Intermittent supply of North Adriatic Dense Water to the deep South Adriatic Pit through gravity currents: an observational study

Severe winters in the northern Adriatic potentially generate gravity currents flowing along the eastern flank of the Adriatic, filling and ventilating the deepest layer of the southern Adriatic Pit with high-density water. The pulses of gravity current observed by data at the moorings in the Canyon of Bari (BB site) and the shelf-slope observation site (FF) are followed by strong fluctuations in the thermohaline properties in the pit observed at the E2M3A site in 2012, 2017, 2018 and 2022. While temperature was the main driver of gravity flow in 2012, salinity played an equal or greater role in the following extreme gravity current events. Thermohaline data from these three moorings show an arrival from mid-February to June and the relaxation phase of the high frequency oscillations (few tens of hours) lasts about two months. During this phase, the gravity current water displaces and mixes with the surrounding water masses. The gravity currents lead to a restratification of the water column, while local convection processes in winter time erode the stratification. The effects of gravity currents in the southern Adriatic have a profound impact on the Eastern Mediterranean circulation, influencing its thermohaline properties and facilitating the ventilation of deep waters. The Adriatic dense water formation adds to and competes with the convection in the Gulf of Lion, forming the dense waters in the Mediterranean which outflow through the Strait of Gibraltar into the northern Atlantic. The European Multidisciplinary Seafloor and water column Observatory (EMSO) South Adriatic Regional Facility (E2M3A in the pit, BB and FF at the edge of the pit) has been providing hourly data on temperature, salinity, oxygen and currents along the water column for about 15 years. This makes it possible to study these high-frequency small-scale processes and their interaction with the surroundings over an extended period of time and to assess their role in a changing climate.