On the Effect of Wind and Turbulence on Ocean Swell Fabrice Ardhuin Centre Militaire Océanographique, Service Hydrographique et Océanographique de la Marine Brest, France Alastair D. Jenkins Bjerknes Centre for Climate Research Bergen, Norway ABSTRACT 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. KEY WORDS: Waves, turbulence, wind, swell, modulation. INTRODUCTION 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. These new applications, along with the traditional problem of wave and surf forecasting, warrant a closer inspection of the theory and practical aspects of swell evolution. 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). These ideas have been gradually abandoned and traded for eddy viscosity analogies (Bowden, 1950; Groen and Dorrestein, 1950) that are used today in some operational wave forecasting models (e.g. Tolman and Chalikov, 1996). The magnitude and the frequency dependence of the associated wave damping are calibrated using buoy and altimeter data, and no theory is available to predict these parameters. Other wave models wishfully assume that swell dissipates in the same way as the wind sea (WAMDI, 1988; Komen et al., 1994). The validation studies on the spectral shape and magnitude of the dissipation are very few. Snodgrass et al. (1966) have demonstrated that swells of periods larger than 16 s are hardly attenuated when crossing the Pacific from south to north, although attenuation of shorter period waves was observed. There is also qualitative evidence of waves blown flat by strong opposing winds, without any satisfactory theory or good observations (Jenkins 2002). We therefore take advantage of recent developments in wave-turbulence interaction theory (Teixeira and Belcher, 2002; Ardhuin and Jenkins, manuscript submitted to J. Phys. Ocenogr.) and observation of short wave modulations by long waves (Hara et al., 2003) to review and combine the existing theories, including the much ignored 30-year old theory on swell-short wave modulations by Hasselmann (1971), and evaluate their relevance for swell forecasting. The paper unfolds as follows. First the theory recent result for wave- turbulence interaction is recalled, followed by an extension of Hasselmann’s (1971) theory for short wave modulation, including now the modulation of the wind forcing. Next, a semi-empirical parameterization is proposed for the short wave modulation, and the different effects are evaluated numerically for typical wind conditions. Perspectives for validation are discussed with our conclusions. WAVE-TURBULENCE INTERACTION Using rapid distortion theory, Teixeira and Belcher (2002) found that waves propagating in a turbulent field produced turbulent kinetic energy locally at the rate of 429 Proceedings of The Fifteenth (2005) International Offshore and Polar Engineering Conference Seoul, Korea, June 19-24, 2005 Copyright © 2005 by The International Society of Offshore and Polar Engineers ISBN 1-880653-64-8 (Set); ISSN 1098-6189 (Set)