A quantitave review of processes contributing to the evolution of swell is proposed, combining direct interactions of swell with the wind and upper ocean turbulence, and interaction with shorter wind waves. The interaction with short waves is based on the extension of Hasselmann's (1971) theory for short wave modulation by long wave to the presence of variable wind stresses. Quantitative estimations of the various effects are performed based on the wave modulation model of Hara et al. (2003) and the wind-over-wave coupling model of Kudryavtsev and Makin (2004). It is found that the observations of swell decay in the Pacific (Snodgrass et al., 1963) are quantitatively consistent with the effects of wind stress modulation and direct wind to wave momentum transfer.
The problem of swell forecasting on the coast of Morocco (Gelci, 1949) led Gelci et al (1957) to develop the first numerical spectral wave models. Half a century later, the forecasting of wind seas has made enormous progress but swells are still the least well predicted part of the wave spectrum (Rogers, 2002). Although these long period waves may be well generated in numerical wave models, what happens next is still much of a mystery. At the same time it is now well recognized that swells play an important role in air-sea interactions (e.g. Drennan et al., 1999; Grachev et al. 2003) and should impact the remote sensing of ocean properties.
It was recognized very early that viscosity had a negligible effect on waves of periods of about 10 s and longer (Lamb, 1932), so that, once generated, swells were supposed to dissipate slowly due to the action of the wind, as represented by Jeffrey's (1925) sheltering theory (Sverdrup and Munk, 1947).