Twenty-first century thermohaline trends and abrupt shifts in the Ionian Sea

Elena Terzić^*Ivica Vilibić

• Rudjer Boskovic Institute, Zagreb, Croatia

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. Throughout the study period, pronounced warming and salinification were observed, occurring at rates much higher than during the 20th century. Between 2022 and 2024, the southern and south-eastern Ionian intermediate waters (300–1200 m) warmed by 0.7–1.8 ◦C and their salinity increased by 0.24–0.40, with maxima near 700–1000 m, reflecting an abrupt shift in water-mass properties. Concurrent ERA5 reanalysis reveals a pronounced negative winter heat-flux anomaly in 2021/2022, intensified wind stress, and elevated evaporation minus precipitation, all of which favored much pronounced convective mixing that transferred warm and saline waters to deeper layers. In parallel, anomalous intermediate-layer properties observed in the southern Adriatic during the same period suggest a broader, interconnected response of the Adriatic–Ionian system. Together, these processes may 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.