Kuroshio path variations south of Japan: Bimodality as a self-sustained internal oscillation

In the two decades following 1975, the Kuroshio path south of Japan was obs erved to oscillate interannually between a straight path state and a meande ring stale. This bimodal condition is in sharp contrast to the decade prior to 1975 when the Kuroshio remained consistently in the straight path state . In situ observations in the upstream East China Sea indicate no obvious c orrespondence between the magnitude (or the temporal change) of the inflow transport and the Kuroshio path, suggesting the Kuroshio's path alternation s in the recent two decades may not be externally determined by the upstrea m inflow. Using a two-layer primitive-equation model of the North Pacific d riven by observed climatological surface wind data, the authors propose tha t the Kuroshio path oscillation since 1975 can be explained by a self-susta ined internal mechanism of the Kuroshio current system. The proposed self-s ustained oscillation is maintained by the accumulation of the low potential vorticity (PV) water carried northward by the upstream Kuroshio. This accu mulation of the low-PV anomalies strengthens the Kuroshio's southern recirc ulation gyre and presses the Kuroshio to how along the coast. The strengthe ning of the southern recirculation gyre increases the velocity shear of the straight-pathed Kuroshio and leads eventually to the meander path developm ent due to baroclinic/barotropic instability of the system. As the meander of the Kuroshio path grows, detachment of cyclonic eddies mixes coastal-ori gin, high-PV water offshoreward, weakening, as a result, the recirculation gyre. The above cycle will restart after the low-PV water is replenished fr om the south and the recirculation gyre spins up again. For the past two de cades, this internal oscillation has been operative because the upstream in flow is relatively large due to the decadal increase in the wind-driven Sve rdrup transport. In the decade before 1975 when the upstream inflow was sma ll, the internal oscillation would cease because the dissipative forces are able to remove the southern-origin low-PV anomalies without resorting to i nstability and the meander development.