'Surface saline lakes' in the Mediterranean Sea

In the Levantine Basin, it has long been known that salinity can reach a maximum in a thin layer near the surface, particularly during the warm season when summer heating, evaporation, and low mixing prevail. This water mass, termed the Levantine Surface Water, has historically been linked to the generation of Levantine intermediate and deep waters, depending on winter heat loss and wind-induced mixing. However, a recent study demonstrated that similar conditions, referred to as 'surface saline lakes' (SSLs), can occur as far north as the Adriatic Sea. To investigate this, we analyzed data from Argo profiling floats across all Mediterranean basins, focusing on the upper layers (up to 200 m in depth), where such lakes are known to form. We developed an objective algorithm to detect SSLs within profiles, defining an SSL by a salinity gradient exceeding -0.01 m⁻¹ at its base, combined with the uppermost salinity value exceeding the base salinity by at least 0.05. This definition allowed us to estimate SSL depth (corresponding to its base), temperature, potential density anomaly (PDA) gradients, and the Schmidt Stability Index, which quantifies the energy needed to mix SSLs. A further condition ensured the quasi-continuity of Argo profiles throughout the year, as SSLs are highly seasonal phenomena. Our analysis revealed that SSLs exhibit minimum or vanishing occurrences between February and April, while peaking between August and October. SSLs were detected in all Mediterranean basins, with the highest prevalence—65–70% of profiles between July and December—occurring in the Levantine Basin. During the August–October peak, SSLs exceeded 35% of monthly profiles in each basin, even in the Western Mediterranean, albeit with varying overall salinity levels and SSL variables ranges. These findings underscore the role of atmospheric heat and water exchange in all Mediterranean basins, influencing deeper thermohaline properties through winter mixing. Despite pronounced interannual and seasonal variability, our analysis of data showed a significant trend in SSL depth, accompanied by decreasing thermohaline gradients (temperature, salinity, PDA) at SSL bases though the investigated period. However, these trends may partly reflect sampling biases due to time-space differences in Argo float coverage, which has been substantial before 2013. The observed changes raise questions about their drivers—whether they indicate ongoing climate-change-induced salinization and shifts in Mediterranean water mass dynamics, or are merely manifestations of decadal variability.