Salinity Trends and Mass Balances in the Mediterranean Sea: The Role of Air-Sea Freshwater Fluxes and Oceanic Exchange

Abstract. Understanding the drivers of salinity and mass variability in the Mediterranean Sea is essential for assessing regional climate impacts and elucidating climate-driven changes in the water cycle. Although it is possible to close the Mediterranean mass and salinity budgets within uncertainty ranges, the relative contributions of key boundary fluxes, namely, surface freshwater fluxes (Evaporation-minus-Precipitation-minus-runoff) and water exchanges through the Strait of Gibraltar, remain unclear. To address this, we analyzed the Mediterranean’s mass and salinity budgets of 2003–2017 using ECCOv4r4. Our findings reveal a delicate balance between boundary fluxes that jointly regulate the Mediterranean’s mass and salinity dynamics. Surface freshwater fluxes play an essential but often understated role in modulating salinity through changes in volume: salinity decreases when precipitation adds volume, and increases when evaporation reduces volume. This insight further clarifies how the inflow of relatively fresh Atlantic water (AW) and the outflow of saltier Mediterranean water (MOW) can, counterintuitively, lead to an overall reduction in salinity. We found that surface fluxes, primarily driven by evaporation, account for most salinity variability, contributing approximately 1.80±0.10 Sv. The Gibraltar exchange is critical for maintaining mass balance, adding 0.30±0.20 Sv of salt. However, due to density differences between AW, MOW, and Mediterranean water, a net salinity reduction of -1.48±0.20 Sv is achieved through the Strait. This results in an overall increase of 0.29±0.09 Sv in salinity over the 15-year period, consistent with the salinification trend reported in previous studies. These findings provide a more comprehensive perspective of regional water cycle dynamics.