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Ulf Högström
,
Erik Sahlée
,
Ann-Sofi Smedman
,
Anna Rutgersson
,
Erik Nilsson
,
Kimmo K. Kahma
, and
William M. Drennan

Abstract

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 , 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 from measurements of H sd, n p , and the 10-m wind speed U 10 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 U 10 alone (e.g., the COARE algorithm), becomes strongly dependent on the magnitude of the swell factor 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.

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Ulf Högström
,
Erik Sahlée
,
Ann-Sofi Smedman
,
Anna Rutgersson
,
Erik Nilsson
,
Kimmo K. Kahma
, and
William M. Drennan

Abstract

Fifteen hours of consecutive swell data from the experiment Flux, État de la Mer, et Télédétection en Condition de Fetch Variable (FETCH) in the Mediterranean show a distinct upward momentum flux. The characteristics are shown to vary systematically with wind speed. A hysteresis effect is found for wave energy of the wind-sea waves when represented as a function of wind speed, displaying higher energy during decaying winds compared to increasing winds. For the FETCH measurements, the upward momentum transfer regime is found to begin for wind speeds lower than about U = 4 m s−1. For the lowest observed wind speeds U < 2.4 m s−1, the water surface appears to be close to dynamically smooth. In this range almost all the upward momentum flux is accomplished by the peak in the cospectrum between the vertical and horizontal components of the wind velocity. It is demonstrated that this contribution in turn is linearly related to the swell significant wave height H sd in the range 0.6 < H sd < 1.4 m. For H sd < 0.6 m, the contribution is zero in the present dataset but may depend on the swell magnitude in other situations. It is speculated that the observed upward momentum flux in the smooth regime, which is so strongly related to the cospectral peak at the dominant swell frequency, might be caused by the recirculation mechanism found by Wen and Mobbs in their numerical simulation of laminar flow of a nonlinear progressive wave at low wind speed.

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