The Ocean Surface in the High Wind Regime
Much of our knowledge of the ocean surface and the surface wave field is based on observations, models and theories that cover the low to moderate wind speed regime, say winds of less than 10-15 m/s. Most theories of ocean surface waves are based on the idealization of irrotational linear free waves; that is, waves that are propagating in an ideal fluid with no leading-order forcing acting on the surface and no departures from the classical free-surface boundary conditions. Such waves satisfy a linear dispersion relationship with gravity and surface tension as restoring forces. Departures from this state are introduced as weak perturbations, including the effects of nonlinearity, wave growth due to the wind, and wave decay due to viscosity, turbulence, or (weak in the mean) intermittent breaking. The advantage of this approach is that it is anchored in the linear dispersion relationship, with all the simplicity that it affords as a basis for perturbation expansions, and all the detailed understanding of the linear kinematics and dynamics. There is an extensive literature on surface waves that follows this approach and it has been the foundation of numerical wind-wave prediction schemes. However, there is growing empirical evidence that under high winds or strongly-forced conditions, in which the time scales for surfacewave growth may be comparable to the wave period, the dynamics of the surface may change significantly, and new approaches must be found. In this talk, I will review some of the evidence for qualitative changes in the ocean surface in high winds, discuss some of the phenomena that may have to be included in improved models and theories, and present initial results of a model that couples a strongly-forced well-mixed shallow surface layer to a deeper lower layer (Fedorov and Melville, 2004). The introduction of new scales in the problem leads to new surface-wave modes that may include quasi-periodic breaking.