John Hillier, Hannah Bloomfield, Colin Manning, Len Shaffrey, Paul Bates, Dhriendra Kumar

Ignoring a correlation between flooding and extreme winds underestimates risk to insurers or providers of critical infrastructure such as railways or electricity. We explore this potential underestimation for Northwest Europe, illustrated using Great Britain (GB), using an event-based analysis in regional 12 km UK Climate Projections (UKCP18, 1981-1999, 2061-2079 – RCP8.5). We derive a new wintertime (Oct-Mar) set of 3,427 wind events to match an existing set of fluvial flow extremes and design innovative multi-event episodes (Dt of 1-180 days long) that reflect how periods of adverse weather are actually experienced (e.g. for damage). Results show the probability of co-occurring wind-flow episodes in GB is underestimated 2-4 times if events are assumed independent. Significantly, this underestimation is greater both as severity increases (e.g. 90th to 99th percentile) and Dt reduces, adding the insight that we need to be most concerned about underestimating co-occurrence in the strongest individual or closely consecutive storms (Dt ~3). In the future, joint extremes are twice as likely as in the present. Statistical modelling demonstrates that changes go significantly beyond thermodynamic expectations (i.e. more high flows in a wetter climate). The largest co-occurrence increases are shown to be in mid-winter (DJF) and changes in the north Atlantic jet stream dynamics are demonstrated to be an important driver; particularly in mid-winter it is strengthened and squeezed into a southward-shifted latitude window (45-50°N), conditions typical of high flows and joint extremes impacting GB in present day simulations. More widely, that work highlights that the recipe of driving large-scale conditions (e.g. jet stream state) for a multi-impact ‘perfect storm’ will vary by country. So, future analyses should work to build area-by-area understanding of how the impact of common drivers varies spatially, which is key to risk mitigation and planning (e.g. diversification, mutual aid across Europe).