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The modulation of the seasonal variability of the surface eddy kinetic energy (EKE) in the Kuroshio extension (KE) region induced by the stable and unstable KE states is analyzed using satellite-based observations and high-resolution ocean reanalysis data. The two KE states are found to strongly modulate the seasonal variability of the local surface EKE: the latter reaches its maximum in May-July if the KE is in its stable state, while it peaks in August-October if the KE is in its unstable state. Further investigation indicates that such behavior is mainly caused by the different baroclinic energy transfers induced by the meridional density gradients associated with the two KE states.

Plain Language Summary Ocean mesoscale eddies have an important effect on physical and biological processes; it is, therefore, important to understand the main features of the mesoscale eddy activity. The Kuroshio extension (KE) region is the most active region of mesoscale eddies in the North Pacific Ocean. On the decadal time scale, the KE yields two main states: a stable and an unstable state. Whether these two states have an effect on the mesoscale eddy activity in the KE region is worth investigating. Our study shows that the KE bimodality affects considerably the seasonality of the mesoscale eddy activity, as represented by the surface eddy kinetic energy (EKE) field: when the KE takes the stable state, the EKE level attains its highest value in summer, while this occurs in autumn if the KE is in its unstable state. The different baroclinic energy transfers induced by the meridional density gradients associated with the two KE states are responsible for the differences in the EKE seasonal cycle.