Study of the near-inertial motions induced by Typhoon"Cempaka"(2021)in the continental shelf of western Guangdong
Near-inertial motion is a type of motion in the ocean that is ubiquitous and has a frequency close to the local inertial frequency.Tropical cyclones are one of the mainmechanisms that generate near-inertial motion.This study established a three-dimensional hydrodynamic model based on COAWST(Coupled Ocean-Atmosphere-Wave-Sediment Transport)numerical model system,which couples waves and currents,covers the northern shelf of the South China Sea,and was fully verified.The model was used to simulate the near-inertial motion triggered by Typhoon"Cempaka",the No.7 typhoon of 2021,on the shelf of western Guangdong.The results indicate that there are spatially two peaks of near-inertial kinetic energy,one in the coastal area with the highest typhoon wind speed,and the other at 130 km 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.As the water depth gradually in-creases offshore,we found that the near-inertial motions exhibit a clear two-layer structure inthe 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 the characteristics of the first baroclinic mode.Through dynam-ic modedecomposition,we found that some areas with obvious two-layer structures are composed of the first and secondbaroclinic 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 in the coastal area with shallow water depth and high wind speed,the balance of momentum equation in the entire water layer is dominated by the vertical turbulent viscous force and pressure gradient force.In offshore areas with deeper water depths and lower wind speeds,vertical turbulent viscous forces are concentrated in the surface and bottom layers,and the balance of momentum equation in the intermediate water body is mainly dominated by the pressure gradi-ent forces,Coriolis forces,and local acceleration.This indicates that the near-inertialmotion in the coastal area is mainly driven by barotropicwave caused bywind stress,while in the continental shelf area,the near-inertial motion in the uppermixed layer is driven by wind stress,and the near-inertial motion below the mixed layeris driven by barotropic pressure gradient force.
near-inertial motiontyphoonCOAWST modelling systemcontinental shelf of western Guangdong
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