Strong near-inertial oscillations in geostrophic shear in the northern South China Sea
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With observational data from three Acoustic Doppler Current Profiler (ADCP) moorings, we detected strong near-inertial oscillations (NIO) in the continental shelf region of the northern South China Sea in July 2008. The amplitude of the near-inertial current velocity is much greater than that of diurnal and semi-diurnal tides. The power of the NIOs is strongest in the intermediate layer, relatively weak in the surface layer, and insignificant in the near-bottom layer. The spectral analysis indicates that the NIOs have a peak frequency of 0.0307 cph, which is 2% lower than the local inertial frequency, i.e., a red-shift. The near-inertial wave has an upward vertical phase velocity, which involves a downward group velocity and energy flux. The estimated vertical phase velocity is about 43 m day−1, corresponding to a vertical wave length of about 58 m. The horizontal scale of the NIOs is at least hundreds of kilometers. This NIO event lasted for about 15 days after a typhoon’s passage. Given the northeastward background flow with significant horizontal shear, both Doppler shift and shear flow modulation mechanisms may be responsible for the red-shift of the observed NIOs. For the shear flow mechanism, the observed negative background vorticity and the corresponding effective Coriolis frequency reduce the lower limit of admissible frequency band for the NIOs, causing the red-shift. Meanwhile, the mooring area with the broadened frequency band acts as a wave-guide. The trapping and amplification effects lead to the relatively long sustaining period of the observed NIOs.
KeywordsNear-inertial oscillation Geostrophic shear Northern South China Sea
This study is supported by the National Basic Research Program of China through project 2009CB421208, the Natural Science Foundation of China through projects 40976013 and 40821063, and the United States National Oceanic and Atmospheric Administration National Environmental Satellite, Data, and Information Service Ocean Remote Sensing Program 05-01-11-000. The authors thank the SCOPE cruise in June–July 2008, co-organized by Profs. J. Zhu, D.X. Wang, X.G. Guo, M.H. Dai, and J.P. Gan, and appreciate R/V Yanping 2 and all the participants for the mooring deployment and recovery. Typhoon data are obtained from Unisys (http://weather.unisys.com/hurricane). We thank the guest editor and three anonymous reviewers for their helpful comments.
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