Conclusions

Infragravity waves can be observed at the ocean bottom broadband seismic station MOBB. When compared to the energy of the short-period ocean waves recorded at the local buoys, infragravity waves in the longer than 20 s period band are found to be mainly locally generated from shorter period waves. Two types of modulation of the infragravity signal are observed. First, the entire infragravity band is modulated in-phase with tides. It is possible that this is a result of the nonlinear exchange of energy between the short-period waves and tidal currents. Second, the low-frequency modulation of the observed infragravity peak is best correlated with the energy of the 14.3 s period ocean waves, suggesting a close relation of infragravity wave generation to that of double frequency microseisms.

Figure 13.29: (a) Theoretical ocean tide at the MOBB location. (b) The power spectral density (PSD) for the vertical MOBB channel as a function of period and time. White lines indicate hours with some missing data. (c) The envelope of the infragravity peak presented in (b), taken at the long-period end, at the PSD value of -136 dB. (d) The significant wave height at the buoy 46042. (e) The spectral wave density (SWD) in the 14.3 s period bin at the buoy 46042. (f) The SWD at the buoy 46042 as a function of period and time. (g) The period of the infragravity peak envelope, taken at the long-period end, at the PSD value of -136 dB, as a function of the significant wave height as observed at the buoy 46042. Gray line shows best linear fit to the data. (h) The period of the infragravity peak envelope as a function of the SWD observed at the buoy 46042 in the 14.3 s period bin. Gray line shows best linear fit to the data. (i) The correlation coefficient between the period of the infragravity peak envelope and the SWD observed in the individual bins at buoy 46042, as a function of the SWD bin period.
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