Energy Transfer Between Mesoscale Eddies and Near-Inertial Waves From Surface Drifter Observations

Ocean circulations are forced at large scales and dissipated at small scales. Forward energy cascades from large to small scales are inevitable in order to maintain the ocean's quasi-equilibrium state. Mesoscale eddies have been known as the dominant reservoir of kinetic energy of the world oceans. However, in the framework of geostrophy, they are characterized by an inverse energy cascade (i.e., from small to large scales). Near-inertial waves (NIWs) are the dominant mode of fast motions in the ocean. Their interaction with mesoscale eddies may contribute significantly to the forward energy cascade process. Previous studies showed that the eddy-to-NIW energy transfer is only 1% of the near-inertial wind work in low resolution simulations. By utilizing hourly satellite-tracked surface drifter data, spatial velocity gradients and thus the eddy-to-NIW energy transfer are calculated, with variable spatial resolutions of O(1 km). Our results show that global integration of the energy transfer from eddies to NIWs is 0.025 TW, about 13% of the near-inertial wind work (0.2 TW). This may still underestimate the actual energy transfer by a factor of 2. This is the first time that this energy transfer is calculated from global drifter observations, providing a useful reference for future studies.