Deep Propagation of Wind-Generated Near-Inertial Waves in the Northern South China Sea

Using a subsurface mooring deployed at a depth of -2290 m in the northern South China Sea, the characteristics of deep -propagating near -inertial waves (NIWs) (>1500 m) generated by a combination of a winter storm and a typhoon were examined. The NIWs had a large vertical wavelength and showed a high coherence in near -full water depth. Correspondingly, the strong vertical shear and energy of the NIWs were concentrated at vertical scales larger than -500 m. The average vertical group velocity was -160 m/day. The horizontal wavelength decreased from -600 km in the upper layer to a few tens of kilometers below a depth of 1500 m. Interestingly, the near -inertial energy did not decrease obviously at depths of 100-1500 m, where the average downward nearinertial energy flux (Fz) was (2.3 +/- 0.9) x 10(-3) W/m(2) and nearly equaled that at 100 m depth. Combined with the slab model, the mean Fz at a depth of 100-1500 m can reach 19%-43% of that in the mixed layer. During the deep propagation of NIWs, the near -inertial shear was not strong in the upper layer. However, in the deep layer, the total shear was dominated by deep -propagating NIWs and was significantly enhanced. Correspondingly, the estimated diapycnal diffusivity increased, reaching a maximum value of -270% of the averaged value. This study contributes to our understanding of the deep propagation of wind -generated NIWs in the open ocean.