A regime diagram for classifying turbulent large eddies in the upper ocean

Abstract

A large eddy simulation (LES) model is used to examine how buoyancy-driven thermal convection, wind-driven shear turbulence and wind/wave-driven Langmuir circulation compete to generate turbulence in the ocean surface mixed layer. The turbulent Langmuir number La t, a ratio of friction velocity to surface Stokes drift velocity, and the Hoenikker number Ho, a ratio of buoyancy forcing to wave forcing, are two controlling dimensionless parameters. We explore low-order turbulence statistics in the La t and Ho parameter space for a wide range of atmospheric forcing conditions and construct a regime diagram to differentiate buoyancy-, shear- and wave-driven turbulence. All three types of turbulent flows are anisotropic but show different orderings of turbulence intensities: vertical > (downwind, crosswind) in convective turbulence; downwind>crosswind>vertical in shear turbulence; crosswind ≈ vertical> downwind in Langmuir turbulence. These orderings of turbulence intensities can be explained by examining the turbulence energy production in three directions. Buoyancy production in the vertical direction dominates turbulence generation in convective turbulence, whereas shear production in the downwind direction dominates turbulence generation in shear-driven turbulence. In Langmuir turbulence, however, Stokes production due to surface waves generates turbulence energy in both crosswind and vertical directions. Turbulence in the wind-driven upper ocean shows a transition from shear to Langmuir turbulence as La t decreases. A fully-developed sea state corresponds to La t≈0.3 and lies within the Langmuir regime. Vertical turbulence intensity in Langmuir turbulence is about two times larger than that in shear turbulence and falls into the range observed in the upper ocean. Hence the wind-driven upper ocean will be dominated by Langmuir turbulence under typical sea state conditions. Transition from Langmuir to convective turbulence occurs around Ho = O ( 1 ) , which is much greater than Ho = O ( 0.01 ) obtained using typical heat fluxes and wind speeds.