Rare event algorithm study of extreme warm summers and heat waves over Europe

TL;DR

This paper demonstrates how rare event algorithms can significantly enhance the statistical analysis of extreme warm summers and heatwaves in climate models, revealing their dynamical patterns and return times.

Contribution

It introduces the application of rare event algorithms to climate modeling, enabling estimation of extreme event probabilities and dynamics beyond traditional sampling limits.

Findings

Extreme warm summers are linked to wavenumber 3 teleconnection patterns.

Rare subseasonal heatwaves contribute to the most extreme summers.

The method estimates return times much larger than direct sampling allows.

Abstract

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.