Abstract

Recent observations from profiles of temperature and salinity in the Algerian Sea showed that salt finger mixing can significantly warm and salinify the deep waters within a period of 2 years, thereby contributing to the erosion of deep water properties formed during winter convection episodes. In this study, heat, salt, and buoyancy fluxes associated with thermohaline staircases are estimated using microstructure observations from four locations of the Western Mediterranean Sea: The Tyrrhenian Sea, the Algerian Sea, the Sardino-Balearic Sea, and the Ligurian Sea. Those fluxes are compared to the rare estimates found in the Mediterranean Sea. Microstructure data show that the temperature variance dissipation rate is one to three orders of magnitude larger in the strong steps that separate weakly stratified layers than in the layers, while the turbulent kinetic energy dissipation rate remains usually weak both in steps and layers. In the steps, the turbulent eddy diffusivity of salt is on average twice as large as that of temperature. The buoyancy flux ratio decreases with the density ratio. It is found that staircases induce a downward heat transfer rate of 46 to 103 × 109 W over the whole western basin, and a downward salt transfer rate of 4.5 to 10.3 × 103 kg s–1 between 1000 and 2000 m. This heat convergence is 2–5 times as large as the western Mediterranean geothermal heat flux in this depth range. Over the whole western basin, heat and salt convergences from salt-fingering staircases are 50% to 100% of those generated by mechanical mixing. Finally, it is found that heat and salt convergences from geothermal heating, salt-fingering and mechanical mixing can balance a deep water upwelling of 0.4 × 106 m3 s–1.

Highlights

  • MATERIALS AND METHODSStaircases form a remarkable characteristic of hydrological profiles of some regions of the western Mediterranean Sea

  • Noticeable is the property that staircases at a given station were not always present when the station was repeated along different cruises (Figure 1, color-filled versus empty markers)

  • Over the whole western Mediterranean Sea, we find that mechanical turbulence produces a heat convergence of 106 GW and a salt convergence of 9.2 × 103 kg s−1 between 1000 and 2000 m (Table 4)

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Summary

Introduction

MATERIALS AND METHODSStaircases form a remarkable characteristic of hydrological profiles of some regions of the western Mediterranean Sea. Interfaces have typical thicknesses of few meters up to few tens of meters, while layer thicknesses can reach several hundreds of meters Such profiles were observed in the Tyrrhenian Sea (Johannessen and Lee, 1974; Molcard and Tait, 1977; Zodiatis and Gasparini, 1996) as well as in the Algerian Sea (Krahmann, 1997; Bryden et al, 2014, B14 hereinafter; Taillandier et al, 2020). Those studies showed that staircases often exhibit persistence over time from months to years, and strong lateral coherence that can reach several tens of kilometers to hundreds of kilometers. The time coherence in the staircases of the central Tyrrhenian Sea was recently remarkably evidenced from a hydrographic station repeated from 2003 to 2016 (Durante et al, 2019)

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