ON THE DETERMINATION OF WIND DIRECTION USING AN UPWARD LOOKING ACOUSTIC DOPPLER CURRENT PROFILER

Schott [1989] demonstrated that velocity estimates of title near-surfa ce ocean, made with an upward looking Acoustic Doppler Current Profile r (ADCP), could be used to estimate wind directions. Subsequent studie s have attempted to repeat Schott's results with limited success [Brow n et al. 1992; Ruiz, 1993; Visbeck and Fischer, 1995]. Considering the behavior of this near-surface velocity with changes in wind speed and other observations of near-surface acoustic scattering, we conclude t hat the acoustic backscatter is caused by subsurface bubbles generated by wave breaking. Data sets from both the equator and the North Pacif ic are examined. The North Pacific data were collected in a region whe re geostrophic flows are relatively small (typically a few centimeters per second), The ADCP measured surface currents in this region are sh own to be directed, on average, at -12 degrees +/- 23 degrees (i,e., t o the right) of the wind direction, The amount of deflection increases with wind speed. We interpret this effect to be a consequence of Ekma n dynamics and increased bubble concentration in the surface range bin at high wind speeds. In principle, the wind speed dependence of the s urface current deflection can be accounted for, although additional da ta will be required to demonstrate that such a correction is generally applicable, At the equator, currents driven by large-scale zonal wind s contribute to the observed surface velocity, We estimate the regiona l current by using ADCP velocity observations from 24 m depth and calc ulate the locally wind-driven surface current as a residual. The resul ts obtained using this technique for equatorial data show an average d ifference between wind direction and wind-driven surface current direc tion to be 1.4 degrees+/-8 degrees.