Decadal thermohaline evolution and water mass variability along the Ligurian and Tyrrhenian shelf under climate forcing (2009–2022)

Abstract. Understanding the impact of climate change on thermohaline dynamics in the western Mediterranean Sea is essential for assessing shifts in regional circulation and water mass formation. This study analyzes data from twelve summer oceanographic surveys conducted between 2009 and 2022 along the Ligurian and Tyrrhenian continental shelf. Results reveal a decadal-scale warming and salinification of both surface and intermediate layers, leading to enhanced water column stratification. On the northern Sicilian shelf, AtlanticWater exhibited a salinity increase of +0.02 yr⁻¹ and a potential temperature rise of +0.40 °C yr⁻¹ between 2011 and 2022, driven by sustained surface warming and advection of Ionian Surface Water from the eastern Mediterranean. This warmer and saltier water mass also affected intermediate depths, modifying vertical density gradients. The influence of Ionian Surface Water peaked around 2016, coinciding with the progressive warming of Tyrrhenian Intermediate Water due to the absence of strong winter cooling after 2014. These hydrographic changes are linked to the Adriatic–Ionian Bimodal Oscillating System and the transition of the Northern Ionian Gyre from a cyclonic to an anticyclonic state, which modulates the inflow of intermediate waters through the Sicily and Messina Straits. On the Ligurian shelf, Western Intermediate Water showed moderate warming (+0.06 °C yr⁻¹) until 2017, followed by cooling, and a salinity increase until 2019. These findings underscore the importance of inter-basin connectivity and the need for integrated monitoring and modeling to understand and anticipate climate-driven variability in the Mediterranean Sea.