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

doi:10.3969/j.issn.1001-909X.2023.04.002

Abstract

Abstract:

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

CLC Number:

P731.2

FU Dianfu, XIE Botao, HUANG Bigui, JIN Weifang, MOU Yong, LIN Feilong. Study on frequency shift of typhoon-excited near-inertial waves in northwestern South China Sea[J]. Journal of Marine Sciences, 2023, 41(4): 12-20.

Figures/Tables 9

Fig.1 The path and intensity of typhoon Doksui

Fig.2 The time series of eastward (a) and northward (b) component of near-inertial waves during typhoon period (The purple triangle indicates the arrival time of the typhoon.)

Fig.3 The vertical distribution of frequency (a) and phase (b) of near-inertial waves

Fig.4 The time series of relative background vorticities from observation at mooring station and the relative theoretical vorticities

Fig.5 Distribution of eastward (a) and northward (b) components of the background current at the mooring station during typhoon period (The vertical black dashed lines mark the time range of typhoons impact. The purple triangle indicates the arrival time of the typhoon.)

Fig.6 Distribution of amplitude (a) and direction (b) for time-averaged background currents during typhoon period

Fig.7 Squared coherence (a) and phase (b) between density perturbation and shear in eastward component (The black dashed lines indicate the position of f0.)

Fig.8 Shear vector hodographs of near-inertial waves at depths of 55 m (a), 105 m (b) and 155 m (c) during typhoon period

Fig.9 Vertical distribution of the effective Coriolis frequency and intrinsic frequency of near-inertial waves (a) and Doppler shift of the waves induced by background current(b) (The shaded areas represent confidence interval.)

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