Energy Transfer between Mesoscale Eddies and Near-inertial Waves from Surface Drifter Observations

Hourly satellite-tracked surface drifter data are utilized to study energy transfer from eddies to Near-Inertial Waves (NIWs). Spatial velocity gradients are computed from two consecutive velocity estimates derived from the same drifter, providing variable spatial resolutions of (1 km). The eddy-to-NIW energy transfer can be positive or negative, with the positive transfer (forward energy cascade) dominant. The global integrated energy transfer rate (ε) is 0.025 TW, with the anticyclonic eddy contribution dominant over the cyclonic eddy contribution. Given that the global near-inertial wind work (W) is 0.2 TW, the eddy-to-NIW energy transfer efficiency (ε/W) is about 13%, which is one order of magnitude larger than that in low resolution simulations. This result may still underestimate the Eulerian energy transfer by a factor of 2. To our knowledge, this is the first time that this energy transfer is calculated from global drifter observations, providing a baseline for comparison in future studies.