Turbulent entrainment in a strongly stratified barrier layer

TitleTurbulent entrainment in a strongly stratified barrier layer
Publication TypeJournal Article
Year of Publication2017
AuthorsPham H.T, Sarkar S.
JournalJournal of Geophysical Research-Oceans
Date Published2017/06
Type of ArticleArticle
ISBN Number2169-9275
Accession NumberWOS:000407088800035
Keywordsboundary-layer; currents; ekman layer; fluid; langmuir turbulence; large-eddy simulation; mixed-layer; model; upper-ocean; wind

Large-eddy simulation (LES) is used to investigate how turbulence in the wind-driven ocean mixed layer erodes the stratification of barrier layers. The model consists of a stratified Ekman layer that is driven by a surface wind. Simulations at a wide range of N-0/f are performed to quantify the effect of turbulence and stratification on the entrainment rate. Here, N-0 is the buoyancy frequency in the barrier layer and f is the Coriolis parameter. The evolution of the mixed layer follows two stages: a rapid initial deepening and a late-time growth at a considerably slower rate. During the first stage, the mixed layer thickens to the depth that is proportional to u(*)/root fN(0) p where u(*) is the frictional velocity. During the second stage, the turbulence in the mixed layer continues to deepen further into the barrier layer, and the turbulent length scale is shown to scale with u(*)/N-0, independent of f. The late-time entrainment rate E follows the law of E = 0.035Ri(*)(-1/2) where Ri(*) is the Richardson number. The exponent of -1/2 is identical but the coefficient of 0.035 is much smaller relative to the value of 2(-3/2) for the nonrotating boundary layer. Simulations using the KPP model (version applicable to this simple case without additional effects of Langmuir turbulence or surface buoyancy flux) also yield the entrainment scaling of E proportional to Ri(*)(-1/2); however, the proportionality coefficient varies with the stratification. The structure of the Ekman current is examined to illustrate the strong effect of stratification in the limit of large N-0/f.

Short TitleJ Geophys Res-Oceans
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