Observed Variability of Ocean Wave Stokes Drift, and the Eulerian Response to Passing Groups

Abstract

Waves and currents interact via exchanges of mass and momentum. The mass and momentum fluxes associated with surface waves are closely linked to their Stokes drift. Both the variability of the Stokes drift and the corresponding response of the underlying flow are important in a wide range of contexts. Three methods are developed and implemented to evaluate Stokes drift from a recently gathered oceanic dataset, involving surface velocities measured continually over an area 1.5 km in radius by 45°. The estimated Stokes drift varies significantly, in line with the occurrence of compact wave groups, resulting in highly intermittent maxima. One method also provides currents at a fixed level (Eulerian velocities). It is found that Eulerian counterflows occur that completely cancel the Stokes drift variations at the surface. Thus, the estimated Lagrangian surface flow has no discernable mean response to wave group passage. This response is larger than anticipated and is hard to reconcile with current theory.

Comments on “The Three-Dimensional Current and Surface Wave Equations”

“…These additional terms are of the same order as the first term and have no flux at the bottom and surface. Thus, the depth-integrated equations including that term also comply with known depth-integrated equations (e.g., Smith 2006). …”

Lagrangian Surface Wave Motion and Stokes Drift Fluctuations

“…The concurrent observation of waves and surface drift is of particular interest in surface dispersion and mixing studies because traditional Eulerian current measurements are difficult to make at the sea surface, and the wave-induced Stokes drift has a completely different profile in the more natural Lagrangian reference frame (e.g., Phillips 1977). Whereas the mean Stokes drift has been the topic of numerous studies (e.g., Hasselmann 1970;Xu and Bowen 1994;Polton et al 2005;Lentz et al 2008;Aiki and Greatbatch 2012), infragravity fluctuations in the wave-induced surface drift on the scale of wave groups have received almost no attention (e.g., Smith 2006). Here, we examine the fluctuating surface drift observed with an idealized drifting buoy in a random sea state using second-order wave theory.…”

The Evolution of Inhomogeneous Wave Statistics through a Variable Medium

“…The dynamics and statistics of ocean waves are important, for example, for upper-ocean dynamics (e.g., Craik and Leibovich 1976;Smith 2006;Aiki and Greatbatch 2011), air-sea interaction (e.g., Janssen 2009), ocean circulation (e.g., McWilliams and Restrepo 1999), and wave-driven circulation and transport processes (e.g., Hoefel and Elgar 2003;Svendsen 2006). Modern stochastic wave models are routinely applied to a wide range of oceanic scales, both in open-ocean applications and the near shore, and either as stand-alone wave prediction models, or as part of coupled ocean-atmosphere models for global circulation and climate studies (e.g., The WAMDI Group 1988;Tolman 1991;Komen et al 1994;Booij et al 1999;Wise Group 2007).…”