California Sea Grant College Program
Influences of Surface Waves on the Open Ocean Wind Stress Vector
- Author(s): Rieder, Karl F.
- et al.
Open ocean measurements of the turbulent wind, directional wave field, and wave breaking are used to investigate the effect of waves on the wind stress, the momentum flux between atmosphere and ocean. Data were collected during the Surface Waves and Processes Program (SWAPP), which took place in February and March of 1990 from the research platform FLIP, situated in the North Pacific.
Traditional formulations of the drag coefficient are reviewed and tested in swell dominated, open ocean conditions in Chapter II. General trends indicate that drag increases with increasing wind speed, inverse wave age, and wave height, but the existence of significant scatter shows that none provides a stable, accurate estimate of the wind stress. However, significant correlations of the drag coefficient with these parameters are seen during the onset of three wind events, when waves are being actively generated. Additionally, the rate of increase of the drag coefficient in each of these periods is most closely linked to the turning rate of the wind, indicating that temporal and directional effects must play an important role.
The directional relationship between waves and the wind stress is investigated in Chapter III. Non-zero angles between the direction of the wind stress and the mean wind are measured. In general, the direction of the wind stress lies between the directions of the mean wind and that of the long period swell. Moreover, a significant trend between the variations of the wind stress and swell directions is found for higher wind speeds. Finally, a relation between the wind stress and wave directions can be noticed as a function of frequency, suggesting a close dynamical link.
A possible connection between wave breaking and the wind stress is investigated in Chapter IV. From comparisons of data from 15 half hour periods, the directions of the whitecap motion and the wind stress are found to be generally co-linear. As well, the speed of whitecap motion is shown to correlate with the drag coefficient. These results suggest that information about wave breaking may be used to estimate both the magnitude and direction of the wind stress.