Projected evolution of dust-favourable weather regimes over the western Mediterranean across different climate scenarios

Large-scale atmospheric circulation and synoptic systems strongly modulate regional environmental variability. Over the western Mediterranean, Saharan dust outbreaks provide a clear circulation-controlled signal, with consequences for aerosol loading, visibility, and air quality. Here we quantify how dust-favourable circulation regimes evolve from recent decades to the end of the 21st century under multiple Shared Socioeconomic Pathways (SSP1-2.6 to SSP5-8.5), using a fixed-centroid synoptic classification applied consistently to reanalyses and daily CMIP6 circulation fields.
Daily atmospheric circulation states were characterized by 850 hPa geopotential height ERA5 reanalysis data fields over North Africa and the western Mediterranean for the period 1980-2014. Each day was assigned to one of 11 pre-defined circulation types (weather regimes) using a non-hierarchical K-means cluster analysis procedure (Salvador et al., 2022). Along with daily regime labels, we retained distances to the nearest centroid and performed internal diagnostics (e.g., centroid stability and assignment consistency) to ensure that the fixed-reference classification remained comparable across datasets.
We linked regimes to an observable regional indicator by evaluating dust relevance using two independent datasets: (I) satellite-constrained dust aerosol extinction optical depth from the MERRA-2 reanalysis (DUEXTTAU) and (II) an observational catalogue of Saharan dust days identified over the Iberian Peninsula. For each regime we quantified conditional dust occurrence and typical dust loading, identifying 6 of the 11 regimes as consistently dust-favourable, i.e., systematically enhancing Saharan dust export and advection into Iberia and the western Mediterranean.
The same classification was applied to 15 CMIP6 models for historical (1980–2014) and future (2020–2100) simulations. We validated each model against ERA5 using metrics that capture agreement in overall regime frequencies, seasonal cycle, interannual variability, and trends, providing an objective basis to interpret uncertainty and to optionally filter or weight models prior to projection.
Finally, we built multi-model ensembles for each SSP and diagnose changes in regime frequency, seasonality, and trend significance through the 21st century. Across SSPs we find a robust increase in dust-favourable regimes, with the strongest changes under SSP5-8.5. In an equal-weight SSP5-8.5 ensemble, the fraction of days assigned to dust-favourable regimes increases from 61.5% (2020s) to 74.5% (2090s), while individual models show larger increases (up to ~20 percentage points), implying sensitivity to model weighting. Regimes typically associated with summer dust transport also become more frequent in spring, indicating a seasonal expansion of dust-conducive synoptic conditions.
By translating projected circulation changes into interpretable regime statistics tied to dust occurrence and loading, this framework provides a transparent bridge between large-scale dynamics and future regional dust-related aerosol variability over the western Mediterranean.