28 / 2023-07-24 15:07:53

Typhoon-enhanced near-inertial waves in the northwestern South China Sea

Near-inertial waves, typhoon, near-inertial kinetic energy, the northwestern South China Sea

Typhoon-enhanced near-inertial waves (NIWs) were recorded by a mooring array (four moorings) in the northwestern South China Sea (SCS) during year 2022 when four consecutive tropical cyclones, namely Typhoon Mulan, Typhoon Ma-on, Typhoon Noru, and Typhoon Nesat, passed through the target area. The characteristics, modes, propagation, and shear properties of NIWs were investigated in this study. Results show that strong NIWs can penetrate through the mixed layer and reach the bottom layer. Clockwise rotating near-inertial currents suggested that NIWs energy propagated predominantly downward. Under impact of background vorticity, NIW’s frequencies varied in spatial in which NIWs showed bule-shift and red-shift in different regions. Vertical mode analysis revealed that the near-inertial kinetic energy (NIKE) was predominantly dominated by the first mode below 800m, by the second mode between 400m and 800m, and by modes three to ten from the sea surface to depths below 200m. These results suggested that the near-bottom NIKE originated from a distant source and propagated towards the equatorial direction. The vertical energy fluxes near the bottom-layer can reach about 5.189 W∙m^2, 4.0791 W∙m^2, 8.2270 W∙m^2, and 1.8921 W∙m^2 at the four locations, respectively, accounting for approximately 13.2%, 18.5%, 15.4%, and 10.1% of the wind-input NIKE at each mooring site. Furthermore, significant enhancements of near-inertial shear were observed during typhoon periods indicating its substantial impact on the deep mixing in the SCS.