Linking the Filomena snowstorm event in Spain to global warming

In early January 2021, an exceptional snowstorm hit Spain, causing widespread damage and chaos across the country. The extreme event was driven by the extratropical cyclone Filomena and left a snow cover of up to 50 cm in the capital, Madrid. In the days following the event, many media outlets hypothesized a link between the episode and climate change. Here we explore this link using a pseudo-global warming attribution approach. Specifically, we reproduce the event using the WRF regional atmospheric model forced with ERA5 reanalysis data. The thermodynamic fields, including temperature and humidity throughout the atmospheric column, skin temperature, and sea surface temperature, are perturbed using five different CMIP6 climate models in order to simulate the event in both a pre-industrial and a future (SSP5-8.5) scenario. In addition, the greenhouse gas concentrations are adjusted to their respective pre-industrial and future values, and the dynamic conditions (i.e., the atmospheric pattern) are fixed using a spectral nudging technique. We found that global warming affects snowfall amounts very unevenly across the country. At higher elevations and especially in northern areas, where the temperature increase is insufficient to convert snow to rain, the amount of snowfall increases due to a general increase in atmospheric water vapor available for precipitation. However, in lower elevations and especially in the south of the country, anthropogenic forcing results in a generalized reduction in snowfall amounts. Under an extreme future warming scenario, increases in snow water equivalent are found to be above +30% in many areas in the north, while in large parts of the south, snow is converted entirely to rain (-100%). Our results demonstrate that the effect of climate change on these cold and wet compound events is highly location-dependent, meaning that in one place the amount of snow recorded during one of these extreme events may increase due to warming, while in another place a few kilometres away the snow may disappear completely.