What do deep sea pressure fluctuations tell about short surface waves?



[1] Short waves centered at the gravity to capillary transition (13.5 Hz) dominate the slope statistics of the sea surface and are responsible for most of the air-sea momentum transfer (wind stress). Little is known about short “gravities.” In contrast, long gravities, with their characteristic κ−4 spectrum, have extensive observational support. We propose a fundamental distinction between long gravities, with their saturated (wind-independent) spectrum, and short gravities, with their wind-dependent spectrum. Evidence comes (surprisingly) from sea-floor pressure fluctuations associated with non-linear interactions between oppositely traveling surface waves of half their frequency. The bottom pressure spectrum shows a transition at about 6 Hz (3 Hz surface wave frequency) from an f−7 to an f−3 dependence that we associate with the long to short surface gravity wave transition. Further, the requirement of oppositely traveling energy places an integral restraint on the directional spread of the surface waves.