Estimates of surface and subsurface forcing for decadal sea surface temperature variability in the mid-latitude North Pacific

The upper ocean, atmosphere and their interaction over the North Pacific exhibit pronounced decadal to interdecadal variations. A diagnostic equation for analyzing the heat budget for decadal variability in winter sea surface temperature (SST) is derived that can properly account for subsurface geostrophic advection, and strong seasonal cycle in the depth and temperature of the ocean mixed layer. A model-assimilated ocean dataset, partially validated for the period of the TOPEX/Poseidon mission, is used to evaluate the relative importance of subsurface advection and surface forcing due to wind-induced turbulent heat flux and Ekman advection. For our analysis, two key regions are chosen where decadal SST variance reaches local maxima, centered at 170degreesE, 42degreesN (Region A) and 155degreesW, 35degreesN (Region B), respectively. Region B is under the direct influence of the Aleutian Low, where the surface effects are dominant. Region A is part of the Kuroshio-Oyashio Extension, where the winter mixed layer is deep and the subsurface geostrophic advection contributes significantly to low-frequency winter SST variations. Our analysis suggests that anomalous geostrophic advection changes signs north and south 38degreesN, presumably as a result of ocean gyre circulation adjustment to wind changes to the east. The surface forcing shows a larger-scale structure covering the entire mid-latitude North Pacific, in response to basin-wide changes in atmospheric circulation.