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

Insurers and risk managers for critical infrastructure such as transport or power networks typically do not account for flooding and extreme winds happening at the same time in their quantitative risk assessments. We explore this potentially critical underestimation of risk from these co-occurring hazards through studying events using the regional 12 km resolution UK Climate Projections for a 1981-1999 baseline and projections of 2061-2079 (RCP8.5). We create 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 (Δt of 1-180 days long) that reflect how periods of adverse weather affect society (e.g. through damage). We show that the probability of co-occurring wind-flow episodes in Great Britain (GB) is underestimated 2-4 times if events are assumed independent. Significantly, this underestimation is greater both as severity increases and episode length reduces, highlighting the importance of considering risk from closely consecutive storms (Δt  3 day[JH1] s) and the most severe storms. In the future (2061-2079), joint wind-flow extremes are twice as likely as during 1981-1999. Statistical modelling demonstrates that changes may significantly exceed thermodynamic expectations of higher river flows in a wetter future climate. The largest co-occurrence increases happen in mid-winter (DJF) with changes in the north Atlantic jet stream an important driver; we find the jet is strengthened and squeezed into a southward-shifted latitude window (45-50°N) giving typical future conditions that match instances of high flows and joint extremes impacting GB today.  This strongly implies that the large-scale driving conditions (e.g. jet stream state) for a multi-impact ‘perfect storm’ will vary by country; understanding regional drivers of weather hazards over climate timescales is vital to inform risk mitigation and planning (e.g. diversification and mutual aid across Europe).