The role of subtle dynamical changes in the European extreme rainfall of July 2021

In July 2021 record-breaking extreme rainfall occurred in Western Europe causing huge damage and loss of life. The event was associated with a persistent cut-off low pressure system. We assess the influence of the large-scale dynamics on the extreme rainfall, evaluating the sensitivity of the rainfall to the specific circulation pattern of this event. Using multiple lines of evidence we show small changes in the position and magnitude of the cut-off low act to reduce the total rainfall over the region. We use reanalysis data, ensemble forecasts of the event, a boosted ensemble of a dynamically similar event, and pseudo global warming simulations of the event. We show that, for an event with analogous large-scale dynamics, future dynamical changes may outweigh thermodynamical effects, reducing the rainfall should such an event occur in a warmer climate.