Stokes drift fluctuations and upper ocean diffusion

Northwest Research Associates, Incorporated, Seattle WA

Investigators

Abstract

The Lagrangian drift associated with ocean surface waves, known as Stokes Drift, plays an important role in the ocean surface boundary layer dynamics and in particular the small-scale (nominally a few kilometers) diffusion of passive tracers. Thus it is important for the break-up and dispersion of oil spills or sewage outflow, and transport of material in the upper ocean and near the coast in general. Considerable theoretical advances have been made over the last few decades, but the lack of direct observations hampers progress on the role of Stokes Drift in material transport in the upper ocean. The collection of new observations and development of theoretical and numerical models will advance the current understanding of these dynamics in realistic wave conditions in both the open ocean and the coastal environment. The nonlinear Lagrangian theory and sea surface kinematics simulations developed in this project will provide a general framework for the analysis and interpretation of measurements of surface-following moored buoys, drifters, and remote sensing systems. The findings from this study will provide a framework towards understanding and predicting wave-driven material transport processes, including the break-up of surface slicks and transport of larvae, and will contribute to more effective disaster mitigation and coastal ecology protection programs. The development of small, low-power, inexpensive drifters with satellite communication and sustainable energy sources, which can resolve both surface waves and surface drift fluctuations, is an important step toward more ambitious, global applications. Large arrays of these types of instruments can be used to monitor ocean waves, surf zone motions, mean surface currents, and surface temperature and salinity. A post-doctoral fellow will be trained in both observational and modeling techniques in the upper ocean and summer undergraduate interns will be exposed to the research process. The Lagrangian near-surface drift of ocean surface waves (Stokes Drift) is still poorly understood and no direct measurements have been reported. In particular, very little is known about random fluctuations in the Stokes Drift that are believed to be important to upper ocean diffusion processes. Detailed observations of the fluctuating surface drift in the upper ocean due to surface waves will be obtained, building on the investigators' recent development of small, wave-resolving drifters. Approximately 25 drifters will be deployed in homogeneous, open ocean conditions and in a more complex coastal environment with variable depth and currents, to observe the near surface kinematics and particle dispersion. To interpret the drifter data and develop a better understanding of the effects of nonlinearity and directional spreading on the surface drift and diffusion statistics, a Lagrangian theory for the surface kinematics in a random wave field will be developed. The theory will be extended to include random, small-scale medium variations (e.g. bottom topography and ambient currents), and augmented with direct numerical integration of the surface momentum balance to simulate more complex conditions with strong nonlinearity and/or rapid medium variations. Then, observations will be compared to theory and numerical simulations, and the relative contribution of Stokes Drift fluctuations to the observed particle diffusion will be evaluated to gain a better understanding of the role of waves in material transport and diffusion, both offshore and near the coast.