Recent thermohaline shifts in the Adriatic Sea and their potential consequences

The Adriatic Sea has warmed and salinified rapidly over the past two decades, and the changes are not uniform with depth. We combine 33 664 quality-controlled shipboard and Argo temperature and salinity casts from 1950–2025 into two climatologies, test the individual casts for regime shifts, and use the Argo profiles from the Southern Adriatic Pit to quantify the vertical structure of the changes. Between the 1950–1999 and 2000–2025 climatologies the southern Adriatic salinified at every depth and season, by 0.11–0.53, and warmed by up to 1.6 °C everywhere except at the uppermost levels in summer and autumn, where the sampling differs between the two periods. Amplification relative to the rest of the Mediterranean is strongly depth-dependent: below 700 m the southern Adriatic warms at 6.2 times the median rate of the other Mediterranean sub-basins, whereas above 200 m its warming is not amplified. The occurrence of near-surface salinity maxima (“surface saline lakes”) rises abruptly after 2014, in both ship and float records, from a few per cent of warm-season casts to about 40 %; this regime shift is only provable if vertical resolution and platform are accounted for. The intermediate salinity maximum has lost its dominant band: the detected maxima no longer cluster in one depth interval. Where two maxima coexist they have moved apart and diverged in density, so they are not one migrating core. Over 2014–2024 the steric contribution in the Southern Adriatic Pit is negative, dominated by halosteric contraction, so that local sea level rise must be mass-driven. Salt fingering has intensified below the classical intermediate core, allowing salt transport towards deeper layers. Three consequences follow, set out here as hypotheses. First, deep ventilation may persist under milder winters, because water that is already saline needs less cooling to sink, but each ventilation event may then deliver less oxygen, because the water that sinks is warmer. Second, exchange of oxygen, nutrients and carbon between the surface and intermediate layers may be either suppressed or enhanced beneath more frequent surface saline lakes, depending on whether stratification or double diffusion prevails. Third, Adriatic ecosystems, in which warming-linked mortality and species redistribution are already documented, face a basin changing faster at depth than at the surface. Testing these hypotheses requires sustained high-resolution observations.