Water mass structure and deep mixing processes in the Tyrrhenian Sea: Results from the VECTOR project

In this study, we analyze data from observations conducted in the southern Tyrrhenian Sea, within the framework of the VECTOR project, initiated in 2006. In the six cruises organized as part of the project, in November 2006, in February, April and June 2007, in February 2008 and in January 2009, repeated hydrological measurements were collected along a transect. Data collected at the same stations in a subsequent cruise in November 2010 were also incorporated into the study. The main Tyrrhenian water masses were clearly identified in vertical sections. In particular, a positive salinity anomaly, observed both in the first (late autumn) and second (winter) cruise, associated with an anti-cyclonic structure, characterized the surface layer. The intermediate layer revealed salinity values higher than the climatological salinities, continuing the rising trend observed in the previous works, because of the influence of the Eastern Mediterranean Transient on the western basin hydrology. At the bottom, both temperature and salinity showed higher values with respect to the historical data, but were nearly constant during the study period. However, the water column between 600 m and 2500 m exhibited a trend of increases in temperature and salinity at a mean rate of 0.025°/y and 0.0075/y, higher than the findings in earlier studies. This discrepancy is likely due to the downward transfer of excess heat and salt from the intermediate depths. In the Tyrrhenian Sea, one of the main mechanisms responsible for transferring heat and salt in the deep layers is double diffusion, which is particularly active here. Double diffusion forms ‘staircase’ structures that are better developed and more stable than in other areas of the world’s oceans. Such structures are clearly seen in the analysis of the CTD data collected at an offshore station. These features occur at depths below 600 m, where the Levantine Intermediate Water (LIW) encounters the Tyrrhenian Dense Water, and mixing processes occur. As the Tyrrhenian Sea represents the main area of deep mixing for the waters from the eastern and western Mediterranean, systematic monitoring and quantification of changes in the Tyrrhenian intermediate and deep hydrological characteristics is critical. These data will prove to be particularly relevant to studies of the impact of climatic variability on the Mediterranean Sea.