Local Energetics Mechanism for the Short‐Term Shift Between Kuroshio Extension Bimodality

Abstract

This study investigates the short‐term transition process between the Kuroshio Extension (KE) stable and unstable states (bimodality) based on eddy energetics analysis. The local dynamic mechanisms modulating the KE transitions are revealed by exploring the eddy kinetic energy (EKE) variability. During the stable‐to‐unstable transitions (low‐to‐high EKE level; SU), baroclinic instability is the major source of EKE growth. As the negative sea surface height anomaly (SSHa) signal propagates westward near the KE domain, the KE jet downstream (150°E–155°E) slows down at first but its upstream (140°E–145°E) remains fast. The great upstream vertical shear is conducive to the strong regional baroclinic instability. The difference in velocity between the KE upstream and downstream leads to the positive velocity anomaly and the strengthening of vertical velocity shears in the midstream (145°E–150°E). Hence, the baroclinic instability in the KE upstream and midstream facilitates the growth of EKE, thereby inducing the KE path to transform from a stable into an unstable state. During the unstable‐to‐stable transitions (high‐to‐low EKE level; US), the combination of advection and eddy wind work plays a key role in reducing EKE. With the positive SSHa signal arriving at the south of the KE axis, the KE jet speeds up. In this situation, more EKE is advected, especially from the KE midstream, into the downstream where wind dissipation takes effect. When most eddies are consumed, the KE path becomes more stable.