On the interpretation of the steric and mass components of sea level variability: The case of the Mediterranean basin

[1] A common practice in sea level research is to analyze separately the variability of the steric and mass components of sea level. However, there are conceptual and practical issues that have sometimes been misinterpreted, leading to erroneous and contradictory conclusions on regional sea level variability. The crucial point to be noted is that the steric component does not account for volume changes but does for volume changes per mass unit (i.e., density changes). This indicates that the steric component only represents actual volume changes when the mass of the considered water body remains constant. This is for instance the case of thermal expansions/contractions due to surface heat fluxes. Conversely, salinity changes are often linked to net mass changes and therefore not only affect the steric component but also imply a change in the mass component that must be considered when quantifying the overall sea level budget. In this paper, we clarify the interpretation of the steric and mass components by identifying the main physical processes contributing to each of them, with particular emphasis on the role of salinity. The case of the Mediterranean Sea is examined as a paradigmatic example of a semienclosed basin where sea level components must be carefully interpreted. Because of the significant salinity changes shown by most projections of the Mediterranean climate, a misinterpretation of the steric and mass components can lead to underestimations of ~40 cm in the sea level of the Mediterranean Sea projected for the end of the 21st century.