Slope-Intensified Storm-Induced Near-Inertial Oscillations in the South China Sea

摘要

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.