Observations and models show that sudden changes in the magnitude of the wind stress which occur within a time interval of one-half inertial period are most effective in increasing surface current speeds and mixing the upper layers of the ocean. The purpose of the present study is to quantify the effects of concurrent time dependent wind direction. The Mellor-Yamada level 2 1/2 turbulence closure model is used. A series of model runs was executed in order to determine the relative sensitivity of mixed layer depth and sea surface temperature to wind speed as compared with the rate of change of wind direction. The results indicate that the accuracy and time resolution of wind direction should be given special consideration in the design and interpretation of field experiments which will be used for testing prognostic mixed layer models.