Characterization of turbulence from a fine-scale parameterization and microstructure measurements in the Mediterranean Sea during the BOUM experiment

Abstract

Abstract.One of the main purposes of the BOUM experiment was to find evidence of the possible impact of submesoscale dynamics on biogeochemical cycles. To this aim physical as well as biogeochemical data were collected along a zonal transect through the western and eastern basins of the Mediterranean Sea. Along this transect 3-day fixed point stations were performed within anticyclonic eddies during which microstructure measurements of the temperature gradient were collected over the top 100 m of the water column. We focus here on the characterization of turbulent mixing. The analysis of microstructure measurements revealed a high level of turbulent kinetic energy (TKE) dissipation rate in the seasonal pycnocline and a moderate level below with mean values of the order of 10 −6 W kg −1 and 10 −8 W kg −1 , respectively. The Gregg Henyey (Gregg, 1989) fine-scale parameterization of TKE dissipation rate produced by internal wave breaking, and adapted here following Polzin et al. (1995) to take into account the strain to shear ratio, was first compared to these direct measurements with favorable results. The parameterization was then applied to the whole data set. Within the eddies, a significant increase of dissipation at the top and base of eddies associated with strong near-inertial waves is observed. Vertical turbulent diffusivity is increased both in these regions and in the weakly stratified eddy core. The stations collected along the East-West transect provide an overview of parameterized TKE dissipation rates and vertical turbulent diffusivity over a latitudinal section of the Mediterranean Sea. Strong TKE dissipation rates are found within the first 500 m and up to 1500 m above the bottom. Close to the bottom where the stratification is weak, the inferred vertical turbulent diffusivity can reach K z 10 −3 m 2 s −1 and may therefore have a strong impact on the upward diffusive transport of deep waters masses.

Influence of anticyclonic eddies on the Biogeochemistry from the Oligotrophic to the Ultraoligotrophic Mediterranean (BOUM cruise)

“…2c). These critical isopycnal interfaces in the eastern and western MS are also shown to be the interfaces where deep vertical diffusive coefficients abruptly change (Cuypers et al, 2012). Our results concerning the intermediate layer need to be discussed.…”

“…0.86 m 2 d −1 ). This K z value, with an estimated error of about 40 % ( Table 2 in Cuypers et al, 2012), was considered constant. N-input by N 2 fixation is the mean integrated value measured in the photic zone, and atmospheric deposition is a mean value taken in the absence of rain (Bonnet et al, 2011 and further references herein, Table 1).…”

“…The top nitracline (D NO 3 ) is reported directly from Table 1, along with the slope of the nitracline (S NO 3 ). N-input by upward diffusion, from the bottom of the water column, is the product of the eddy diffusion coefficient (K z ) measured at the level of the shallow nitracline (Cuypers et al, 2012) by the concentration gradient S NO 3 . The overall averaged K z found in each eddy and within eddies between the pycnocline basis and 100 m depth is ≈10 −5 m 2 S −1 (i.e.…”

“…Real measurements using high-frequency temperature measurements were carried out for the first time during the BOUM cruise (Cuypers et al, 2012) in order to provide accurate Kz. The mean value measured (0.86 m 2 j −1 ) is in the large 0.1-2 m 2 d −1 (i.e.…”

Introduction to the Biogeochemistry from the Oligotrophic to the Ultraoligotrophic Mediterranean (BOUM) experiment

“…The first estimations of turbulent dissipation inside anticyclonic eddies give evidence for a significant increase at the top and base of eddies which can be associated with strong near-inertial waves (Cuypers et al, 2012). Vertical turbulent diffusivity is increased both in these regions and in the weakly stratified eddy core (Cuypers et al, 2012).…”