Energetic Wind-Generated Near-Inertial Waves Observed in a Cyclonic Eddy

Abstract The propagation of wind-generated near-inertial waves (NIWs) is well documented in anticyclonic eddies (ACEs) but remains rarely reported in cyclonic eddies (CEs). Here, subsurface mooring observations in the northwestern Pacific captured two distinct NIW packets (WP1 and WP2) generated by Typhoon Sanvu within a CE. In the typhoon’s wake, WP1 was confined to the upper 80 m, with little energy penetrating deeper despite strong wind forcing. A linear internal wave model and ray-tracing experiments suggest that the shallow mixed layer and the repelling effect of the CE favor lateral propagation of near-inertial energy rather than downward penetration. About 2 weeks later, WP2 emerged at 450–900-m depth with a velocity amplitude of 0.3 m s −1 and persisted for nearly 1 month. Wind work and Wentzel–Kramers–Brillouin (WKB)-scaled near-inertial kinetic energy indicate that WP2 was decoupled from local surface energy and unrelated to WP1. Spectral and modal analyses show that WP2 had the characteristics of low-mode and long-range-propagating NIWs. Ray-tracing and dynamical analyses suggest a plausible remote-source interpretation for WP2, in which it may have originated from a remote cyclonic region 250–350 km away and reached the mooring along a pathway modulated by mesoscale eddies. However, because the present analysis does not resolve the wave packet’s horizontal extent, spatial inhomogeneity of NIW energy may also affect the mooring observations and thus the interpretation of the decay of WP1, the later arrival of WP2, and the apparent lack of downward-moving energy between them. These results highlight the complexity of NIW propagation in eddy-rich environments. Significance Statement Wind-generated near-inertial waves (NIWs) are ubiquitous in the ocean. Using a subsurface mooring, we document two dynamically distinct NIW packets in a cyclonic eddy (CE). One is confined to the surface layer of the CE in the wake of the typhoon, with minimal downward energy penetration. Two weeks later, another energetic NIW packet appears in the deep layer of the CE, and its timing is consistent with arrival from a remote source region via long-range propagation. These observations provide a useful reference for future model refinement and also suggest that spatial inhomogeneity of NIW energy may strongly influence how subsurface wave packets are recorded at a single-mooring site.