The Relationship of Near‐Surface Flow, Stokes Drift and the Wind Stress

Abstract

AbstractMany years of simultaneous hourly buoy wind and directional wave spectra data in the Gulf of Mexico and the Pacific were used to estimate Stokes drift and u* ew where u* = (magnitude of the local windstress/water density)1/2 and ew is a unit vector in the direction of the local wind. Stokes drift and u* ew were strongly vectorally correlated, the two vectors on average being within a few degrees of one another. This result remained valid even when there was evidence of remotely forced swell. Extension of the observed wave spectra above 0.35 Hz to the u*‐dependent wave breaking frequency shows that typically the e‐folding scale of the Stokes drift with depth is less than 1.8 m, much smaller than the Ekman layer e‐folding scale. Therefore, there is negligible induced Eulerian cancellation of the Stokes drift, and the surface particle movement is governed by the Eulerian velocity + |uStokes|ew. Taking into account wave spreading, |uStokes| typically ranges from about 3 to 13 cm/s. Thus, the Stokes drift, which can be estimated directly from the wind stress, is an order one contributor to the surface transport of particles.