Validation of Thorpe-scale-derived vertical diffusivities against microstructure measurements in the Kerguelen region

[摘要] The Thorpe scale is an energy-containing vertical overturning scale of largeeddies associated with shear-generated turbulence. This study investigatesindirect estimates of vertical diffusivities from the Thorpe scale method inthe polar front region east of the Kerguelen Islands based on fine-scaledensity profiles gathered during the 2011 KEOPS2 (KErguelen Ocean and Plateau compared Study 2) cruise. These diffusivitiesare validated in comparison with diffusivities estimated from the turbulencedissipation rate directly measured via a TurboMAP (Turbulence ocean Microstructure Acquisition Profiler) microstructure profiler.The results are sensitive to the choice of the diffusivity parameterizationand the overturn ratio R_o, and the optimal results have been obtained from theparameterization by Shih et al. (2005) and the R_o = 0.25 criterion, ratherthan the parameterization by Osborn (1980) and the R_o = 0.2 criterionoriginally suggested by Gargett and Garner (2008).

The Thorpe-scale-derived diffusivities in the KEOPS2 region show a highdegree of spatial variability, ranging from a canonical value ofO(10⁻⁵) m² s⁻¹ in the Winter Water layer and in the areaimmediately north of the polar front to a high value of O(10⁻⁴) m² s⁻¹in the seasonal thermocline between the surface mixed layer andthe Winter Water. The latter high diffusivities are found especially overthe shallow plateau southeast of the Kerguelen Islands and along the polarfront that is attached to the escarpment northeast of the islands. Theinteraction of strong frontal flow with prominent bottom topography likelycauses the observed elevated mixing rates.

[学科分类] 地球化学与岩石