Extreme precipitation events in the Mediterranean: Spatiotemporal characteristics and connection to large-scale atmospheric flow patterns

Abstract

The Mediterranean is strongly affected by Extreme Precipitation Events (EPEs), sometimes leading to negative impacts on society, economy, and the environment. Understanding such natural hazards and their drivers is essential to mitigate related risks. Here, EPEs over the Mediterranean between 1979 and 2019 are analyzed, using ERA5 dataset from ECMWF. EPEs are determined based on the 99th percentile of the daily distribution (P99). The different EPE characteristics are assessed, based on seasonality and spatiotemporal dependencies. To better understand the connection to large-scale atmospheric flow patterns, Empirical Orthogonal Function (EOF) analysis and subsequent K-means clustering are used to quantify the importance of weather regimes to EPE frequency. The analysis is performed for three different variables, depicting atmospheric variability in the lower and middle troposphere: Sea level pressure (SLP), temperature at 850 hPa (T850), and geopotential height at 500 hPa (Z500). Results show a clear spatial division in EPEs occurrence, with winter (autumn) being the season of highest EPEs frequency for the eastern (western) Mediterranean. There is a high degree of temporal dependencies with 20% of the EPEs (median value of all studied grid-cells), occurring up to 1 week after a preceding P99 event at the same location. Local orography is a key modulator of the spatiotemporal connections and substantially enhances the probability of co-occurrence of EPEs even for distant locations. The clustering clearly demonstrates the prevalence of distinct synoptic-scale atmospheric conditions during the occurrence of EPEs for different locations within the region. Results indicate that clustering based on a combination of SLP and Z500 can increase the conditional probability of EPEs by more than three (3) times (median value for all grid cells) from the nominal probability of 1% for the P99 EPEs.