Wind stress vector over sea waves

Previous investigations of the wind stress in the marine surface layer have primarily focused on determination of the stress magnitude (momentum flux) and other scalar variables (e.g. friction velocity, drag coefficient, roughness length). However, the stress vector is often aligned with a direction different from the mean wind flow. In this paper we focus on the study of the stress vector direction relative to the mean wind and surface wave directions. We discuss results based on measurements made during three field campaigns onboard the R/P FLIP in Pacific. In the general case the stress is a vector sum of the (i) pure shear stress (turbulent and viscous) aligned with the mean wind, (ii) wind wave-induced stress aligned with the direction of the pure wind sea waves, and (iii) swell-induced stress aligned with the swell direction. The magnitude of each constituent depends upon the measurement level. The direction of the wind wave-induced stress and the swell-induced stress components may coincide with, or be opposite to, the direction of wave propagation (pure wind waves and swell respectively). As a result the stress vector may deviate widely from the mean wind flow including cases when stress is directed across or even opposite to the wind.