Enhanced turbulent mixing in mesoscale eddies near the critical latitude of the M2 internal tides

Mesoscale eddies can elevate turbulent mixing, but the mechanisms have not been fully understood. Based on a field experiment in two mesoscale eddies near the critical latitude of the M2 internal tides in the northwestern Pacific Ocean, enhanced turbulent mixing was found both in and below the mixed layer. Microstructure observations indicate that enhanced turbulent mixing mainly appeared outside the eddy center in the mixed layer (approximately 117 m), which was speculated to be related to geostrophic current shear. By analyzing the CTD and Argo measurements through a fine-scale parameterization method, enhanced turbulent mixing was found to be concentrated at the southern flank of the anticyclonic eddy below 300 m, which showed little correlation with topographic roughness and local winds. Based on a ray-tracing model, the wind-generated near-inertial waves at higher latitudes were also excluded as the cause of the enhanced turbulent mixing. The enhanced turbulent mixing below 300 m was generally consistent with the features of subthermocline eddies and eddy-modulated parametric subharmonic instability (PSI) of the M2 internal tides. However, due to the limitation of observations, the eddy-modulated PSI mechanism was not fully validated.