A Composite View of Surface Signatures and Interior Properties of Nonlinear Internal Waves: Observations and Applications

Ming-Huei Chang Institute of Oceanography, National Taiwan University, Taipei, Taiwan

Search for other papers by Ming-Huei Chang in
Current site
Google Scholar
PubMed
Close
,
Ren-Chieh Lien Applied Physics Laboratory, University of Washington, Seattle, Washington

Search for other papers by Ren-Chieh Lien in
Current site
Google Scholar
PubMed
Close
,
Yiing Jang Yang Department of Marine Science, Naval Academy, Kaohsiung, Taiwan

Search for other papers by Yiing Jang Yang in
Current site
Google Scholar
PubMed
Close
,
Tswen Yung Tang Institute of Oceanography, National Taiwan University, Taipei, Taiwan

Search for other papers by Tswen Yung Tang in
Current site
Google Scholar
PubMed
Close
, and
Joe Wang Institute of Oceanography, National Taiwan University, Taipei, Taiwan

Search for other papers by Joe Wang in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

Surface signatures and interior properties of large-amplitude nonlinear internal waves (NLIWs) in the South China Sea (SCS) were measured during a period of weak northeast wind (∼2 m s−1) using shipboard marine radar, an acoustic Doppler current profiler (ADCP), a conductivity–temperature–depth (CTD) profiler, and an echo sounder. In the northern SCS, large-amplitude NLIWs propagating principally westward appear at the tidal periodicity, and their magnitudes are modulated at the spring–neap tidal cycle. The surface scattering strength measured by the marine radar is positively correlated with the local wind speed when NLIWs are absent. When NLIWs approach, the surface scattering strength within the convergence zone is enhanced. The sea surface scattering induced by NLIWs is equivalent to that of a ∼6 m s−1 surface wind speed (i.e., 3 times greater than the actual surface wind speed). The horizontal spatial structure of the enhanced sea surface scattering strength predicts the horizontal spatial structure of the NLIW. The observed average half-amplitude full width of NLIWs λη/2 is 1.09 ± 0.2 km; the average half-amplitude full width of the enhanced scattering strength λI/2 is ∼0.57 λη/2. The average half-amplitude full width of the enhanced horizontal velocity convergence of NLIWs λxu/2 is approximately equal to λI/2. The peak of the enhanced surface scattering leads the center of NLIWs by ∼0.46 λη/2. NLIW horizontal velocity convergence is positively correlated with the enhancement of the surface scattering strength. NLIW amplitude is positively correlated with the spatial integration of the enhancement of the surface scattering strength within the convergence zone of NLIWs. Empirical formulas are obtained for estimating the horizontal velocity convergence and the amplitude of NLIWs using radar measurements of surface scattering strength. The enhancement of the scattering strength exhibits strong asymmetry; the scattering strength observed from behind the propagating NLIW is 24% less than that observed ahead, presumably caused by the skewness and the breaking of surface waves induced by NLIWs. Above the center of NLIWs, the surface scattering strength is enhanced slightly, associated with isotropic surface waves presumably induced or modified by NLIWs. This analysis concludes that in low-wind conditions remote sensing measurements may provide useful predictions of horizontal velocity convergences, amplitudes, and spatial structures of NLIWs. Further applications and modification of the presented empirical formulas in different conditions of wind speed, surface waves, and NLIWs or with other remote sensing methods are encouraged.

Corresponding author address: Tswen Yung Tang, Institute of Oceanography, National Taiwan University, Taipei, Taiwan. Email: tyt@ntu.edu.tw

Abstract

Surface signatures and interior properties of large-amplitude nonlinear internal waves (NLIWs) in the South China Sea (SCS) were measured during a period of weak northeast wind (∼2 m s−1) using shipboard marine radar, an acoustic Doppler current profiler (ADCP), a conductivity–temperature–depth (CTD) profiler, and an echo sounder. In the northern SCS, large-amplitude NLIWs propagating principally westward appear at the tidal periodicity, and their magnitudes are modulated at the spring–neap tidal cycle. The surface scattering strength measured by the marine radar is positively correlated with the local wind speed when NLIWs are absent. When NLIWs approach, the surface scattering strength within the convergence zone is enhanced. The sea surface scattering induced by NLIWs is equivalent to that of a ∼6 m s−1 surface wind speed (i.e., 3 times greater than the actual surface wind speed). The horizontal spatial structure of the enhanced sea surface scattering strength predicts the horizontal spatial structure of the NLIW. The observed average half-amplitude full width of NLIWs λη/2 is 1.09 ± 0.2 km; the average half-amplitude full width of the enhanced scattering strength λI/2 is ∼0.57 λη/2. The average half-amplitude full width of the enhanced horizontal velocity convergence of NLIWs λxu/2 is approximately equal to λI/2. The peak of the enhanced surface scattering leads the center of NLIWs by ∼0.46 λη/2. NLIW horizontal velocity convergence is positively correlated with the enhancement of the surface scattering strength. NLIW amplitude is positively correlated with the spatial integration of the enhancement of the surface scattering strength within the convergence zone of NLIWs. Empirical formulas are obtained for estimating the horizontal velocity convergence and the amplitude of NLIWs using radar measurements of surface scattering strength. The enhancement of the scattering strength exhibits strong asymmetry; the scattering strength observed from behind the propagating NLIW is 24% less than that observed ahead, presumably caused by the skewness and the breaking of surface waves induced by NLIWs. Above the center of NLIWs, the surface scattering strength is enhanced slightly, associated with isotropic surface waves presumably induced or modified by NLIWs. This analysis concludes that in low-wind conditions remote sensing measurements may provide useful predictions of horizontal velocity convergences, amplitudes, and spatial structures of NLIWs. Further applications and modification of the presented empirical formulas in different conditions of wind speed, surface waves, and NLIWs or with other remote sensing methods are encouraged.

Corresponding author address: Tswen Yung Tang, Institute of Oceanography, National Taiwan University, Taipei, Taiwan. Email: tyt@ntu.edu.tw

Save
  • Alpers, W., 1985: Theory of radar imaging of internal waves. Nature, 314 , 245247.

  • Chang, M-H., Lien R-C. , Tang T. Y. , D’Asaro E. A. , and Yang Y. J. , 2006: Energy flux of nonlinear internal waves in northern South China Sea. Geophys. Res. Lett., 33 .L03607, doi:10.1029/2005GL025196.

    • Search Google Scholar
    • Export Citation
  • Cushman-Roisin, B., 1994: Introduction to Geophysical Fluid Dynamics. Prentice Hall, 320 pp.

  • Dankert, H., 2003: Measurement of waves, wave groups, and wind fields using nautical radar image sequences. Ph.D. dissertation, Department of Earth Sciences, University of Hamburg, 115 pp.

  • Dankert, H., Horstmann J. , and Rosenthal W. , 2005: Wind- and wave-field measurements using marine X-Band radar-image sequences. IEEE J. Oceanic Eng., 30 , 534542. doi:10.1109/JOE.2005.857524.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Hughes, B. A., 1978: The effect of internal waves on surface wind waves. 2. Theoretical analysis. J. Geophys. Res., 83 , 455465.

  • Hughes, B. A., and Grant H. L. , 1978: The effect of internal waves on surface wind waves. 1. Experimental measurements. J. Geophys. Res., 83 , 443454.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Hughes, B. A., and Gower J. F. R. , 1983: SAR imagery and surface truth comparisons of internal waves in Georgia Strait, British Columbia, Canada. J. Geophys. Res., 88 , 18091824.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kropfli, R. A., Ostrovski L. A. , Stanton T. P. , Skirta E. A. , Keane A. N. , and Irisov V. , 1999: Relationships between strong internal waves in the coastal zone and their radar and radiometric signatures. J. Geophys. Res., 104 , C2. 31333148.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lee, P., and Coauthors, 1995: X-band microwave backscattering from ocean waves. J. Geophys. Res., 100 , C2. 25912611.

  • Lien, R-C., Tang T. Y. , Chang M. H. , and D’Asaro E. A. , 2005: Energy of nonlinear internal waves in the South China Sea. Geophys. Res. Lett., 32 .L05615, doi:10.1029/2004GL022012.

    • Search Google Scholar
    • Export Citation
  • Lyzenga, D. R., 1998: Effects of intermediate-scale waves on radar signatures of ocean fronts and internal waves. J. Geophys. Res., 103 , C9. 1875918768.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ramp, S. R., and Coauthors, 2004: Internal solitons in the northeastern South China Sea. Part I: Source and deep water propagation. IEEE J. Oceanic Eng., 29 , 11571181.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Smith, S. R., Bourassa M. A. , and Sharp R. J. , 1999: Establishing more truth in true winds. J. Atmos. Oceanic Technol., 16 , 939952.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Thompson, D. R., and Gasparovic R. F. , 1986: Intensity modulation in SAR images of internal waves. Nature, 320 , 345348.

  • Trizna, D., and Carlson D. , 1996: Studies of dual polarized low grazing angle radar sea scatter in nearshore regions. IEEE Trans. Geosci. Remote Sens., 34 , 747757.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yang, Y-J., Tang T. Y. , Chang M. H. , Liu A. K. , Hsu M-K. , and Ramp S. R. , 2004: Solitons northeast of Tung-Sha Island during the ASIAEX pilot studies. IEEE J. Oceanic Eng., 29 , 11821199.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Zhao, Z., Klemas V. , Zheng Q. , and Yan X-H. , 2004: Remote sensing evidence for baroclinic tide origin of internal solitary waves in the northeastern South China Sea. Geophys. Res. Lett., 31 .L06302, doi:10.1029/2003GL019077.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 431 143 18
PDF Downloads 331 54 4