Slope-Intensified Storm-Induced Near-Inertial Oscillations In The South China Sea

The South China Sea (SCS) is a region vulnerable to tropical cyclones (TCs). An array of moored current meters and meteorological buoys was deployed within major pathways of TCs across the slope of the northern shelf of the SCS from September 1, 2017 through September 30, 2018. Three TCs, Khanun, SonTinh, and Mangkhut, traversed by the storm-monitoring array in October 2017, July, and September 2018, respectively. Storm-generated near-inertial oscillations (NIOs) were observed at the moorings. These oscillations exhibited a slope-intensified feature regardless of the trajectories and intensities of TCs. In the stratified area, the NIOs was characterized by a two-layer structure, with large amplitudes in the mixed layer and beneath thermoclines. The phases of these oscillations changed rapidly through thermoclines. No significant nonlinear interactions were found between near-inertial and tidal currents. The low-frequency flow over the slope was characterized by a southwestward current in the along-shelf direction and an onshore current above the bottom boundary, and a weak offshore current near the bottom in the cross-shelf direction. The flow-induced negative vorticity was intensified during the TCs and appearances of mesoscale eddies. Slope-intensified NIOs can be explained by topographic inertial waves and energy trapping due to the negative vorticity of the slope flow. The strongest NIOs always occur during an adjustment period of the low-frequency flow to a pre-storm condition, suggesting a significant energy transfer from the low-frequency flow to the inertial motion.Plain Language Summary Near-inertial oscillations (NIOs) are a common feature caused by restoring force due to Earth's rotation. In the South China Sea (SCS), NIOs are often observed over the continental shelf during tropical cyclone (TC) events as oceanic responses to the sudden change of the storm's wind direction. Due to a lack of simultaneous observations, no efforts have been made to examine the cross-shelf variation of storm-induced NIOs over the slope of the SCS. An array with eight moored current meters and two meteorological buoys was deployed over the northern SCS shelf from September 1, 2017 through September 30, 2018. Three TCs, Khanun, SonTinh, and Mangkhut, traversed by the storm-monitoring array in October 2017, July, and September 2018, respectively. Regardless of the trajectories and intensities of TCs, the storm-generated NIOs exhibited a slope-intensified feature. The SCS slope is characterized by the persistent density front and along-shelf subtidal flow. The flow-induced negative vorticity was intensified during the storms. The slope-intensified NIOs are likely associated with topographically trapped inertial waves and energy-trapping by the flow-induced negative vorticity. The strongest NIOs occur during the restoring period of subtidal flow to the pre-storm condition, suggesting an energy transfer from the low-frequency flow to the inertial motion.