Directional attributes of the ocean surface wind stress vector

Abstract

In a recent paper by Geernaert (1988) it was suggested that the stress and wind vectors are rarely aligned and that the deviation of the stress vector direction from the wind vector is related to the surface heat flux. It was argued that since cold air advection over the open ocean generally corresponds to upward heat flux, the consequent rotation of the winds to the right with height during cold air advection could be responsible for observations of the stress vector to be to the right of the wind flow for unstable stratifications. Conversely, observations of the stress vector to the left of the flow for stable stratifications were suggested to be associated with the turning of the wind vector to the left of the flow for warm air advection, or downward heat flux. In that study only 30% of the statistical variance of the angle between the wind and wind stress vectors could be explained by air‐sea heat flux, and it was suggested that the orbital velocity spectrum of surface waves (i.e., data unavailable in that study) could be important for explaining at least part of the remaining variance. In this study we report on measurements of the wind stress vector and wave directional spectrum collected during the Synthetic Aperture Radar X‐Band Ocean Nonlinearities experiment. Conducted on the Chesapeake Light Tower during 1988, the experiment yielded data reported herein that provide convincing evidence that the stress vector direction, during light winds and near‐zero air‐sea heat flux, is governed in large part by the direction of long waves on the ocean surface. Further, the measurements suggest that the stress vector has a direction which is a blend between the wind direction and the swell direction.