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Abstract. A compilation of data from several cruises between 1998 and 2013 was used to derive polynomial fits that estimate total alkalinity (A T ) and total dissolved inorganic carbon (C T ) from measurements of salinity and temperature in the Mediterranean Sea surface waters. The optimal equations were chosen based on the 10-fold cross-validation results and revealed that second-and third-order polynomials fit the A T and C T data respectively. The A T surface fit yielded a root mean square error (RMSE) of ± 10.6 µmol kg −1 , and salinity and temperature contribute to 96 % of the variability. Furthermore, we present the first annual mean C T parameterization for the Mediterranean Sea surface waters with a RMSE of ± 14.3 µmol kg −1 . Excluding the marginal seas of the Adriatic and the Aegean, these equations can be used to estimate A T and C T in case of the lack of measurements. The identified empirical equations were applied on the 0.25 • climatologies of temperature and salinity, available from the World Ocean Atlas 2013. The 7-year averages (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012) showed that A T and C T have similar patterns with an increasing eastward gradient. The variability is influenced by the inflow of cold Atlantic waters through the Strait of Gibraltar and by the oligotrophic and thermohaline gradient that characterize the Mediterranean Sea. The summer-winter seasonality was also mapped and showed different patterns for A T and C T . During the winter, the A T and C T concentrations were higher in the western than in the eastern basin. The opposite was observed in the summer where the eastern basin was marked by higher A T and C T concentrations than in winter. The strong evaporation that takes place in this season along with the ultra-oligotrophy of the eastern basin determines the increase of both A T and C T concentrations.

Measurements of the CO 2 system parameters in the Mediterranean Sea are relatively scarce and not representative for all its sub-basins. High quality data collected on May 2013 during the 2013 MedSeA cruise covering the whole basin were used to provide for the first time linear relationships estimating the total alkalinity (A T) and the total dissolved inorganic carbon (C T) from salinity in each Mediterranean basin and sub-basin at different depth layers. These correlations show that a substantial quantity of alkalinity is added to the seawater during its residence time in the Mediterranean Sea, whereas the biological processes, the air-sea exchange and the high remineralization rate are responsible of the high C T concentrations in this sea. Moreover, these fits could be used to estimate the A T and C T from salinity where there are not available measurements of the carbonate system parameters.

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