Booms and Busts in the Deep

Deep sea (5 km) pressure and velocity at the Hawaii-2 Observatory (H2O), midway between Hawaii and California, exhibit a number of remarkable features that are interpreted using the Longuet-Higgins theory of acoustic radiation from oppositely directed surface waves. A change in the slope of the bottom spectra near 5 Hz can be associated with a transition near 2.5 Hz (25-cm wavelength) of the surface wave spectrum from the classical kappa(-4) saturated (wind independent) Phillips spectrum to a distinct band of ultragravity waves. Bottom spectra are remarkably stable. Occasional 15-dB busts in the gravities and booms in the ultragravities are prominent features in the bottom records and can be associated with calms and storms at the sea surface. For strong winds, two broad lobes in the directional spectrum of the gravity waves are nearly perpendicular to the wind; as the wind drops, the lobes become narrower and more nearly aligned with the wind, leading to busts.