Influence of the Decadal Variability of the Kuroshio Extension on the Atmospheric Circulation in the Cold Season

The Kuroshio Extension (KE) exhibits large decadal fluctuations between relatively stable and unstable states. When in the stable state, the KE jet is strengthened, its path is shifted northward and the regional eddy kinetic energy is lower. The reverse holds in the unstable state. This variability affects the Sea Surface Temperature (SST) and may therefore have an impact on the atmospheric circulation. In this study, we investigate the cold season atmospheric response to the decadal KE fluctuations during 1979-2012 using the KE index derived by Qiu et al. (2004) from altimetry and an eddy-resolving ocean general circulation model hindcast. Regression analysis shows that there is a coherent atmospheric response between October and January. When the KE is in the stable state, the Kuroshio-Oyashio Extension region is anomalously warm, and this generates an upward surface heat flux that leads to local ascending motions and a northeastward shift of the zones of maximum baroclinicity, eddy heat and moisture fluxes, and the storm track. The heating and transient eddy anomalies excite stationary Rossby waves that propagate the signal poleward and eastward. The atmospheric response consists of an equivalent barotropic large-scale signal, with a downstream high and a low over the Arctic. One month later, in November-December-January-February, a high appears over northwestern Europe, and the hemispheric teleconnection bears some similarity with the Arctic Oscillation. Composite analysis shows that the atmospheric response primarily occurs during the stable state of the KE, while no evidence of a significant large-scale atmospheric response is found in the unstable state. The stationarity of the atmospheric response is investigated by repeating the analysis in 1958-1977.