New Coupled Reanalysis Reconstructs 32 Years of Mediterranean Climate Change

The Mediterranean Sea is warming faster than almost anywhere else on Earth, and the consequences ripple far beyond its shores, touching water security, fisheries, coastal cities, and the intensity of summer heatwaves across three continents. Yet reconstructing exactly how this climate hotspot has changed over the past three decades has long required stitching together separate atmospheric and oceanic products, each with its own physics, resolution, and blind spots. A team at Italy’s National Research Council has now changed that. In a study published in Ocean Science, Andrea Storto, Vincenzo de Toma, and Chunxue Yang introduce MESMAR-R, a coupled regional reanalysis that reconstructs the Mediterranean climate system continuously from 1993 to 2024, weaving together weather, ocean, and river-discharge models with decades of satellite and in-situ observations.

Reanalyses occupy a special place in climate science. They are essentially the most honest simulations a modelling system can produce: a numerical model runs forward in time while a data assimilation scheme continuously nudges it back toward reality using observations. The result is a physically consistent, gap-free record of the past. Global products such as ERA5 for the atmosphere or GLORYS12 for the ocean do this superbly at planetary scales, but their horizontal resolution is too coarse to capture the Mediterranean’s notoriously complex geography, where steep mountains, narrow straits, and intense land-sea contrasts generate mesoscale processes that global grids simply blur out. Regional reanalyses exist for the Mediterranean, but until now they have been either ocean-only or atmosphere-only, never both at once.

MESMAR-R closes that gap by coupling three model components through the OASIS3-MCT coupler at half-hourly intervals. The atmospheric core is the Weather Research and Forecasting model running on a grid of roughly 15 kilometres with 41 vertical levels, while the ocean component uses the NEMO model at approximately 7 kilometres with 72 vertical levels spanning the entire basin from the Atlantic box to the Dardanelles. A hydrological discharge model routes surface and subsurface runoff collected by the atmospheric model directly into coastal ocean points, meaning rivers genuinely interact with the sea rather than being prescribed from climatology. Air-sea fluxes of momentum, heat, and freshwater are computed within the atmospheric model to guarantee consistency with its boundary-layer physics, then passed conservatively to the ocean, while sea surface temperature and currents flow back the other way.

The assimilation strategy is deliberately asymmetric. The ocean receives a full three-dimensional variational analysis every three days, ingesting temperature and salinity profiles from Argo floats, gliders, moorings, and expendable bathythermographs, along with along-track satellite altimetry, weak surface relaxations, and a large-scale bias correction for the deep layers. The atmosphere, by contrast, is constrained through spectral nudging toward ERA5: only spatial scales longer than roughly 850 kilometres are relaxed, with timescales of about an hour for winds and temperature and a day for humidity. This leaves the regional model free to develop its own mesoscale weather, including Mediterranean tropical-like cyclones, while remaining anchored to the large-scale circulation of the global reanalysis. The authors describe the system as weakly coupled, since observations in one medium do not instantly correct the other, but information propagates between components dynamically through the exchanged fluxes during each model integration.

The evaluation against observations is where the new product earns its credentials. Over the common comparison period of 2000 to 2020, MESMAR-R delivers the lowest root-mean-square error of any tested product for sea surface height, with typical errors below four centimetres basin-wide, and it more than halves the sea surface salinity error relative to the global GLORYS12 reanalysis. For sea surface temperature it is essentially indistinguishable from the dedicated regional ocean reanalysis MEDREA24, and both substantially outperform the global product. In the upper ocean, the layer that matters most for air-sea interaction, MESMAR-R shows the lowest temperature errors in the 20 to 60 metre range and leads or ties for best salinity performance in the top 210 metres. Bias profiles reveal only a small fresh bias near the surface and near-zero thermal drift below 200 metres, indicating that the system stays honest at depth.

The atmospheric component, handicapped by its simplified nudging-based assimilation, naturally trails the dedicated ERA5 and CERRA reanalyses in area-averaged skill. Root-mean-square errors for two-metre temperature, relative humidity, wind speed, and pressure are moderately higher, and the gap is largest for precipitation over land. But the comparison maps tell a subtler story: MESMAR-R achieves localized improvements near complex coastlines and orography, precisely where the 15-kilometre model resolves gradients that coarser reference datasets smooth away. Crucially, the atmospheric fields remain internally consistent with the ocean beneath them, which is the entire point of a coupled product. For studying compound events, where a marine heatwave coincides with an atmospheric heat dome, that consistency matters more than shaving a few hundredths of a degree off an error metric.

Sensitivity experiments disentangle the contributions of coupling and data assimilation, and they deliver one of the study’s most instructive findings: coupling alone does not improve a reanalysis. A fully coupled simulation with no data assimilation showed the largest error growth, particularly in the subsurface ocean, while an uncoupled ocean reanalysis forced by ERA5 slightly outperformed the full coupled system for most parameters. Yet coupling measurably improved sea surface salinity, especially in the Aegean Sea, where the interactive exchange of freshwater fluxes enhances realistic evaporation and cools and salinifies the surface in ways prescribed forcing cannot replicate. Ocean data assimilation, meanwhile, proved to be the dominant control on basin-scale heat content, with the reanalysis reproducing the observed warming rate of roughly 1.3 watts per square metre in the upper 700 metres, against an observational estimate of about 1.4 watts per square metre, and correlations with observed heat content reaching 0.93 to 0.94.

The long-term climate trends extracted from the reconstruction read like a diagnosis of a basin under stress. Total Mediterranean sea level rose at approximately 3.06 millimetres per year over 1993 to 2024, in excellent agreement with independent estimates, and a striking 94 percent of that rise is mass-driven, reflecting the basin’s response to Atlantic sea-level rise through the Strait of Gibraltar rather than local thermal expansion. Beneath that total, a powerful thermo-halo compensation plays out: the halosteric component of sea level falls at nearly 3.9 millimetres per year basin-wide as the sea salinifies, almost exactly cancelling the thermosteric rise from warming. The eastern basin warms fastest in heat content, consistent with the documented abrupt warming and salinification of Levantine Intermediate Water since the early 2000s, while the western basin shows the strongest salinity-driven density increase, a hotspot centred on the Alboran Sea and the western gyre.

The atmospheric trends confirm the region’s reputation as a climate change amplifier. Winter warming averages about 0.040 degrees Celsius per year across the Mediterranean domain, accelerating to roughly 0.047 degrees per year in summer, with the strongest signals over southern Europe. Precipitable water increases almost everywhere, by about 0.023 percent per year in winter and a remarkable 0.100 percent per year in summer, yet relative humidity near the surface declines across large parts of Europe and mean precipitation does not follow the moisture upward. The picture is a warming, drying regime in which the atmosphere holds more water but delivers it in fewer, more intense bursts, favouring longer dry spells punctuated by extreme events. Winter wind trends even leave fingerprints in the ocean, with localized strengthening over the Gulf of Lion deepening the mixed layer while the southeastern basin shoals as buoyancy loss weakens.

The authors are candid about limitations. Observational coverage is not homogeneous across the 32 years, and because the atmospheric component is nudged toward ERA5, it partly inherits the time-varying information content of the global observing system, so decadal trends should be interpreted with that caveat in mind. Mid-depth temperature skill trails MEDREA24, and localized degradations near the northern Adriatic and the Dardanelles point to riverine and boundary-condition challenges. Future versions may adopt observation-space atmospheric assimilation and fully balanced air-sea corrections. Even so, MESMAR-R stands as one of the first regional coupled reanalyses ever produced, and its balanced, physically coherent reconstruction of the Mediterranean’s recent past arrives at a moment when the region needs it most, offering researchers, forecasters, and even machine-learning emulators a single dataset in which the sea and the sky finally tell the same story.

Subject of Research: A coupled regional atmosphere-ocean-hydrology reanalysis of the Mediterranean Sea covering 1993-2024, evaluated against observations and used to quantify regional climate trends

Article Title: Evaluation of a coupled regional reanalysis for the Mediterranean region covering the period 1993–2024

Article References: Storto, A., de Toma, V., & Yang, C. (2026). Evaluation of a coupled regional reanalysis for the Mediterranean region covering the period 1993–2024. Ocean Science, 22(5), 2809-2834. https://doi.org/10.5194/os-22-2809-2026

Image Credits: AI Generated

DOI: 10.5194/os-22-2809-2026

Keywords: Mediterranean Sea, reanalysis, data assimilation, coupled modelling, climate change, sea level rise, ocean heat content, salinification, air-sea interactions, NEMO, WRF, marine heatwaves