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

Many 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* e(w) where u* = (magnitude of the local windstress/water density)(1/2) and e(w) is a unit vector in the direction of the local wind. Stokes drift and u* e(w) 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 + vertical bar u(Stokes)vertical bar e(w). Taking into account wave spreading, vertical bar u(Stokes)vertical bar 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. Plain Language Summary Although crucial for the movement of oil spills, red tide, fish eggs and larvae, and floating garbage, much still has to be learned about net particle movement in the top 1 or 2 m of the ocean. George Gabriel Stokes showed mathematically in 1847 that ocean surface waves may affect the net movement of particles at the ocean surface, but later it was shown that because we live on a rotating Earth, the net particle movement in the direction of the waves (the "Stokes drift") might be canceled by another opposite flow. In this paper we demonstrate that because of the ocean turbulence generated by the wind, the main part of the Stokes drift in the top 2 m of the ocean is not canceled by an opposing flow. Furthermore, analysis of simultaneous hourly wind and wave measurements for many years at Christmas Island in the equatorial Pacific, ocean station Papa in the north Pacific, and 10 stations in the Gulf of Mexico shows that Stokes drift is strongly related to the local wind and is in the direction of the wind. Stokes drift is therefore not primarily due to remotely driven swell; rather, it is mainly due to the much shorter waves that the local wind generates. By taking into account when the short waves break, it is shown how Stokes drift can be approximately estimated directly from the local wind.