Energetic near-inertial waves induced by winter storms and mesoscale eddies in the subtropical Northwestern Pacific Ocean

Abstract. Near-inertial waves (NIWs) play a fundamental role in transferring wind energy into the ocean interior and sustaining diapycnal mixing, yet their wintertime characteristics and interactions with mesoscale eddies remain insufficiently understood. Using subsurface mooring observations and reanalysis products in the subtropical Northwestern Pacific Ocean, we investigate the generation, downward propagation, and modal characteristics of NIWs associated with two winter storm events. Although the wind energy input into mixed layer during the first storm is approximately three times larger than that during the second storm, the observed near-inertial kinetic energy (NIKE) in the thermocline is comparable between the two events. Energy transfer analyses show that mesoscale eddies extract about 46 % of the wind-generated near-inertial energy during the first event, whereas they supply approximately 43 % of the wind input to NIWs during the second event, leading to similar observed NIKE intensity. Additionally, the two NIW events exhibit distinct vertical wavelengths, group velocities, and modal structures. The first event is characterized by a larger vertical wavelength, faster downward group velocity, and dominance of low baroclinic modes, with the first four modes accounting for nearly half of the total NIKE. In contrast, the second event displays shorter vertical wavelengths and enhanced high-mode energy, with modes five to eight contributing about 41 % of the total NIKE. These differences are attributed to the combined effects of mesoscale eddy modulation and the modal projection of wind energy. Our results highlight the critical roles of winter storms and eddy-wave interactions in shaping NIW propagation and characteristics in wintertime.