The speed factor and deflection angle of the wind-driven flow has been discussed for more than a century. However, these parameters are quite uncertain. Large measurement errors due to surface waves, geostrophic flows and tidal currents are responsible for this large uncertainty.

Recently developed HF radar is capable of measuring surface velocity with good spatial and temporal resolutions. In this study, the speed factor and deflection angle of the wind-driven flow were estimated by comparing winds with surface ageostrophic velocities, which are estimated from detided surface velocities measured with HF radar and geostrophic velocities estimated from sea level differences.

Large seasonal variations of both the speed factor and the deflection angle were found from our analysis; The speed factor is 1.1-1.3 % in winter (November - February) and 1.5-2.0 % in summer (May - August), while the deflection angle is 15-27 deg in winter and 41-65 deg in summer. It was also found that the speed factor increases with wind speed (or the friction velocity). Possible cause(s) of these variations will be discussed in the presentation.