Deep propagation of wind-generated near-inertial waves in the Northern South China Sea

Abstract

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 near-inertial energy flux (Fz) was (2.3 ± 0.9) × 10−3 W/m2 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.