Eddy-mean flow interaction in the decadally modulating Kuroshio Extension system

Satellite altimeter sea-surface height (SSH) data of the past 16 years are used to investigate the decadal changes of the Kuroshio Extension (KE) system that oscillated between a stable and an unstable dynamic state. During the stable state of 10/1992–06/1995 and 01/2002–12/2004, the KE jet was intense and had a northerly zonal mean path and a well-defined southern recirculation gyre. During the unstable state of 07/1995–12/2001 and 01/2005–present, the KE jet had a reduced eastward transport and a more southerly flow path. Transitions between the two dynamic states are caused by the basin-scale wind-stress curl forcing in the eastern North Pacific related to the Pacific decadal oscillations (PDOs) or the North Pacific Gyre Oscillations (NPGOs). During the positive PDO (or negative NPGO) phase, the intensified Aleutian Low generates negative SSH anomalies in the eastern North Pacific through Ekman divergence. As these wind-induced negative SSH anomalies propagate to the west as baroclinic Rossby waves, they weaken the zonal KE jet and shift its path southward. As its path is pushed southward (∼32∘N)(∼32∘N), the deep-reaching KE jet has to ride over the shallow Shatsky Rise, generating localized disturbances that lead to their subsequent development along the KE jet west of the Shatsky Rise. The sequence opposite to that listed above occurs when the PDO (NPGO) changes to its negative (positive) phase. After the KE system transitions from a stable to unstable state, the enhanced eddy interaction is found to strengthen both the southern recirculation gyre and quasi-stationary meanders along the KE's upstream path. This nonlinear eddy interaction is important in determining the amplitude of the observed decadal KE variability.