midsummer to early autumn the anomalous net shortwave radiation flux associated with a meridional shift of the storm track acts to induce strong seasonal damping of the decadal SST anomaly in the northern Kuroshio–Oyashio Extension region. In contrast, in the north of the subtropical frontal region, the net shortwave radiation flux anomaly, which results from changes in low-level stratiform cloud cover, plays a major role in seasonally enhancing the decadal SST anomaly. Consequently, the SST anomalies formed by these radiative forcings cause significant variations in the local thermal damping rate at the sea surface over the period from late summer to early autumn.