Interannual-to-Decadal Variability in the Oyashio and Its Influence on Temperature in the Subarctic Frontal Zone: An Eddy-Resolving OGCM Simulation

Abstract

Output of an eddy-resolving OGCM simulation is used to investigate mechanisms for interannual-to-decadal variability in the Oyashio and its influence on the subarctic frontal zone in the western North Pacific. Lag correlation analysis reveals that positive anomalies both in basin-scale wind stress curl and in local Ekman pumping can intensify the southward Oyashio almost simultaneously via barotropic and baroclinic Rossby wave propagations, respectively. The Oyashio strength can also be influenced by anomalous Ekman pumping that is exerted in the western portion of the basin through the baroclinic wave propagation with the lag of 3 yr, which appears to arise from a periodicity in the wind field. The intensification of the Oyashio is accompanied by negative anomalies both in the sea surface temperature and height off of Hokkaido Island of Japan and is followed by their eastward development along the southern branch of the Oyashio Extension and associated subarctic frontal zone in association with a southward displacement of their axes. These changes are associated with cool sea surface temperature anomalies and low potential vorticity anomalies at the thermocline level in the frontal zone. The surface cooling, thus induced in the frontal zone by those oceanic processes, accompanies anomalous downward surface heat fluxes, indicative of ocean-to-atmosphere feedback forcing associated with the Oyashio variations.