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Ocean Dynamics

, Volume 69, Issue 7, pp 787–807 | Cite as

Mixing efficiency from microstructure measurements in the Sicily Channel

  • Anda VladoiuEmail author
  • Pascale Bouruet-Aubertot
  • Yannis Cuypers
  • Bruno Ferron
  • Katrin Schroeder
  • Mireno Borghini
  • Stephane Leizour
  • Sana Ben Ismail
Article

Abstract

The dissipation flux coefficient, a measure of the mixing efficiency of a turbulent flow, was computed from microstructure measurements collected with a vertical microstructure profiler in the Sicily Channel. This hotspot for turbulence is characterised by strong shear in the transitional waters between the south-eastward surface flow and the north-westward deep flow. Observations from the two deep passages in the channel showed a contrast in turbulent kinetic energy dissipation rates, with higher dissipation rates at the location with the strongest deep currents. This study investigated the dissipation flux coefficient variability in the context of mechanically driven turbulence with a large range of turbulence intensities. The dissipation flux coefficient was shown to decrease on average with increasing turbulence intensity Reb, with median values of 0.74 for low Reb (< 8.5), 0.48 for moderate Reb (8.5≤ Reb < 400) and 0.30 for high Reb (≥ 400). The dissipation flux coefficient inferred from the measurements was systematically higher on average than the parameterisation as a function of turbulence intensity suggested by Bouffard and Boegman (Dyn Atmos Oceans 61:14–34, 2013). A plateau at moderate turbulence intensities was observed, followed by a decrease in the dissipation flux coefficient with increasing turbulence intensity as predicted by the parameterisation, but at higher turbulence intensity. The dissipation flux coefficient showed a strong variability with the water column stability regime for the different water masses. In particular, high dissipation flux coefficient (median 0.40) was found at Reb between 400 and 104 for the transitional waters at the northeastern passage, where dissipation rates were high, stratification and shear were strong but the Richardson number Ri was sub-critical. Vertical diapycnal diffusive fluxes were computed, and upward salinity sustained density fluxes of the order of 9 × 10−6 and 4 × 10−6 kg m−2 s−1 were found to be characteristic of the transitional (28 < σ < 29 kg m−3) and intermediate (σ > 29 kg m−3) waters, respectively. Turbulent mixing led to a lightening of the transitional and intermediate waters, which was consistent with previous estimates (Sparnocchia et al. J Mar Syst 20:301–317, 1999), but an order of magnitude lower when inferred from the (Bouffard and Boegman Dyn Atmos Oceans 61:14–34, 2013) parameterisation.

Notes

Acknowledgements

The authors wish to thank all crew members of the R/V Urania (CNR-ISMAR), as well as Alberto Ribotti (CNR-IAMC, Oristano) and Stefania Sparnocchia (CNR-ISAMR, Trieste), Chief Scientists during the Ichnussa2013 and Emso01 cruises, respectively. The data used in this paper were acquired within the framework of a project funded by CNR-ISMAR, LOCEAN, LOPS, and the HyMeX (HYdrological cycle in The Mediterranean EXperiment) and INSU-MISTRALS (Mediterranean Integrated STudies at Regional And Local Scales) programs. The microstructure profiler was funded by the French Agence Nationale de la Recherche (ANR) through Grant ANR-310 JC05-50690 and by the French Research Institute for Exploitation of the Sea (IFREMER). The authors are grateful to Ilker Fer, Stephen Monismith and an anonymous reviewer for their valuable input which improved the original manuscript.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  • Anda Vladoiu
    • 1
    • 2
    Email author
  • Pascale Bouruet-Aubertot
    • 1
  • Yannis Cuypers
    • 1
  • Bruno Ferron
    • 3
  • Katrin Schroeder
    • 4
  • Mireno Borghini
    • 5
  • Stephane Leizour
    • 3
  • Sana Ben Ismail
    • 6
  1. 1.Sorbonne Université - UPMC Paris VI - LOCEANParisFrance
  2. 2.Applied Physics LaboratoryUniversity of WashingtonSeattleUSA
  3. 3.CNRS, IFREMER, IRD, Laboratoire d’Océanographie Physique et Spatiale, IUEMUniversity of BrestBrestFrance
  4. 4.CNR-ISMARVeniceItaly
  5. 5.CNR-ISMARLericiItaly
  6. 6.Institut des Sciences et Technologies de la MerTunisTunisia

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