The Tyrrhenian Sea exchanges water with the rest of the Mediterranean Sea through the Sardinia Channel,the Sicily Strait and the Corsica Channel, that represent morphologic constraints for the circulation of the intermediate and deep waters MILLOT87, ASTRALDI94, SPARNOCCHIA99, ASTRALDI01. The surface water (0-200 m) entering the Tyrrhenian Sea through the Sardinia Channel is the Modified Atlantic Water (MAW) from the Algerian Current (AC). The MAW is characterized by low salinity (on average less than 38 PSU), and flows cyclonically along the Italian coast. Through the Sicily Strait and deeper than 200 m down to about 700 m, the basin receives the Levantine Intermediate Water (LIW), which is marked by a subsurface temperature maximum and by a higher salinity (on average 38.8 PSU), and mixes with the surface MAW and deeper water masses. From about 700 m to the bottom the Tyrrhenian Deep Water (TDW) is present, being the result of the modification of the West Mediterranean Deep Water (WMDW) that crosses the Sardinia Channel. The circulation pattern in the Tyrrhenian Sea is normally characterized by two cyclonic gyres in the south and in the northern basins, and by the presence of cyclonic and anticyclonic eddies in the central basin. Interesting features in the TDW ZODIATIS95 are the thermoaline 'staircase' formations.
The physical oceanographic objective of this survey was to conduct seismic oceanography
[e.g. BUFFETT2009], that is, the method of using multi-channel seismic (MCS)
reflection profiling to image thermohaline finestructure in the ocean. In addition to the MCS profiling, a series of strategically located Expendable Bathythermograph (XBT) probes (which consist of a missile-shaped device with a thermocouple located at its nose cone) were launched. The XBT is attached to a pistol-shaped launching device that is connected to a personal computer. Once the probe is launched it falls under its own weight, while maintaining connection with the ship via a thin copper wire. As it falls, it records temperature variations in the ocean. It is capable of measuring vertical resolutions as small as 65 cm and temperature variations as small as 
0.1
C, BOYD1993.
Recently, the Mediterranean Occidental (MEDOC) survey RANERO2010, found significant thermohaline staircases in the deeper parts of the Tyrrhenian basin (Figure 1)
Thermohaline staircases are regular, well-defined, step-like variations in vertical temperature and salinity gradients that form when temperature and salinity increase with depth and nearly compensate with density, KELLEY1984. Turbulent mixing can disrupt the regular step-like structures, so they are typically found in regions where the Prandtl number (the ratio of viscous to thermal diffusion rates) is near unity and turbulent mixing is unusually weak MERRYFIELD2000. Therefore, isopycnal (equal density) stratification is more static than in regions dominated by turbulence.
Figure 1 - Map of the lateral distribution of XBT profiles in the Tyrrhenian Sea. Three main water masses are identified: a) Modified Atlantic Water (pink), b) Levantine Water (green) and c) Western Mediterranean Deep Water (blue), which is characterized by staircase formation.