Salinity-driven dynamics in the central Mediterranean in the era of ocean warming

Surface currents play a key role in determining the thermohaline structures in the upper ocean and thus influence its mean state and interannual variability. In turn, the thermohaline structure and its variability can also affect surface currents by changing density and pressure gradients in the ocean. At the present, the thermohaline effects on surface currents are poorly understood, particularly with regard to the role of salinity, which is considered less important than other factors influencing the circulation field (such as wind and sea level). However, in an era characterised by ocean warming, which affects ocean stratification and raises sea level, the contribution of ocean density to circulation in the upper layer becomes significant, and salinity could play a leading role in driving thermohaline variability and dense water formation processes.

Within the Mediterranean, the Central Mediterranean Sea (CMed), consisting of the Ionian and Adriatic Seas, is a good indicator of variability for several reasons. Firstly, it is a crossing point of all waters making part of the zonal Mediterranean overturning circulation, i.e. the Atlantic Water (AW) and the Levantine Intermediate Water (LIW). Secondly, the CMed can be considered a connecting point between the zonal and the meridional overturning cells, as it is also one of the sites of open-ocean deep convection and dense water formation of the Mediterranean Sea. It is also considered a gauge of the quasi-decadal variability of the whole Mediterranean Sea.

In recent decades, the water column of the CMed experienced significant positive trends in temperature and salinity. From 2012 onwards, there was a steep increase in salinity, with record breaking values observed in 2021. At the same time, the vorticity field in the upper layer of the Southern Adriatic Gyre (SAG) increased significantly, with the mean value doubling from the end of 2012 compared to the previous period. In this work, the main sources of this enhanced vorticity field in the SAG are analysed using in-situ (Argo-float and ocean glider), satellite and model products. Wind, horizontal advection and baroclinic terms interact to cause the increase of vorticity in 2012, but this new state is mainly supported by advection and baroclinicity in the following period (2012-2023). The baroclinic contribution associated with density gradients is comparable in magnitude to wind stress and shows the largest correlation with the temporal variability of relative vorticity in the period 2012-2023. A high-resolution analysis performed using glider data highlights the greater influence of the salinity field compared to the temperature field in modulating the shape of the isopycnals in the surface layer of the SAG. This condition leads to an enhanced positive contribution of the circulation field to the vorticity field, especially during periods of AW inflow along the edges of the SAG.

The results of this work emphasise the role of salinity in shaping the thermohaline variability of the CMed with direct effects on the surface currents field.