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

Anda Vladoiu1, Ren-Chieh Lien1, Barry Ma1, Je-Yuan Hsu2, Ming-Huei Chang2, Leif Thomas3, Sen Jan2, Yu-Hsin Cheng4, Yiing Jang Yang2, Wei-Chuan Chiang5
1Applied Physics Laboratory, University of Washington, Seattle, WA, USA
2National Taiwan University, Taipei, Taiwan
3Department of Earth System Science, Stanford University, Stanford, CA, USA
4National Taiwan Ocean University, Keelung, Taiwan
5Eastern Fishery Research Center, Fisheries Research Institute, Taitung, Taiwan

As the Kuroshio encounters Green Island ($\sim 7$ km diameter) off the SE coast of Taiwan, a forward energy cascade is generated, with enhanced turbulent mixing modifying local and far-field water mass properties and nutrient supplies in a region with economically significant fisheries.

Clusters of 6 to 9 EM-APEX profiling floats measured CTD, horizontal velocity and microstructure temperature variance during four deployments, each of $\sim 1$ day period, performed at the same locations in the lee of Green Island where anticyclonic wake eddies occur. Large turbulent kinetic energy dissipation rates $\varepsilon \sim 10^{−7}−10^{−6}$ W/kg were observed in the near-field wake and extended up to $\sim 30$ km downstream of the island, suggesting a sustained forward energy cascade. Enhanced turbulent mixing occurs at Richardson number $Ri<0.25$, suggesting strong mixing is driven by vertical shear instabilities. Dissipation rates and $Ri$ within an isopycnal layer extending from the upstream bottom boundary layer are modulated by the semidiurnal tide.

Horizontal divergence $\Gamma$, relative vorticity $\zeta_z$, lateral strain $\alpha$ and potential vorticity $PV$ were estimated from float clusters. Large $\varepsilon$ coincides with large $\alpha$, suggesting horizontal shear instability is also important. $PV$ is dominated by vertical relative vorticity. Strong negative $\zeta_z=O(-10f)$, where $f$ is Coriolis frequency, suggests inertial instability and strong negative $PV$ symmetric instability, though the measurements are in a nonlinear transitional imbalance so conventional submesoscale instability criteria, assuming geostrophic (thermal or gradient wind) balance, are not suitable. Observed $\varepsilon$ increases with decreasing $Ri$ and increasing magnitude of negative $\zeta_z$. We hypothesize that inertial and symmetric instabilities lead to the observed vertical shear instability.