Energetics of Eddy-Mean Flow Interactions along the Western Boundary Currents in the North Pacific

The energetics of eddy-mean flow interactions along two western boundary currents of the North Pacific, the Kuroshio and Ryukyu Currents, are systematically investigated using 22 years of numerical data from the Ocean General Circulation Model for the Earth Simulator (OFES). For the time-mean and time-varying flow fields, all the energy components and conversions exhibit inhomogeneous spatial distributions. In the two currents, complex cross-stream and along-stream variations are seen in the eddy-mean flow energy conversions. East of Taiwan, the kinetic energy is mainly transferred from the mean flow to the eddy field through barotropic instability, whereas the baroclinic energy conversions form a meridional dipole structure caused by the topographic constraint. In the northern area, particularly, the eddy field drains 2.25 x 10(8) W of kinetic energy and releases 2.82 x 10(8) W of available potential energy when interacting with the mean flow, indicating that mesoscale eddies impinging on the Kuroshio decay with baroclinic inverse energy cascades. In the Ryukyu Current, inverse energy conversions from the eddy field to the mean flow also dominate the power transfer in the subsurface layer. The eddy field transfers 0.16 x 10(8) W of kinetic energy and 1.89 x 10(8) W of available potential energy to the mean flow, suggesting that meososcale eddies play an important role in maintaining the velocity and hydrographic structure of the current. In other areas, both barotropic and baroclinic instabilities contribute to the generation of eddy kinetic energy with the latter one providing more than 3 times as much power as the former one.