Unveiled the Black Sea Spring dynamics using Underwater Glider Observations

The Black Sea is a semi-enclosed basin with substantial river discharges. These inflows are crucial for the Black Sea's hydrography, nutrient supply, and ecological dynamics. The interplay of atmospheric forcing, river inflows, and mesoscale dynamics contribute to the formation of distinct water masses in the Black Sea. In the northern part, the extended shelf is prone to seasonal hypoxia and eutrophication, while the southern part is deep and stratified, marked by anoxic waters below 100m, rendering the Black Sea an immense meromictic sea. In the framework of the DOORS project (Developing Optimal and Open Research Support), during the DOORS field campaign, a first glider mission was performed in the Romanian Exclusive Economic Zone from May 6 to June 17, 2023, covering 288 nautical miles and conducting 863 physical and biogeochemical profiles at 1000m. The glider performed ten repeated transects perpendicular to the shelf parallel to the Danube Cone. Each transect has been completed approximately within four days, revealing the spatial and temporal characteristics of the region. At the shelf break, the isopycnals notably steepen and relax. In addition, intense surface heat gain of up to 3 ^oC induced strong stratification in two weeks, decreasing the density and MLD. The glider observations also captured small-scale eddies that contribute to the re-stratification process. These re-stratification events are essential to be monitored as they provide insights into the dynamic processes that affect the thermohaline characteristics of the water column and impact the nutrient availability in the euphotic layer. Understanding these events is essential for predicting and managing changes in the stratification, which plays a fundamental role in the upper layer circulation. By integrating glider-based observations with the broader regional Earth system dynamics context, our research supports a comprehensive understanding of the Black Sea's role in the global ocean climate system.