Observed Impact of Typhoon Mangkhut (2018) on a Continental Slope in the South China Sea

Mooring observations of nearly the full water column on the continental slope in the South China Sea revealed the ocean's response to Typhoon Mangkhut (2018). Mangkhut induced sea surface cooling similar to 4 degrees C that was biased to the right side of its track, which recovered with an e-folding time after approximately 1 week. Mangkhut was a relatively fast-moving typhoon and caused a fast near-inertial response throughout the entire depth in its lee. The typhoon-induced upper ocean (deep-water) near-inertial current velocities were >1.5 m/s (similar to 0.08 m/s), with an e-folding time of approximately a week (2 weeks) and frequency of 1.04f (1.08f, where f is the local inertial frequency). The near-inertial currents were near-circular polarized in the upper ocean and near-rectilinear polarized with the main axis in the across-slope direction in deep water. The deep-water near-inertial waves amplified the vertical excursions of isotherms from similar to 120 to similar to 200 m, reduced the stratification, elevated vertical current shears, and enhanced turbulent dissipation rate, especially during 14-17 September when the effects of near-inertial waves and diurnal spring tides overlapped. A net cooling similar to 0.15 degrees C and salinity increase similar to 0.05 psu were observed in the deep ocean after Mangkhut. Typhoon-induced near-inertial waves may further contribute to the intensification of southwestward along-slope mean near-bottom currents. This study indicates the immediate influence of typhoon in deep-water through near-inertial waves and their contribution to the bottom mixing on the continental slope.