Wind Stress Vector over Ocean Waves

Abstract

Previous investigations of the wind stress in the marine surface layer have primarily focused on determining 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 that of the mean wind flow. In this paper, the focus is on the study of the stress vector direction relative to the mean wind and surface-wave directions. Results based on measurements made during three field campaigns onboard the R/P Floating Instrument Platform (FLIP) in the Pacific are discussed. In general, the wind stress is a vector sum of the 1) pure shear stress (turbulent and viscous) aligned with the mean wind shear, 2) wind-wave-induced stress aligned with the direction of the pure wind-sea waves, and 3) swell-induced stress aligned with the swell direction. 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 in which stress is directed across or even opposite to the wind.