Horizontal variations of typhoon-forced near-inertial oscillations in the south China sea simulated by a numerical model

Abstract

Horizontal variations of near-inertial oscillations (NIOs) in the northeastern South China Sea after the passage of typhoon Kalmaegi were investigated by using a three-dimensional Price-Weller-Pinkel (3DPWP) model. The 3DPWP model was modified by considering simple viscosity terms and the real topography to better simulate the typhoon-forced NIOs. The model results can generally match the observations. According to the model results, the horizontal structure of typhoon-forced NIOs is somewhat like a series of ellipses propagating northwestward with their centers parallel to the track of typhoon. The order of near-inertial wavelength is about 500 km. On the left side of the track, the near-inertial kinetic energy (NIKE) is weak and decreases quickly with distance. However, on the right side, NIKE increases from center of typhoon, gets the maximum at about one and a half radius of maximum wind speed (Rmax), and then decreases quickly. Several sensitivity experiments were designed to investigate the effect of Rmax, the maximum wind speed (Vmax) and the moving velocity of typhoon (Um) on NIOs. As Rmax increases, the influence scope of NIOs narrows, whilst increases of Vmax and Um tend to enlarge the influence scope. When three parameters increase by 10%, the corresponding NIKE increases by 18%, 38% and 19%, respectively, revealing that Vmax has the most effect. The moving of typhoon is a major reason for horizontal propagation of NIOs. When typhoon is stationary, the time-averaged and depth-integrated NIKE is about only 3.5% of the total kinetic energy input by typhoon, and the induced NICs are much less than those induced by a moving typhoon.