Temporal and Spatial Variability of Near-Inertial Waves in the East/Japan Sea From a High-Resolution Wind-Forced Ocean Model

Abstract

Near-inertial waves (NIWs), a fundamental oceanic response to wind forcing, play an important role in dynamical processes related to ocean mixing and hence have attracted sustained interest. Herein, we investigate temporally and spatially varying NIW distributions in the mixed and deep layers of the East/Japan Sea (EJS) using high-resolution hourly-wind-forced data-assimilated ocean model outputs. Temporally, the kinetic energy of NIWs in the mixed and deep layers is higher in fall and winter than in spring and summer, showing maxima in December, corresponding to wind forcings of both wind stress and wind-current resonance. Spatially, the NIW energy in the mixed layer is higher on the northern side of the subpolar front (SPF), although there are no significant spatial differences in the wind forcings. Because of intensive background currents and their vorticities in the upper layer on the southern side of the SPF, vertical transfer of NIW energy is facilitated, shown by a shorter e-folding decay time scale of NIWs in the mixed layer on the southern side of the SPF. The NIW kinetic energy in the deep layer of 400-1,000 m is higher on the southern side than on the northern side, an opposite spatial pattern to that in the mixed layer, but consistent with a previous observational study. Our results confirm that energetic anticyclonic circulations with negative relative vorticity in the upper layer on the southern side enable vertical penetration of NIW energy from the mixed layer to the deep layer more effectively. Plain Language Summary Near-inertial waves (NIWs) are an important source for vertical ocean mixing, which is associated with the maintenance of stratification and overturning circulation of the East/Japan Sea (EJS); however, the spatiotemporal variability of NIWs is unknown. We investigate the distribution of NIWs in the EJS focusing on the surface mixed layer and deep layer using a data-assimilative numerical ocean model. Temporally, NIW energy in the mixed and deep layers shows seasonality, higher in fall and winter than in spring and summer, maxima in December and minima in June, due to wind forcings. Spatially, mixed-layer NIWs are stronger (weaker) on the northern (southern) side of the subpolar front, whereas deep-layer NIWs are stronger (weaker) on the southern (northern) side. Predominant clockwise mesoscale circulations on the southern side play a key role in energy distribution by facilitating vertical propagation of NIWs. Variation of the NIWs due to wind and meridionally different mesoscale circulation implies spatiotemporal differences of NIW-induced ocean mixing.