Abstract: Based on the observation data of Xisha submerged marker in the South China Sea from October to November 2020, the article investigates in detail the characteristics of near-inertial waves (NIWs) excited by consecutive typhoons, and analyses the decay time scales of the NIWs induced by different typhoons, as well as the modulation effect of the background vorticity and the flow field on the near-inertial frequency. The vertical wavelength of NIWs induced by Typhoon Molave is the largest at 133m. NIWs induced by Saudel propagate downward the fastest, with a vertical group velocity of 23 md-1, and the buoyancy frequency N determines the magnitude of the vertical group velocity, which is the largest during Typhoon Saudel. The energy decay times of NIWs caused by the three typhoons are significantly different, about 3 days for Saudel, 7 days for Molave, and 15 days for Vamco. This difference is due to the fact that with the weakening of the upper oceanic laminar knot, the horizontal flow of NIWs during the Vamco period is the weakest in terms of vertical shear, and is affected by the propagation of the NIWs from the far field. The results of the dynamical modal decomposition show that the near-inertial kinetic energy (NIKE) is dominated by the first mode from 50 to 90 m, while the third mode is dominated from 90 to 150 m and from 350 to 640 m, and the second mode is dominated from 150 to 350 m. In general, the NIKE is dominated by the third mode from 90 to 150 m and from 350 to 640 m, and the second mode is dominated by the second mode from 150 to 350 m. Overall, NIKE has the largest energy in the first mode, and the proportion of first mode NIKE is more than 25% in three consecutive typhoon events. Meanwhile, the results of the empirical orthogonal function analysis also show that the near-inertial flow (NIW) is significantly dominant in mode 1, with a contribution of more than 40% of the variance. The observed frequencies of the NIWs are all blueshifted, and the blueshifts in Typhoon Saudel and Molave are due to the combination of the background flow and background vorticity, while the positive background vorticity brought by the cyclonic vortex during the Vamco period is the main reason for the blueshift. The results of the study are useful for a deeper understanding of the characteristics of the structure, propagation, attenuation and energy distribution of strong wind-induced NIWs and their generation mechanisms, and provide a reference for the simulation and rational characterisation of NIWs in numerical models.

Key words: Successive typhoons, Near-inertial waves, Near-inertial frequencies, Modal decomposition