Quantifying High-Frequency Wind Energy Flux into Near-Inertial Motions in the Southeast Pacific

Abstract

ind-forced internal waves close to the inertial frequency are ubiquitous throughout the world?s oceans, but observational constraints on their global energetics and impact on subsurface mixing remain scarce. This study reports on velocity measurements from three Electromagnetic Autonomous Profiling Explorers (EM-APEX) deployed in February 2009. These floats observed downward-propagating near-inertial internal waves near the Subantarctic and Polar Fronts of the Antarctic Circumpolar Current. These waves were episodic and enhanced at middepth between 500 and 1000 m. Depth-integrated kinetic energy varied between 1 and 7 kJ m?2 and averaged 1.6 kJ m?2 with typical group velocities of 40 m day?1, implying an average energy flux of 3 mW m?2 at the mixed layer base decreasing to approximately 25% of that value at 1500 m. Modeled currents forced by reanalysis winds along each float track agree with observed surface currents from EM-APEX, provided that mixed layer depth is restricted to the layer of weakest observable stratification (interpreted as the maximum depth that can remain mixed over an inertial period given the continual balance between mixing and restratification). This model estimates an average wind power of 3 mW m?2. Shipboard wind and current observations during a strong storm show an integrated wind work of 3.5 kJ m?2, comparable to the vertically integrated kinetic energy over the following month. Model wind work estimates are considerably less, likely because of the mixed layer depth used. A model with varying stratification in response to the wind provides a better match to the observations, emphasizing the importance of stratification within the mixed layer in amplifying wind energy input.