Downward Propagation and Trapping of Near-Inertial Waves by a Westward-moving Anticyclonic Eddy in the Subtropical Northwestern Pacific Ocean

Abstract

Mesoscale eddies can alter the propagation of wind-generated near-inertial waves (NIWs). Different from previous studies, the subsurface mooring observed NIWs are generated outside an anticyclonic eddy (ACE) and then interact with the arriving ACE. It is found that with the arrival of the ACE, the NIWs accelerate to propagate downward and the maximum vertical wavelength and group velocity of NIWs reach ~500 m and ~35 m/day, respectively. When entering the core of the ACE, the near-inertial energy is trapped, and finally stalls at a critical depth, which basically corresponds to the base of the ACE located at around 750 m depth. Through a ray-tracing model and dynamic analyses, this critical depth is much deeper than that of NIWs generated directly inside an ACE. By using depth-time varying stratification and relative vorticity, ray-tracing experiments further demonstrate that NIWs generated outside and passed over by an ACE can propagate to deep depths. Furthermore, energy budget analyses indicate that the net energy transfer from the ACE to NIWs plays an important role in the enhancement of downward-propagating near-inertial energy and its long-term persistence (~45 days) in the critical layer. Within the critical layer, the enhancement of shear instability and nonlinear interactions among internal waves account for the loss of the trapped near-in ertial energy and provide energy for furnishing deep ocean mixing.