Thermohaline staircase formations in the Tyrrhenian Sea

Hydrographic measurements taken in the Tyrrhenian Sea in two different seasons revealed well pronounced thermohaline stepped structures lying between the Levantine Intermediate Water (LIW) and the Deep Water (DW). The investigated staircase, found in the depth range of 600–2500 m, was composed of four to eight homogeneous layers, each about 20–54 m thick, which were separated by thinner (about 2–27 m) interfaces. It encompassed an area of about 5 × 104 km2 with a lateral coherence of 150 km. However, while the structure of the staircase was strongly discernible in the deepest part of the basin, it became less defined towards the basin boundaries. Finestructure considerations enable the formation mechanism of the Tyrrhenian stepped structure to be more precisely understood: the stepped structure in this region has its origin in the buoyancy flux released by a double-diffusive process and primarily by salt fingers; the estimated values of heat-salt fluxes across the interfaces were sufficient to maintain the Tyrrhenian stepped structure over the entire period of observation. In the deepest part of the Tyrrhenian Basin the heat-salt fluxes, associated with the double-diffusive activity, appear to be one of the main mechanisms of exchange between the LIW and DW. The comparison with the available historical data, besides confirming the persistence of the stepped structure in this area, evidences for the period 1973–1992 a progressive reduction of step number together with a progressive increase of the step thickness. The relevance of long-term changes in the Tyrrhenian step-structure can give important information on the Mediterranean climate evolution.