39 / 2023-07-24 15:08:01

The Near-inertial Motions Induced by Typhoons on the Northern Shelf of the South China Sea: A Case Study of Typhoon

Near inertial motion; The northern shelf of the South China Sea; Typhoon; Numerical simulation

Near-inertial motion is a type of motion in the ocean that is ubiquitous and has a frequency close to the local inertial frequency. By using a COAWST(Coupled Ocean-Atmosphere-Wave-Sediment Transport) modeling system covering the northern shelf of the South China Sea, we chose the typhoon Cempaka to study the near-inertial motions in the nearshore area. It is found that there are spatially two energy peaks, one in the coastal area with the highest typhoon wind speed, and the other at a certain distance offshore, with the second energy peak lasting longer. In the coastal area with water depth shallower than 40 m, the near-inertial motion is mainly in a barotropic mode, with vertical uniform phases, and energy decreasing from the surface to the bottom. As the water depth gradually increases offshore, the near-inertial motions exhibit a clear two-layer structure in the regions with depths ranging from 70-100 m, with opposite directions of near-inertial flow in the surface and bottom layers, and two energy peaks in the vertical direction, showing obvious first baroclinic mode characteristics. Some areas with obvious two-layer structures are not dominated by the first baroclinic mode but are jointly dominated by the first and second baroclinic modes. As the water depth continues to increase, higher modes of near-inertial flow account for an increasing proportion of the total near-inertial kinetic energy. Momentum balance analysis shows that the near-inertial motion in the coastal area is mainly driven by barotropic fluctuations caused by wind stress, while in the continental shelf area, the near-inertial motion in the upper mixed layer is driven by wind stress, and the near-inertial motion below the mixed layer is driven by barotropic gradient force. The two near-inertial motions superimposed on each other, resulting in higher near-inertial energy and a longer duration in the area.