Seasonal variations in the speed factor and deflection angle of the wind-driven surface flow in the Tsushima Strait

[1] The speed factor and deflection angle of the wind-driven surface flow were estimated for the Tsushima Strait from long (≥3.5 years) records of surface velocities measured with HF radar, geostrophic velocities estimated from sea level differences, and analyzed wind velocities. Large seasonal variations in both the speed factor and deflection angle were found. The speed factor, which is defined here as the ratio of the wind-driven surface flow to the (waterside) friction velocity, is 7.7–9.5 in winter and 11.4–14.3 in summer, while the deflection angle is 17.7°–27.3° in winter and increases to 48.5°–67.3° in summer. A likely cause of these seasonal variations is the seasonal variation of the eddy viscosity profile. It was also found that the speed factor in winter increases with the friction velocity. This dependence is possibly explained by deepening of the wind-driven surface flow with increasing friction velocity: as the friction velocity increases, HF radar (whose measurement depth is fixed) tends to measure a shallower part of the wind-driven surface flow whose speed factor is larger.