Meridional shift of the Kuroshio Extension induced by response of recirculation gyre to decadal wind variations

Abstract

Abstract The meridional shift of the Kuroshio Extension to decadal wind variations is examined by analyzing the hindcast simulation of a high-resolution ( 1 / 12 ∘ × 1 / 12 ∘ ) North Pacific ocean general circulation model (OGCM) and by conducting a series of idealized OGCM experiments. The meridional shift of the Kuroshio Extension in the hindcast is consistent with observations and former studies. Using a series of sensitivity experiments in idealized settings, the following possible mechanism for this shift is proposed. The baroclinic Rossby waves that arise in the northern half of the subtropical gyre cannot directly enter the Kuroshio–Oyashio Extension region due to the strong Kuroshio Extension jet. When the Rossby waves accompanying the negative (positive) sea-surface height anomalies propagate westward and reach the region to the east of the recirculation gyre, the difference in thermocline depth between the recirculation gyre and the region to its east becomes larger (smaller), resulting in stronger (weaker) baroclinic instabilities. These stronger (weaker) instabilities cause a shallower (deeper) thermocline in the recirculation gyre. To compensate for this shrinking (stretching), the recirculation gyre migrates southward (northward), leading to the southward (northward) shift of the Kuroshio Extension.