Propagation and mixing of the 2023/24 inflow: impacts on stratification and deep-water ventilation in the Southern Baltic Sea

Introduction Major Baltic Inflows (MBIs) episodically ventilate deep Baltic basins, but detailed observations of mixing during their propagation remain scarce. The 2023/24 inflow introduced saline, oxygen-rich North Sea water into the Southern Baltic. Methods We combined autonomous ocean glider observations with ship-based CTD data across the Bornholm Basin (BB), Slupsk Furrow (SF), and Gdansk Basin (GB). Water-mass analysis was performed using T–S diagrams, while turbulence was estimated using Thorpe-scale methods. Results The inflow propagated as a bottom-intensified gravity current and under-halocline layers. Deep-water oxygen concentrations increased markedly (>6 mg L − ¹ in BB), while salinity rise remained modest (~0.5 PSU), indicating strong dilution. Elevated dissipation rates (ϵ > 10 −7 W kg − ¹) below the halocline and in the bottom boundary layer reveal active shear-driven mixing and internal wave processes. Oxygen inventory analysis shows an initial dominance of advective supply, followed by vertical mixing as the main oxygenation mechanism. Discussion The 2023/24 inflow caused temporary basin-wide deep-water oxygenation, but persistence depends on post-inflow stratification and biogeochemical demand.