| 09 Apr 2026
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.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Ocean Science.
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General comments
This paper employs a variational method originally developed by some of the authors as an interpolating tool and later adapted for gap-filling High Frequency Radar (HFR) surface currents. The study's novelty lies in incorporating data from three distinct observing platforms (HFRs, drifters, and satellite altimetry) to generate a monthly surface current dataset for the Mediterranean Sea. The authors conclude that their newly generated dataset improves upon DUACS, the L4 gridded geostrophic surface current product derived from altimetric SSH observations. They state that their dataset captures additional dynamics compared to DUACS by integrating HFR and drifter data. Furthermore, they report observing a previously undocumented dynamical pattern in the Ionian Sea.
The overall aim of the paper is compelling. Using observational data for a basin-wide study of the Mediterranean is a valuable approach, because even though very relevant literature based on observations of the whole Mediterranean is available, research at such scale is more often based on numerical simulations. However, despite the interesting application of this novel methodology, the presentation of the results is unconvincing, and the writing quality currently falls a bit short of the standard expected for a scientific publication.
Specific comments
One of my main concerns relates to the spatial homogeneity and nature of the derived surface currents. While the authors assume their method captures additional dynamics than DUACS (supporting this claim by demonstrating an improved correlation with drifters compared to DUACS), the manuscript lacks a visualization of the drifter distribution used for the interpolation and for validation. Aside from the two trajectories in the eastern basin shown in Figure 3, the spatial coverage remains unclear. This omission raises the question of whether the claimed improvement over DUACS is heavily skewed by the western basin, where HFRs are concentrated, and perhaps drifters are more abundant (or maybe not, but I think a figure is missing with this information).
The authors mention that the selection of the validation drifters is random. Can we be sure the selected drifters are well distributed across the basin? I recommend showing the drifter positions and investigating the regional differences between the DIVAnd product and DUACS. Do differences increase in the Western Mediterranean, where HFRs are present and drifters might be more abundant? I recommend the authors to review Poulain et al. (2012) (10.1175/JPO-D-11-0159.1) who already use drifters together with altimetry to study the circulation of the Mediterranean and make a good representation of the data they use.
About the nature of the results: what are the calculated currents: total currents, something in between total and geostrophic? And, as mentioned earlier: would this definition be homogeneous throughout the entire basin, or does it depend on the density of HFR and drifter data versus altimeter observations? I think this question should be clarified.
The authors focus some of their subsequent analysis on the eastern basin, concluding their product outperforms the "less variable" geostrophic product. I believe it would be more interesting if the authors provided a robust quantitative analysis rather than relying solely on their figures to claim that the observed differences are genuinely attributable to their product capturing a wider range of dynamics than DUACS.
It would be interesting to relate the "twin gyres" section and, the conclusions the authors make, with existing literature. Menna et al., 2026 (https://doi.org/10.1088/2515-7620/ae558a) and references within could provide information to make a more complete discussion of the effects of warming waters on the dynamics of the Mediterranean Sea, and particularly the Ionian Sea.
Other comments