State-of-the-art climate models project a substantial decline in precipitation for the Mediterranean region in the future¹. Supporting this notion, several studies based on observed precipitation data spanning recent decades have suggested a decrease in Mediterranean precipitation^2,3,4, with some attributing a large fraction of this change to anthropogenic influences^3,5. Conversely, certain researchers have underlined that Mediterranean precipitation exhibits considerable spatiotemporal variability driven by atmospheric circulation patterns^6,7 maintaining stationarity over the long term^8,9. These conflicting perspectives underscore the need for a comprehensive assessment of precipitation changes in this region, given the profound social, economic and environmental implications. Here we show that Mediterranean precipitation has largely remained stationary from 1871 to 2020, albeit with significant multi-decadal and interannual variability. This conclusion is based on the most comprehensive dataset available for the region, encompassing over 23,000 stations across 27 countries. While trends can be identified for some periods and subregions, our findings attribute these trends primarily to atmospheric dynamics, which would be mostly linked to internal variability. Furthermore, our assessment reconciles the observed precipitation trends with Coupled Model Intercomparison Project Phase 6 model simulations, neither of which indicate a prevailing past precipitation trend in the region. The implications of our results extend to environmental, agricultural and water resources planning in one of the world’s prominent climate change hotspots¹⁰.

Mountain systems within the Mediterranean region, e.g., the Pyrenees, are very sensitive to climate change. In the present study, we quantified the magnitude of extreme precipitation events and the number of days with torrential precipitation (daily precipitation ≥ 100 mm) in all the rain gauges available in the Pyrenees for the 1981–2015 period, analyzing the contribution of the synoptic scale in this type of event. The easternmost (under Mediterranean influence) and north-westernmost (under Atlantic influence) areas of the Pyrenees registered the highest number of torrential events. The heaviest events are expected in the eastern part, i.e., 400 mm day⁻¹ for a return period of 200 years. Northerly advections over the Iberian Peninsula, which present a low zonal index, i.e., implying a stronger meridional component, give rise to torrential events over the western Pyrenees; and easterly advections favour extreme precipitation over the eastern Pyrenees. The air mass travels a long way, from the east coast of North America, bringing heavy rainfall to the western Pyrenees. In the case of the torrential events over the eastern Pyrenees, the trajectory of the air mass causing the events in these areas is very short and originates in the Mediterranean Basin. The North Atlantic Oscillation (NAO) index has no influence upon the occurrence of torrential events in the Pyrenees, but these events are closely related to certain Mediterranean teleconnections such as the Western Mediterranean Oscillation (WeMO).

Aerosol concentration, size and composition are fundamental in hydrometeor formation processes. Meteorological models often use prescribed aerosol concentrations and a single substance. In this study, we analyze the role of aerosol concentration, acting both as CCN and IN, in the development of precipitation in a mixed phase system in numerical weather simulations. To this end, Storm Filomena was selected as the case study. In such a mixed-phase system, the coexistence of supercooled water with ice crystals, as well as the particular existence of a thermal inversion, led to the formation of precipitation in the form of rain, snow and graupel. Several high resolution experiments varying the fixed background aerosol concentration as well as a simulation with an interactive aerosol calculation were performed by means of the WRF-Chem model, using the same physics suite, domain and driving conditions. Results show that the total precipitation remains basically unaltered, with maximum changes of 5%; however, the production of snow is heavily modified. The simulation with maximum prescribed aerosol concentration produced 27% more snow than the interactive aerosol simulation, and diminished the graupel (74%) and rain production (28%). This redistribution of precipitation is mainly linked to the fact that under fixed ice crystal population the variation of aerosol concentration translates into changes in the liquid water content and droplet size and number concentration, thus altering the efficiency of precipitation production. In addition, while modifying the prescribed aerosol concentration produces the same precipitation pattern with the concentration modulating the precipitation amount, interactive aerosol calculation leads to a different precipitation pattern due to the spatial and temporal variability induced in the dynamical aerosol distribution.

Aerosols can strongly influence atmospheric circulation, and categorizing it into circulation types (CTs) helps in understanding the relationship between atmospheric forcing and surface conditions. However, few studies have considered the impact of interactive aerosols on atmospheric dynamics from a climatic perspective. This contribution aims to assess whether simulations with interactive aerosols (online solving of aerosol–radiation interactions, ARI, and aerosol–radiation–cloud interactions, ARCI) significantly impact atmospheric dynamics over Europe during winter compared to conventional regional climate models with prescribed aerosols. For that, Principal Component Analysis (PCA) has been applied to reduce the dimensionality of the problem in order to cluster different weather patterns. Results showed significant differences in the two predominant patterns, characterized by a western zonal flow (CT1) and a low-pressure system centered in Italy (CT2). The ARI experiment revealed a substantial reduction of surface level pressure over central-eastern Europe for CT1, resulting in a southward shift of the flux direction, and an increase in pressure over Scandinavia for CT2. The ARCI experiment exhibited a similar, but weaker effect. Furthermore, the study demonstrated the impact of aerosols on the frequency of different CTs and on the concentration of black and white aerosols. The findings of this study emphasize the significant role of aerosols in the atmospheric system and the need for further research to reduce uncertainty in meteorological and climatic experiments, particularly in the context of mitigating climate change.

Although the literature still debates how several anthropogenic and natural factors have contributed to the recent streamflow decline in the Iberian Peninsula, a continuing decrease in winter precipitation (WP) has been noticed in this area since the 1980s and has been associated with large-scale atmospheric drivers. This contribution assesses the potential propagation of this WP deficit into water resource variability. For this purpose, the novel “NEar-Natural Water Inflows to REservoirs of Spain” (NENWIRES) dataset was created. The results highlight that higher decreases in winter water inflows (WWIs) are always related to WP reductions. However, while the decline in WP was strongly provoked by the enhancement of the North Atlantic Oscillation index (NAOi) during the study period, WWI reductions could not be essentially linked to the NAOi behaviour in several NENWIRES catchments. Instead, the intensification of drought conditions and forest extension promoted WWI decreases over the target area and aided in understanding why WWI reductions were generally higher than WP decreases. In summary, most humid catchments registered a WWI decline that was mainly promoted by NAOi enhancement, whereas the extension of forest and evapotranspiration increases seem to explain WWI losses in semi-arid environments. This contribution sheds light on the recent debate regarding the magnitude and drivers of water resource decline over southern European regions.