Modulation of infragravity signal

The PSD for the vertical MOBB component for a 10-day period is shown in Figure 13.29b. As before, the strongest infragravity signal (days 18-20) coincides with the increased energy of 10-20 s ocean waves as recorded at the local buoy 46042 (Figure 13.29f). In addition, two types of modulation of the infragravity peak can be observed (Dolenc et al., 2005).

The modulation with a period equal to the diurnal tide and to a lesser extent the semidiurnal tide is best seen at the short-period end of the infragravity peak (30-40 s periods) as well as throughout the entire infragravity band. This modulation correlates with the amplitude of the ocean tides at MOBB, shown in Figure 13.29a.

Previous studies of the nonlinear interaction between short-period waves and currents (Longuet-Higgins and Stewart, 1960, 1964) found that the energy variations of the short-period waves correspond to work done by the currents against the radiation stress of the short-period waves. The magnitude of the energy exchange between the short-period waves and tidal current depends on the pattern of the tidal currents, but in simple situations, the energy of the short-period waves is in phase with the tidal elevations (Longuet-Higgins and Stewart, 1964). This agrees with our observations.

Another effect that the tides have on the generation of the infragravity waves is through different topography that is brought into play at the same water depth during different tide heights. The topography around MOBB is very complex and it is possible that already a small water depth change can significantly perturb the conditions for generation and reflection of infragravity waves.

We also observe a low-frequency modulation which is best seen as the variation of the period on the long-period side of the infragravity peak at which the infragravity peak rises above the noise from other sources (Figure 13.29b,c). First we compare this to the significant wave height measured at the local buoy 46042 (Figure 13.29d). The two agree well in the first half of the 10-day period, but then significant wave height has a peak in the second half of the day 25, when most of the wave energy was in the waves with periods shorter than 10 s (Figure 13.29f). Correlation between the period of the infragravity peak envelope and the significant wave height is shown in Figure 13.29g. Next, we looked at the correlation between the period of the infragravity peak envelope and the wave energy in individual frequency bins as observed at the local buoy. The best correlation was observed with the ocean waves with 14.3 s period for which the SWD is shown in Figure 13.29e,h. 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, is presented in Figure 13.29i, and confirms that the infragravity peak long-period modulation correlates the strongest with the ocean wave energy at $\sim $14 seconds.

A similar result can be obtained for other stormy periods at MOBB. This suggests that the short-period ocean waves are essential for the generation of the infragravity waves. It is interesting that the same period ocean waves are also the source of the microseisms noise, observed at the double frequency, at 6-7 s. This suggests that the generation mechanisms of infragravity waves and double frequency microseisms could be closely related.

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