Making the Mediterranean region since the late nineteenth century: From aridity indices to synoptic oscillations and the ‘return of the sea’

This article reconstructs how climatic sciences contributed to making the Mediterranean region through successive systems of classification, climatic indices, and synoptic circulation metrics. It does not seek to establish a new definitive Mediterranean boundary. Instead, it examines how particular variables, operations, thresholds, temporal scales, observational infrastructures, and institutional uses produced different climatic Mediterraneans. We combine quantitative mapping of three historically consequential index traditions on a common contemporary archive (ERA5-Land, 1994–2023) with a historical-epistemological analysis of their formulation and circulation. We compare De Martonne's aridity index, which represents a surface thermopluviometric balance; Rivas-Martínez's mediterraneity indices, which diagnose summer evaporative stress as a bioclimatic constraint; and a synoptic attribution diagnostic based on the Western Mediterranean Oscillation index, which relates wet-season precipitation variability to Atlantic and western Mediterranean pressure configurations. These operators do not offer interchangeable representations of a complete Mediterranean climate. They target different material processes and therefore generate different regional geometries. Aridity and bioclimatic thresholds produce relatively expansive terrestrial regions, whereas the synoptic diagnostic produces a narrower and more coastal western Mediterranean gradient. We argue that this divergence reveals the historicity of climatic regionalisation and a conceptual shift in which the Mediterranean Sea moves from an implicit boundary condition to an explicit causal actor.