Observations of infragravity waves at the Monterey ocean bottom broadband station (MOBB)

Infragravity waves can be observed at the 1000 m deep ocean bottom broadband seismic station MOBB on stormy as well as quiet days. 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 signal is modulated in-phase with tides, possibly as a result of the nonlinear exchange of energy between the short-period waves and tidal currents. Second, a longer-period modulation of the infragravity signal is observed and is best correlated with the energy of the 14 s period ocean waves. This correlation indicates that the mechanism of generation of double frequency microseisms and infragravity waves are likely strongly related. Previously recorded data during the Oregon ULF/VLF experiment at 600 m water depth also indicate that infragravity waves are primarily locally generated.