New vision on the Ionian Sea abyssal variability through integrated approaches.

The processes involved in abyssal variability, their connections to the water column above, and their vulnerability to climate change are still unknown. This is due to a dearth of long-term observations below 2 km of depth. To fill these gaps, our approach leverages existing datasets using different methodologies. The Ionian Sea in the eastern Mediterranean is a unique location to investigate abyssal dynamics considering that it serves as a gateway between two major deep-water formation zones i.e., the Adriatic and Aegean seas. Moreover, the observed severe thermohaline variations experienced by the bottom layers of the Ionian Sea during the last decades, had dramatic effects on the deep circulation, leaving their signature on the whole Mediterranean Sea. The variation of the deep stratification, the causes that generate it, and the impact it produces are still under debate. The mechanisms involved in such variability, as well as the connection with the abyss and the above layers of the water column, are not fully understood. Through tidal and fine-scale mixing analysis, seafloor observations, and a tailored semi-analytical model we depicted the critical role of stratification, morphology, and tides in enhancing local diffusivity in the abyssal layer. Stratification, in particular, influences stability and the rotation of vorticity among different density layers, linking deep-sea variability to the entire water column. We achieved these results by combining different indirect methodologies exploiting all available resources at best with a synergic approach, getting a possible representation of what happens at depths.