Kuroshio-induced island wake instabilities and turbulent mixing from EM-APEX floats

Abstract

Clusters of 6 to 9 EM-APEX profiling floats measured CTD, horizontal velocity and microstructure thermal variance during four deployments at the same locations in the Green Island wake induced by the Kuroshio. Large turbulent kinetic energy dissipation rates ε ∼ O (10 −7 – 10 −6 ) W/kg were observed in the near-field wake. Enhanced turbulent mixing is robustly explained by vertical shear instability with subcritical Richardson number Ri = N 2 / S 2 < 0.25 (where N is buoyancy frequency and S vertical shear). Horizontal divergence δ , relative vorticity ζ z , lateral strain α and potential vorticity PV were estimated from float clusters. Strong negative ζ z = O (−10 f ) potentially suggests inertial instability and strong negative PV , symmetric instability, but the measurements are in a nonlinear transitional imbalance so conventional submesoscale instability criteria, assuming geostrophic balance, are not sufficient. Observed ε increases with decreasing Ri and increasing magnitude of negative ζ z , suggesting inertial instability leading to secondary vertical shear instability. Dissipation rates, strain, and negative relative and potential vorticity magnitudes decrease from near- to far-field. In the upper 200 m, temperature-salinity deviations from typical Kuroshio properties coincide with large ε and thermal eddy diffusivity, implying significant watermass modifications. Turbulent mixing and wake eddy formation are modulated by the semidiurnal tides. The origin of the observed strong negative vorticity, effects of internal tidal modulation on the formation of anticyclones/cyclones and their downstream evolution, and the generation and evolution of near-inertial waves in the wake warrant further investigation.