Regimes of precipitation change over Europe and the Mediterranean

The Mediterranean region is experiencing pronounced aridification and in certain areas higher occurrence of intense precipitation. In this work, we analyze the evolution of the precipitation probability distribution in terms of precipitating days (or “wet-days”) and all-days quantile trends, in Europe and the Mediterranean, using the ERA5 reanalysis. Looking at the form of wet-days quantile trends curves, we identify four regimes. Two are predominant: in most of northern Europe the precipitation quantiles all intensify, while in the Mediterranean the low-medium quantiles are mostly decreasing as extremes intensify or decrease. The wet-days distribution is then modeled by a Weibull law with two parameters, whose changes capture the four regimes. Assessing the significance of the parameters' changes over 1950–2020 shows that a signal on wet-days distribution has already emerged in northern Europe (where the distribution shifts to more intense precipitation), but not yet in the Mediterranean, where the natural variability is stronger. We extend the results by describing the all-days distribution change as the wet-days’ change plus a contribution from the dry-days frequency change, and study their relative contribution. In northern Europe, the wet-days distribution change is the dominant driver, and the contribution of dry-days frequency change can be neglected for wet-days percentiles above about 50%. In the Mediterranean, however, the change of precipitation distribution comes from the significant increase of dry-days frequency instead of an intensity change during wet-days. Therefore, in the Mediterranean the increase of dry-days frequency is crucial for all-days trends, even for heavy precipitation.