Linking Euro-Atlantic Weather Regimes to Precipitation Patterns and Major Regional Flood Events in the Greater Alpine Region

This study investigates the relationship between Euro-Atlantic large-scale atmospheric circulation, characterized using year-round weather regimes (WRs), and major flood events in the Greater Alpine Region (GAR). With the goal of characterizing atmospheric conditions leading to major flood events and therefore gaining insights on their predictability, we identify the WR paths most commonly linked to these events in the GAR.

To this aim, we identify on average one major flood event per year over the GAR in the period 1951-2023 using discharge simulations from a regionalized rainfall-runoff model (a modified version of the TUW model) for the region. Of all the events identified, those selected were chosen based on three features: spatial extent, duration and intensity. These events cover large portions of the GAR, allowing the exploration of the role of large-scale atmospheric dynamics rather than local or convective processes.

WRs classification is based on daily geopotential height data at 500 hPa (Z500) from ERA5 reanalysis, following the methodology outlined by Grams et al. (2017), that enables the year-round characterization of atmospheric patterns.

Given the prevalence of regional floods during autumn, our analysis focuses on SON (September–October–November). We link flood events to their corresponding WRs at the time window included between peak day and two days before and compare their frequency of occurrence against the seasonal WRs climatology to isolate statistically significant associations. Results reveal that floods occur predominantly under Scandinavian Blocking (ScBL) and Scandinavian Trough (ScTr) regimes, which favor events of large-scale precipitation that can, in turn, lead to flood events during this season. Categorizing their magnitude using a quantile-based approach, we observe that precipitation events happening in proximity of floods associated with ScTr and ScBL regimes can be classified as extremes (magnitude exceeding 95th quantile). This result is also supported by the integrated water vapor (IVT) analysis, showing the presence of a larger south-north vapor transport from the Mediterranean basin towards the Alps during flood events when compared to the characteristic IVT patterns of ScBL and ScTr regimes.

Linking autumn floods over the GAR with specific large-scale atmospheric circulation regimes would support the potential use of WRs diagnostics as predictive tools to improve early warning systems for extreme hydrological events at present and, in principle, under future climate scenarios.