Evaluating Langmuir turbulence parameterizations in the ocean surface boundary layer

Abstract

It is expected that surface gravity waves play an important role in the dynamics of the ocean surface boundary layer (OSBL), quantified with the turbulent Langmuir number ( La=u*/us0, where u* and us0 are the friction velocity and surface Stokes drift, respectively). However, simultaneous measurements of the OSBL dynamics along with accurate measurements of the wave and atmospheric forcing are lacking. Measurements of the turbulent dissipation rate ϵ were collected using the Air-Sea Interaction Profiler (ASIP), a freely rising microstructure profiler. Two definitions for the OSBL depth are used: the mixed layer derived from measurements of density (hρ), and the mixing layer (hϵ) determined from direct measurements of ϵ. When surface buoyancy forces are relatively small, ϵ∝La−2 only near the surface with no dependency on La at mid-depths of the OSBL when using hρ as the turbulent length scale. However, if hϵ is used then the dependence of ϵ with La−2 is more uniform throughout the OSBL. For relatively high destabilizing surface buoyancy forces, ϵ is proportional to the ratio of the OSBL depth against the Langmuir stability length LL. During destabilizing conditions, the mixed and mixing layer depths are nearly identical, but we have relatively few measurements under these conditions, rather than any physical implications. Observations of epsilon are compared with the OSBL regime diagram of Belcher et al. (2012) and are generally within an order of magnitude, but there is an improved agreement if hϵ is used as the turbulent length scale rather than hρ.