A Mesoscale Ocean-Atmosphere Coupled Pathway for Quasi-decadal Variability of the Kuroshio Extension System

It has been known that the Kuroshio Extension (KE) system undergoes distinct interannual-to-decadal variability; however, the generation mechanism of its preferred time scale remains unclear. This study points out the active role of interactions among mesoscale sea surface temperature (SST), storm tracks and large-scale atmospheric circulation in the quasi-decadal fluctuations of the wintertime KE system between stable and unstable regimes. Results are obtained by analyzing the high-resolution satellite observations, FORA-WNP30, and ERA-Interim during 2002–2016. When the winter KE jet is in its stable state, the mesoscale eddy activity is higher-than-normal in the KE downstream region, which generates prevalent mesoscale SST warming through eddy-induced horizontal heat convergence of the mean flow temperature. Such warming further produces large upward turbulent heat flux and sufficient moisture supply primarily in the zonal belt of 30°–40°N, which in turn intensifies the storm-track activity along this belt (corresponding to the southern flank of storm-track climatology) through the enhanced latent heat energy. The resultant southward shift of storm tracks over the eastern basin favors anomalous cyclonic circulation with equivalent-barotropic structure via synoptic eddy feedback. Through upward Ekman pumping in the central basin induced by such cyclonic response, negative sea surface height anomalies are invoked and propagate westward into the KE region in ~4 years via baroclinic Rossby waves. This acts as a delayed negative feedback to instigate an unstable state of the KE jet, resulting in a quasi-decadal cycle.