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Journal of Oceanography

, Volume 67, Issue 4, pp 377–384 | Cite as

Strong near-inertial oscillations in geostrophic shear in the northern South China Sea

  • Zhenyu Sun
  • Jianyu Hu
  • Quanan Zheng
  • Chunyan Li
Special Section: Original Article Regional Environmental Oceanography in the South China Sea and Its Adjacent Areas (REO-SCS): I

Abstract

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.

Keywords

Near-inertial oscillation Geostrophic shear Northern South China Sea 

Notes

Acknowledgments

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.

References

  1. Chaigneau A, Pizarro O, Rojas W (2008) Global climatology of near-inertial current characteristics from Lagrangian observations. Geophys Res Lett 35:L13603. doi: 10.1029/2008GL034060 CrossRefGoogle Scholar
  2. Chen S, Ma J (1991) Oceanographic data processing methods and its application. Ocean Publishing, Beijing, p 660 (in Chinese)Google Scholar
  3. Chu PC, Veneziano JM, Fan CW, Carron MJ, Liu WT (2000) Response of the South China Sea to tropical cyclone Ernie. J Geophys Res 105:13991–14009CrossRefGoogle Scholar
  4. D’Asaro EA, Perkins H (1984) A near-inertial internal wave spectrum for the Sargasso Sea in late summer. J Phys Oceanogr 14:489–505CrossRefGoogle Scholar
  5. Deans SR (1983) The Radon transform and some of its applications. Wiley, Newyork, p 289Google Scholar
  6. Fu LL (1981) Observations and models of inertial waves in the deep ocean. Rev Geophys Space Phys 19:141–170CrossRefGoogle Scholar
  7. Gill AE (1984) On the behavior of internal waves in the wake of storms. J Phys Oceanogr 14:1129–1151CrossRefGoogle Scholar
  8. Gonella J (1971) A local study of inertial oscillations in the upper layers of the ocean. Deep Sea Res 21:895–909Google Scholar
  9. Hu J, Kawamura H, Hong H, Qi Y (2000) A review on the currents in the South China Sea: seasonal circulation, South China Sea Warm Current and Kuroshio intrusion. J Oceanogr 56:607–624CrossRefGoogle Scholar
  10. Hu J, Kawamura H, Li C, Hong H, Jiang Y (2010) Review on current and seawater volume transport through the Taiwan Strait. J Oceanogr 66:591–610CrossRefGoogle Scholar
  11. Kunze E (1985) Near-inertial wave propagation in geostrophic shear. J Phys Oceanogr 15:544–565CrossRefGoogle Scholar
  12. Leaman KD, Sanford TB (1975) Vertical energy propagation of inertial waves: a vector spectral analysis of velocity profiles. J Geophys Res 80:1975–1978CrossRefGoogle Scholar
  13. Mooers CNK (1975) Several effects of a baroclinic current on the cross-stream propagation of inertial-internal waves. Geophys Fluid Dyn 6:245–275CrossRefGoogle Scholar
  14. Orvik KA, Mork M (1995) A case study of Doppler-shifted inertial oscillations in the Norwegian Coastal Current. Cont Shelf Res 15:1369–1379CrossRefGoogle Scholar
  15. Park JJ, Kim K, King BA (2005) Global statistics of inertial motions. Geophy Res Lett 32:L14612. doi: 10.1029/2005GL023258 CrossRefGoogle Scholar
  16. Perkins H (1972) Inertial oscillations in the Mediterranean. Deep Sea Res 19:289–296Google Scholar
  17. Perkins H (1976) Observed effect of an eddy on inertial oscillations. Deep Sea Res 23:1037–1042Google Scholar
  18. Pollard RT (1970) On the generation by winds of inertial waves in the ocean. Deep Sea Res 17:795–812Google Scholar
  19. Shay LK, Elsberry RL (1987) Near inertial ocean current response to hurricane Frederic. J Phys Oceanogr 17:1249–1269CrossRefGoogle Scholar
  20. Su J (2004) Overview of the South China Sea circulation and its influence on the coastal physical oceanography near the Pearl River Estuary. Cont Shelf Res 24:1745–1760CrossRefGoogle Scholar
  21. Webster F (1968) Observation of inertial period motions in the deep sea. Rev Geophys 6:473–490CrossRefGoogle Scholar
  22. Xie XH, Shang XD, Chen GY, Sun L (2009) Variations of diurnal and inertial spectral peaks near the bi-diurnal critical latitude. Geophys Res Lett 36:L02606. doi: 10.1029/2008GL036383 CrossRefGoogle Scholar
  23. Xu Z (1999) Dynamics of internal waves in the ocean. Science Press, Beijing, p 336Google Scholar
  24. Zhang X, Liang X, Tian J (2005) Study on internal tides and near-inertial motions in the upper 450 m ocean in the northern South China Sea. Chin Sci Bull 50:2027–2031Google Scholar
  25. Zheng Q, Lai RJ, Huang NE, Pan J, Liu TW (2006) Observation of ocean current response to 1998 Hurricane Georges in the Gulf of Mexico. Acta Oceanol Sin 25:1–14Google Scholar
  26. Zhu D, Li L (2007) Near inertial oscillations in shelf break of northern South China Sea after passage of typhoon Wayne. J Trop Oceanogr 26:1–7 (in Chinese with English abstract)Google Scholar

Copyright information

© The Oceanographic Society of Japan and Springer 2011

Authors and Affiliations

  • Zhenyu Sun
    • 1
  • Jianyu Hu
    • 1
  • Quanan Zheng
    • 2
  • Chunyan Li
    • 3
  1. 1.State Key Laboratory of Marine Environmental ScienceXiamen UniversityXiamenChina
  2. 2.Department of Atmospheric and Oceanic ScienceUniversity of MarylandCollege ParkUSA
  3. 3.Department of Oceanography and Coastal Science, Coastal Studies InstituteLouisiana State UniversityBaton RougeUSA

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