Seasonal variability of eddy characteristics and energetics in the Kuroshio Extension

Based on the eddy-resolving Four-dimensional variational Ocean Re-Analysis for the Western North Pacific over 30 years (FORA-WNP30) product, this study investigates the seasonal evolution of mesoscale eddy characteristics and underlying mechanisms in the Kuroshio Extension (KE) region. In the upstream region, eddies are stretched in the zonal direction and characterized by higher activity in summer and autumn. Energy analysis illustrates that baroclinic instability associated with the horizontal buoyancy flux is responsible for the seasonal variability. In the downstream region, in comparison, eddies tend to be stretched in the meridional direction. Eddy kinetic energy (EKE) level in this region is mainly regulated by the upstream through pressure work. During 1993–2013, the EKE level does not change in the upstream region but it depicts an increasing trend in the downstream region. This evolution reflects the decadal variability of the KE system associated with the Pacific Decadal Oscillation.