Journal of Marine Sciences ›› 2023, Vol. 41 ›› Issue (4): 12-20.DOI: 10.3969/j.issn.1001-909X.2023.04.002

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Study on frequency shift of typhoon-excited near-inertial waves in northwestern South China Sea

FU Dianfu1(), XIE Botao1, HUANG Bigui1, JIN Weifang2, MOU Yong2, LIN Feilong2,*()   

  1. 1. CNOOC Research Institute Ltd., Beijing 100028, China
    2. State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, MNR, Hangzhou 310012, China
  • Received:2022-09-27 Revised:2022-11-23 Online:2023-12-15 Published:2024-01-30


Near-inertial waves (NIWs) play an important role in the response of ocean to typhoon. Their frequency varies with the depth and is the main factor in determining the propagation rate of near-inertial energy to the ocean interior. Based on the observation data from mooring, the factors affecting the blue-shift frequency of NIWs excited by typhoon were investigated in northwestern South China Sea. By analyzing the vorticity effect and Doppler effect caused by background currents, this study suggests that the Doppler effect of background currents was the main factor in the blue-shift frequency of NIWs. As depth increased, inertial wave frequencies increased. Quantitative calculations further demonstrated that within the upper 200 meters, the Doppler effect of the background currents was negative, approaching zero in depth around 200 meters. However, in the depth range of 230 to 400 meters, the Doppler effect became positive. This depth range exhibited the maximum strength of the background currents, with their direction aligned with the propagation direction of inertial waves. Consequently, the positive Doppler shift induced by the background currents was most pronounced. The results of this study are important for improving the understanding of the ocean response to typhoons, especially the propagation of near-inertial waves in areas with complex background current structure (e.g., the western boundary current region).

Key words: typhoon, near-inertial waves, Doppler effect

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