Twenty-first century thermohaline trends and abrupt shifts in the Ionian Sea, a key water mass exchange region in the Mediterranean

Being centrally located within the basin and exhibiting internally driven quasi-decadal variability, the Ionian Sea serves as a pivotal conduit for water-mass exchange between the Western and Eastern Mediterranean. Using 23 years (2001–2024) of Argo float profiles, we quantify recent thermohaline changes across six sub-basins of the Ionian. On top of enhanced warming and salinification during the whole period, with much higher rates than during the 20th century, intermediate waters (100–1000 m) instantaneously warmed by 0.7–1.8 °C between 2022 and 2024, with salinity increasing by 0.24-0.40. The strongest anomalies were found in the southern and south-eastern Ionian sectors. Concurrent ERA5 reanalysis reveals a pronounced negative winter heat-flux anomaly in 2021/2022, intensified wind stress, and elevated evaporation–precipitation, all of which favored much pronounced convective mixing that transferred warm and saline waters to deeper layers. In parallel, the deep southern Adriatic experienced locally-driven unprecedented warming and salinification, weakening dense-water renewal and reducing the Adriatic contribution to the Ionian. Together, these processes indicate a transition toward a warmer, more saline deep-Ionian state. If sustained, such a regime could weaken dense-water formation, alter the Mediterranean overturning circulation, and propagate anomalies westward into the Atlantic through the Gibraltar outflow.