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

Abstract

Abstract. The Thorpe scale is an energy-containing vertical overturning scale of large eddies associated with sheargenerated turbulence. This study investigates indirect estimates of vertical diffusivities from the Thorpe scale method in the polar front region east of the Kerguelen Islands based on fine-scale density profiles gathered during the 2011 KEOPS2 (KErguelen Ocean and Plateau compared Study 2) cruise. These diffusivities are validated in comparison with diffusivities estimated from the turbulence dissipation rate directly measured via a TurboMAP (Turbulence ocean Microstructure Acquisition Profiler) microstructure profiler. The results are sensitive to the choice of the diffusivity parameterization and the overturn ratio R o , and the optimal results have been obtained from the parameterization by Shih et al. (2005) and the R o = 0.25 criterion, rather than the parameterization by Osborn (1980) and the R o = 0.2 criterion originally suggested by Gargett and Garner (2008).The Thorpe-scale-derived diffusivities in the KEOPS2 region show a high degree of spatial variability, ranging from a canonical value of O(10 −5 ) m 2 s −1 in the Winter Water layer and in the area immediately north of the polar front to a high value of O(10 −4 ) m 2 s −1 in the seasonal thermocline between the surface mixed layer and the Winter Water. The latter high diffusivities are found especially over the shallow plateau southeast of the Kerguelen Islands and along the polar front that is attached to the escarpment northeast of the islands. The interaction of strong frontal flow with prominent bottom topography likely causes the observed elevated mixing rates.

Significant mixed layer nitrification in a natural iron‐fertilized bloom of the Southern Ocean

“…Satellite Chl a observations shows a clear spring peak in biomass with less summer continuation. The vertical NO 3 − diffusive flux is also estimated to be 0.2 ± 0.2 mol m −2 , from the NO 3 − concentration gradient (5.2 ± 3.4 µmol L −1 ; between 0–100 and 140–160 m depth stratum) and the estimated vertical diffusivity (3.0 ± 1.5 × 10 −4 m −2 s −1 ) above the southeast Kerguelen Plateau [ Park et al ., , ].…”

Particulate matter stoichiometry driven by microplankton community structure in summer in the Indian sector of the Southern Ocean

“…The turbulent diffusivity coefficient was computed with the Thorpe scale method using the Shih et al () parameterization as previously described in Park et al (). The robustness of the Thorpe scale calculation using this indirect method depends on the level of CTD processing prior to the computation (Park et al ). The diffusivity coefficient (Kz in m 2 s −1 ) was calculated as follows: where ν is the cinematic viscosity of seawater (1.5–1.8 10 −6 m 2 s −1 for T = 0–5°C), L t is the Thorpe scale (vertical density overturning scale, in m) and N is the Brunt‐Väisalä buoyancy frequency (s −1 ) defined as: where g is the gravitational acceleration (9.81 m s −1 ), ρ e is a constant reference density for seawater, ρ is the seawater density and z is the depth (m).…”

“…Each Kz profile was averaged in 10 m bins. The Thorpe scale method cannot resolve overturns smaller than 20 cm, consequently Kz values < 10 −5 m 2 s −1 were set to this minimal value based on in situ measurements around the Kerguelen plateau with a Turbo MAP profiler (Park et al ).…”

“…The turbulent diffusivity coefficient was computed with the Thorpe scale method using the Shih et al (2005) parameterization as previously described in Park et al (2014). The robustness of the Thorpe scale calculation using this indirect method depends on the level of CTD processing prior to the computation (Park et al 2014). The diffusivity coefficient (Kz in m 2 s 21 ) was calculated as follows:…”

“…Each Kz profile was averaged in 10 m bins. The Thorpe scale method cannot resolve overturns smaller than 20 cm, consequently Kz values < 10 25 m 2 s 21 were set to this minimal value based on in situ measurements around the Kerguelen plateau with a Turbo MAP profiler (Park et al 2014). The mixed layer depth (MLD) was calculated using a 0.02 kg m 23 density-difference criterion relative to the density at 20 m (Park et al 1998).…”