Spontaneous near-inertial wave generation from mesoscale eddy: Energy transformation

The energy transformation between inertial oscillations (IOs), near-inertial waves (NIWs), and mesoscale eddies during spontaneous NIW generation is analyzed by the kinetic energy equations of the multiple temporal–spatial scale interaction between slow and fast motions and then quantitatively estimated by numerical simulations. The evolution of perturbed quasi-geostrophic mesoscale eddies is accompanied by IOs. The nonlinear interaction of IOs and mesoscale eddies results in spontaneous energy transfer from mesoscale eddies to NIWs. However, IOs act as catalysts in the spontaneous NIW energy transformation process, because there is no energy transfer between NIWs and IOs for longer than one inertial period and NIW energy is entirely transferred from mesoscale eddies. The energy conversion rate (ECR) from a propagating eddy to NIWs is significantly enhanced by the resonance of NIWs and the nonlinear coupling of IOs and mesoscale eddies during the spontaneous NIW generation. The time-averaged NIW ECR from a propagating an anti-cyclonic mesoscale eddy (AE) is approximately 5.725 × 10−6 Wm−2 and nearly 16 times higher than those from the standing AE. The magnitude of global near-inertial kinetic energy generated spontaneously from mesoscale eddies is approximately on the order of 2 GW. Therefore, the spontaneous adjustment of mesoscale eddies is a non-negligible NIW generation mechanism and plays a crucial role in the process of supplying energy from mesoscale eddies to the diapycnal mixing in the ocean interior.