Surface stress over the ocean in swell-dominated conditions during moderate winds

Atmospheric and surface wave data from several oceanic experiments carried out on the Floating Instrument Platform (FLIP) and the Air-Sea Interaction Spar (ASIS) have been analyzed with the purpose of identifying swell-related effects on the surface momentum exchange during near-neutral atmospheric conditions and wind-following or crosswind seas. All data have a pronounced negative maximum in uw cospectra centered at the frequency of the dominant swell n_p, meaning a positive contribution to the stress. A similar contribution at this frequency is also obtained for the corresponding crosswind cospectrum. The magnitude of the cospectral maximum is shown to be linearly related to the square of the orbital motion, being equal to 1:25H_sd²n_p², where H_sd is the swell-significant wave height, the effect tentatively being due to strong correlation between the surface component of the orbital motion and the pattern of capillary waves over long swell waves. A model for prediction of the friction velocity u_*from measurements of H_sd, n_p, and the 10-m wind speed U₁₀ is formulated and tested against an independent dataset of ~400 half-hour measurements during swell, giving good result. The model predicts that the drag coefficient C_D, which is traditionally modeled as a function of U₁₀ alone (e.g., the COARE algorithm), becomes strongly dependent on the magnitude of the swell factor 1:25H_sd²n_p² and that C_D can attain values several times larger than predicted by wind speed-only models. According to maps of the global wave climate, conditions leading to large effects are likely to be widespread over the World Ocean.