Abstract

Thermohaline staircase structures are commonly observed in the western Mediterranean Sea within the halocline-thermocline connecting the Levantine Intermediate Water at about 400 m depth with the western Mediterranean deep waters below 1,500 m. In this halocline-thermocline where warmer, saltier waters overlie colder, fresher deep waters, salt finger mixing processes are thought to be active and produce staircases with layers of order 75 m thickness containing nearly constant properties separated by sharp steps of order 6 m thickness with jumps in properties between the layers. While the layers have nearly constant salinity, potential temperature, and potential density, each property decreases very slightly downward through the layer so that it appears that salinity, heat, and density are being put into the top of each layer and then convectively mixing downward through the layer. Such observations are consistent with salt finger processes that transport salinity, heat and density downward through the halocline-thermocline. Using repeat occupations of stations across the southern western Mediterranean Sea in 2006, 2008, and 2010, we have calculated downward salt transport, FS, of 5.35 × 10–8 psu m s–1, and downward heat transport, FT, of 12.4 × 10–8 °C m s–1. After multiplying these fluxes by haline contraction (β) and thermal expansion (α) coefficients respectively, the buoyancy flux ratio, αFT/βFS, is found to be 0.74 and there is a downward density flux of 1.0 × 10–10 W kg–1. The halocline-thermocline in this region between 600 and 1,400 dbar has a background vertical salinity gradient of 0.95 ×10–4 m–1 and a vertical temperature gradient of 4.1 × 10–4 °C m–1 so the background density ratio is Rρ = (αd/dz)/(βdS/dz) is 1.28. Dividing the downward fluxes by the background vertical gradients yields vertical diffusivities kS = 5.6 × 10–4 m2 s–1 and kT = 3.0 × 10–4 m2 s–1. These downward fluxes of salt and heat are compared with estimates based on salt finger experiments and theory and with the long-term increases in salinity and temperature in the deep western Mediterranean Sea over the past 40 years.

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