What happens in the deep ocean? Observations, parameterizations and models of the deep ocean and in the Mediterranean.

The exchange between the ocean's turbulent surface boundary layer and the underlying stratified water column can occur over a wide range of timescales. On the other hand, deep water takes many decades to return to the surface, acting as reservoir of heat and $CO_{2}$, contributing to the climate decadal variability. The bottom dynamics has been re-evaluated in recent papers, in which the interplay between downward and upward energy propagation has updated the original vision of Munk (1966). The Mediterranean basin is a good example to explore this physical (upside-down) mechanism. Recently, in the Ionian Sea, a significant positive shift of the ocean heat content between 2000 and 4000 m has been observed during thirty years, from 1977 to 2011. This intriguing heat storage is due to the bottom-driven mixing processes, produced by the ventilation of the deep layers of two different deep-water sources, originating from the Adriatic and the Aegean, respectively. From the above, we intend to discuss the role of the deep mixing in the abyssal circulation in the Mediterranean Sea, considering new in situ data exploring the deep dynamics and modeling results.