Near-Inertial Wave Interactions and Turbulence Production in a Kuroshio Anticyclonic Eddy

Abstract

Abstract Interactions between near-inertial waves and the balanced eddy field modulate the intensity and location of turbulent dissipation and mixing. Two EM-APEX profiling floats measured near-inertial waves generated by Typhoons Mindulle, 22 August 2016, and Lionrock, 30 August 2016, near the radius of maximum velocity of a mesoscale anticyclonic eddy in the Kuroshio–Oyashio confluence east of Japan. High-vertical-wavenumber near-inertial waves exhibit energy fluxes inward toward eddy center, consistent with wave refraction/reflection at the eddy perimeter. Near-inertial kinetic energy tendencies are nearly two orders of magnitude greater than observed turbulent dissipation rates ε, indicating propagation/advection of wave packets in and out of the measurement windows. Between 50 and 150 m, ε ∼ (10−10) W kg−1, more than an order of magnitude weaker than outside the eddy, pointing to near-inertial wave breaking at different depths or eddy radii. Between 150 and 300 m, small-scale inertial-period patches of intense turbulence with near-critical Ri occur where comparable near-inertial and eddy shears are superposed. Three-dimensional ray-tracing simulations show that wave dynamics at the eddy perimeter are controlled by radial gradients in vorticity and Doppler shifting with much weaker contributions from vertical gradients, stratification, and sloping isopycnals. Surface-forced waves are initially refracted downward and inward, consistent with the observed energy flux. A turning-point shadow zone is found in the upper pycnocline, consistent with weak observed dissipation rates. In summary, the geometry of wave–mean flow interaction creates a shadow zone of weaker near-inertial waves and turbulence in the upper part while turning-point reflections amplify wave shear leading to enhanced dissipation rates in the lower part of the eddy.