Observations of Turbulence Mixing and Vorticity in a Littoral Surface Boundary Layer

Abstract

Measurements of small-scale vorticity, turbulence velocity, and dissipation rates of turbulence kinetic energy ε were taken in a littoral fetch-limited surface wave boundary layer. Drifters deployed on the surface formed convergence streaks with ?1-m horizontal spacing within a few minutes. In the interior, however, no organized pattern of velocity, vorticity, or turbulence mixing intensity was found at a similar horizontal spatial scale. The turbulent Langmuir number La was 0.6?1.3, much larger than the 0.3 of the typical open ocean, suggesting comparable importance of wind-driven turbulence and Langmuir circulation. Observed ε are explained by the wind-driven shear turbulence. The production rate of turbulence kinetic energy associated with the vortex force is about 10?7 W kg?1, slightly smaller than that generated by the wind-driven turbulence. The rms values of the streakwise component of vorticity σ?

and the vertical component of vorticity σ?z have a similar magnitude of ?0.02 s?1. Vertical profiles of ε, σ?

, and σ?z showed a monotonic decrease from the surface. Traditionally, surface convergence streaks are regarded as signatures of Langmuir circulation. Two large-eddy simulations with and without Stokes drift were performed. Both simulations produced surface convergence streaks and vertical profiles of ε, vorticity, and velocity consistent with observations. The observations and model results suggest that the presence of surface convergence streaks does not necessarily imply the existence of Langmuir circulation. In a littoral surface boundary layer where surface waves are young, fetch-limited, and weak, and La = O(1), the turbulence mixing in the surface mixed layer is primarily due to the wind-driven shear turbulence, and convergence streaks exist with or without surface waves.