Rare Event Algorithm Study of Extreme Warm Summers and Heatwaves Over Europe

The analysis of extremes in climate models is hindered by the lack of statistics due to the computational costs required to run simulations long enough to sample rare events. We demonstrate how rare event algorithms can improve the statistics of extreme events in state-of-the-art climate models. We study extreme warm summers and heatwaves over France and Scandinavia with CESM1.2.2 in present-day climate. The algorithm concentrates the simulations on events of importance, and shifts the probability distributions of regional temperatures such that warm summers become common. We estimate return times of extremes orders of magnitude larger than what feasible with direct sampling, and we compute statistically significant composite maps of dynamical quantities conditional on the occurence of the extremes. We show that extreme warm summers are associated to wavenumber 3 hemispheric teleconnection patterns, and that the most extreme summers are related to the succession of rare subseasonal heatwaves. Plain Language Summary The impact of extreme climatic events is often dominated by the rarest events. These events have return times (a measure of how often they occur on average) of hundreds of years or more, but they could, and do, happen anytime. These events are poorly understood because of lack of statistics. Climate models are computationally expensive, and cannot be run for long enough to study events with return times longer than a few decades. We use a new computational technique that allows simulations to focus only on trajectories leading to extreme heatwaves over a target region, optimizing the use of computational resources. We thus gather robust statistics for seasonal heatwaves with return times of hundreds or thousands of years, and observe even rarer events impossible to observe otherwise. We find that extreme warm summer over France or Scandinavia are synchronized with extreme warm summers in specific regions of Asia and North-America by a teleconnection pattern extending over the entire Northern hemisphere. We also find suggestions that extreme warm summers over Scandinavia may occur in two distinct ways. This new method can be used to better study the impact of global warming on the risk of catastrophic events, and to improve their predictability.

Automated summary

The lack of statistics in climate models hinders the analysis of extreme events, as direct sampling is not feasible due to computational costs. By using rare event algorithms, we can improve the statistics of extreme events in state-of-the-art climate models. Our study on extreme warm summers and heatwaves over France and Scandinavia shows that extreme warm summers are associated with specific hemispheric teleconnection patterns, with the most extreme summers linked to rare subseasonal heatwaves.