PhD student 2014-2017 : Observation and modelling of short ocean surface gravity waves directional properties
Abstract: Short surface gravity waves are ubiquitous at the ocean surface, with lengths from a few tens of meters to a meter typically. Knowing their propagation directions at sea is important in several respects, especially for the understanding of sea-state dynamics, air-sea interactions and particles surface drift.
Their directional distributions are here investigated in the light of the recent progress made in instrumentation techniques. The analysis of ocean bottom seismo-acoustic noise records allows for the extraction of a quasi-universal behavior which indirectly depends on this distribution through the so-called overlap integral. It is coherent with direct observations of the wave field obtained from 3-dimensional reconstructions of the ocean surface elevation field. While the propagation direction of long waves aligns with the wind direction, short waves progressively detach from it towards small scales (bimodality).
Comparing those measurements with the predictions of a spectral numerical wave model, based on the WAVEWATCH®III environment, allows to realize that they provide qualitatively correct but quantitatively incorrect predictions. One of the possibilities here explored to correct for it, is by accounting for the sources of energy at ±90° to the wind direction, which could be associated with the breaking of long waves. This source term on its own does not explain the shapes of the observed directional distributions. Other mechanisms could come into play that future investigations will help clarify.