Kuroshio Path Variations South of Japan: Bimodality as a Self-Sustained Internal Oscillation

Abstract

In the two decades following 1975, the Kuroshio path south of Japan was observed to oscillate interannually between a straight path state and a meandering state. 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 correspondence between the magnitude (or the temporal change) of the inflow transport and the Kuroshio path, suggesting the Kuroshio’s path alternations in the recent two decades may not be externally determined by the upstream inflow. Using a two-layer primitive-equation model of the North Pacific driven by observed climatological surface wind data, the authors propose that the Kuroshio path oscillation since 1975 can be explained by a self-sustained internal mechanism of the Kuroshio current system. The proposed self-sustained oscillation is maintained by the accumulation of the low potential vorticity (PV) water carried northward by the upstream Kuroshio. This accumulation of the low-PV anomalies strengthens the Kuroshio’s southern recirculation gyre and presses the Kuroshio to flow along the coast. The strengthening of the southern recirculation gyre increases the velocity shear of the straight-pathed Kuroshio and leads eventually to the meander path development due to baroclinic/barotropic instability of the system. As the meander of the Kuroshio path grows, detachment of cyclonic eddies mixes coastal-origin, high-PV water offshoreward, weakening, as a result, the recirculation gyre. The above cycle will restart after the low-PV water is replenished from the south and the recirculation gyre spins up again. For the past two decades, this internal oscillation has been operative because the upstream inflow is relatively large due to the decadal increase in the wind-driven Sverdrup transport. In the decade before 1975 when the upstream inflow was small, the internal oscillation would cease because the dissipative forces are able to remove the southern-origin low-PV anomalies without resorting to instability and the meander development.