Wind-driven and intrinsic interannual-to-decadal variability in eddy activity in the Kuroshio Extension

Eddies in the Kuroshio Extension have been shown to affect the atmosphere aloft through large heat and moisture released from the ocean to the atmosphere. It is therefore important to explore variability in the current and associated eddy activity in the region to improve our understanding of air-sea interactions over the North Pacific region. While interannual-to-decadal variability in the eddy activity in the Kuroshio Extension could be partly driven by atmospheric variability over the central/eastern North Pacific, it has also intrinsic variability independent from the atmospheric forcing. To separate the wind-driven and intrinsic components of the variability under realistic conditions, we analyze a ten-member ensemble, of fifty-year integration of an eddy-resolving OGCM driven by time-varying JRA55 reanalysis data. We consider the ensemble mean as the wind-driven, and the spread of ensemble members from their average as the intrinsic components. Focusing on interannual-to-decadal variability by the 13-month running mean, eddy activity in the downstream Kuroshio Extension region (32-38N, 153-165E) shows rather limited ensemble spread and ensemble mean has significant correlation with the observation (correlation coefficient r=0.58). Also, the eddy activity variability highly correlates with the local current speed variability that propagates westward from the central North Pacific, inducing predictability of the eddy activity to some extent. In contrast, in the upstream Kuroshio Extension region (32-38N, 141-153E), the spread is large and the ensemble mean does not correlate to the observation at all. While possibly model dependent, thus result implies that the observed eddy activity in the upstream Kuroshio Extension may have a significant intrinsic component on the interannual time scale.