Permanent Thermohaline Staircases in the Tyrrhenian Sea.

The longest historical time series (14 years, from 2003 to 2016) of temperature and salinity of thermohaline staircases with highly homogeneous and reliable data ever observed is here presented and studied. The thermohaline staircase system of the central Tyrrhenian Sea is due to double diffusion in salt finger regime, and our study reveals its conservative behavior, oscillating among slightly different shapes, passing through merging processes, with a systematic upward drift of the interfaces. Data also show enhanced salt finger processes after 2010, near the bottom, promoted by the ingression from the Western Mediterranean of a new denser water mass due to the Western Mediterranean Transition. Our results are relevant for studying the mixing in the intermediate and deep region and open the way for modeling and theoretical follow‐up studies aimed to reproduce and explain these observations. Plain Language Summary: This study deals with a microscale mixing process usually denoted as double diffusion. Seawater masses contain different quantities of heat and salt and can be recognized by their resulting density. The molecular diffusion of heat is quicker than that of salt, resulting in a slow but very efficient mixing at the interface between two different water masses. In the Tyrrhenian Sea this peculiar phenomenon is well visible and shows very high persistency and progression: Vertical profiles of temperature and salinity look, in fact, like a staircase; such profiles have been first observed in the 1970s. Tyrrhenian staircases show variability; in particular, they have been used as a proxy to infer general water mass property changes (in turn linked to climate change), confirming the presence of a new, denser water at the bottom of the basin, and its ability to mix very quickly. Moreover, this study shows the longest time series ever observed. Key Points: This study deals with the longest thermohaline staircases time series ever observed, steps characterization and behaviorWe provide evidence of self‐regulating and merging in a thermohaline staircase systemWe evidence the quick mixing due to salt fingers in the Tyrrhenian interior [ABSTRACT FROM AUTHOR]