Past and projected weather pattern persistence with associated multi-hazards in the British Isles

Hazards such as heatwaves and floods are often linked to persistent weather patterns. Atmosphere-Ocean General Circulation Models (AOGCMs) are important tools for evaluating projected changes in extreme weather. Here, we demonstrate that 2-day weather pattern persistence is a useful concept for both investigating climate risks from multi-hazard events as well as for assessing AOGCM realism. This study evaluates the ability of a Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model sub-ensemble of 10 AOGCMs at reproducing seasonal weather pattern persistence and frequencies over the British Isles. Changes in persistence are investigated under two Representative Concentration Pathways (RCP8.5 and RCP4.5) up to 2100. Broadly, the ensemble replicates historical weather type persistence observed in reanalyses (1971-2000). Future persistence and frequency of summer anticyclonic patterns are found to increase, implying heightened risk of drought and heatwaves. On the other hand, the cyclonic weather type decreases in autumn suggesting reduced likelihood of flooding and severe gales. During winter, AOGCMs suggest increased risk of concurrent flood-wind hazards by 2100, however, they also tend to over-estimate such risks when compared to reanalyses. In summer, the strength of the nocturnal Urban Heat Island (UHI) of London could intensify, enhancing the likelihood of combined heatwave-poor air quality events. Further research is needed to explore other multi-hazards in relation to changing weather pattern persistence and how best to communicate such threats to vulnerable communities.