Mediterranean Sea surface currents obtained with a variational inverse method, insight on the central Ionian Sea

Abstract. The surface currents of the Mediterranean Sea are interpolated with a variational inverse method for nine years (2013–2021). The general method is called DIVAnd and was adapted here specifically for surface currents as it considers the presence of the coastline and a divergence constraint. For the interpolation, three data sources are used; satellite altimetry, drifters and high-frequency radars. A subset of drifters (10 %) is used to calibrate the parametrization and to validate the results. Moreover, the results are compared with the geostrophic product computed per DUACS and was shown to be less accurate than the DIVAnd results. Using the new results, summer and winter seasons were averaged and compared to capture the most consistent patterns. At the basin scale, we concluded that the winter to summer variation is a shift between a coastal, large and well-defined currents to a variable and gyral circulation. In addition, we decomposed the Ionian Sea into orthogonal functions to investigate the appearance of what we called the Twin Gyres (TGs). These gyres, that settle around 36° N–17° E on the second part of the year, first appeared in the central Ionian Sea in 2020. The TGs were found to modify consequently the temperature profile of the central Ionian Sea. The mode decompositions showed it to be associated with the Atlantic Ionian Stream velocity and being associated with a temperature front, situated between the Adriatic waters and the southern Ionian waters.