Abstract
Abstract. In the western Mediterranean Sea, Levantine intermediate waters (LIW), which circulate below the surface productive zone, progressively accumulate nutrients along their pathway from the Tyrrhenian Sea to the Algerian Basin. This study addresses the role played by diffusion in the nutrient enrichment of the LIW, a process particularly relevant inside step-layer structures extending down to deep waters – structures known as thermohaline staircases. Profiling float observations confirmed that staircases develop over epicentral regions confined in large-scale circulation features and maintained by saltier LIW inflows on the periphery. Thanks to a high profiling frequency over the 4-year period 2013–2017, float observations reveal the temporal continuity of the layering patterns encountered during the cruise PEACETIME and document the evolution of layer properties by about +0.06 ∘C in temperature and +0.02 in salinity. In the Algerian Basin, the analysis of in situ lateral density ratios untangled double-diffusive convection as a driver of thermohaline changes inside epicentral regions and isopycnal diffusion as a driver of heat and salt exchanges with the surrounding sources. In the Tyrrhenian Sea, the nitrate flux across thermohaline staircases, as opposed to the downward salt flux, contributes up to 25 % of the total nitrate pool supplied to the LIW by vertical transfer. Overall, however, the nutrient enrichment of the LIW is driven mostly by other sources, coastal or atmospheric, as well as by inputs advected from the Algerian Basin.
Highlights
The Mediterranean Sea is an ultraoligotrophic basin where primary production is generally weak because nutrient fluxes into the sunlit surface layers are very low during most of the year
We focus on the role played by diapycnal diffusion in the distribution of nutrients in the western Mediterranean Sea considering nutrient fluxes between DW, Levantine intermediate waters (LIW) and surface waters, given the regional characteristics of mixing processes
During the cruise PEACETIME, repeated casts were carried out at a station located in the central Tyrrhenian Sea, a well-characterized deep area where intense thermohaline staircases are confined (Molcard and Tait, 1977; Zodiatis and Gasparini, 1996; Falco et al, 2016; Durante et al, 2019)
Summary
The Mediterranean Sea is an ultraoligotrophic basin where primary production is generally weak because nutrient fluxes into the sunlit surface layers are very low during most of the year. In the western Mediterranean, deep waters (hereinafter DW) result from winter convection in the Provençal Basin and in the Ligurian Sea (Medoc Group, 1970; Prieur et al, 1983). Levantine intermediate waters (hereinafter LIW) originate mostly through shallow convection in different sites of the Levantine Basin (Nittis and Lascaratos, 1999; The LIWEX Group, 2003). The thermohaline circulation cells involving DW are distinct in the eastern and the western Mediterranean basins, whereas the one driven by LIW encompasses the whole Mediterranean. They regionally distribute nutrient stocks, through DW that spread from
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