Role of mixed layer depth in decadal variability of the Kuroshio Extension region in the western North Pacific

Using a high-resolution ocean reanalysis product, decadal variations in the Kuroshio Extension region is investigated, paying special attention to mixed layer depth anomalies. Based on a mixed layer heat budget analysis, it is shown that the strength of the sea surface temperature (SST) front associated with the Kuroshio Extension undergoes large decadal variations owing to the surface heat flux term. Although the contribution of surface heat fluxes to surface frontogenesis/frontolysis depends on the horizontal gradient of both surface heat flux and mixed layer depth, the latter is found to be more important. More specifically, when the mixed layer becomes anomalously thick to the south and thin to the north of the SST front associated with incoming Rossby waves, the mixed layer becomes less sensitive to the wintertime surface cooling to the south, and more sensitive to the north. As a result, the SST front is anomalously strengthened, with positive SST anomalies to the south and negative SST anomalies to the north. This, in turn, forces wind anomalies to the east, which induces Rossby waves that reverse the sign of mixed layer depth anomalies across the SST front, leading to a cycle of variability on decadal timescales.