Advancing our understanding of compound weather and climate events via large ensemble model simulations

Abstract

Most societally relevant weather impacts result from compound events, that is, rare combinations of weather and climate drivers. Focussing on four event types arising from different combinations of climate variables across space and time, we illustrate that robust analyses of compound events – such as frequency and uncertainty analysis under present-day and future conditions, event attribution, exploration of low-probability-high-impact events – require very large sample sizes. In particular, the required sample is much larger than that needed for routinely considered univariate extremes. We demonstrate how large ensemble simulations from multiple climate models are crucial for advancing our understanding of compound events and for constructing robust projections. Importantly, this will avoid providing practitioners with potentially misleading information on climate risks.

The New Max Planck Institute Grand Ensemble With CMIP6 Forcing and High‐Frequency Model Output

“…SMILEs are also used to investigate ocean ecosystem drivers (Rodgers et al., 2015), and to identify systematic differences between simulated and observed patterns of sea‐surface temperature and sea‐level pressure change that are very unlikely to occur due to internal variability (Olonscheck et al., 2020; Wills et al., 2022). Furthermore, recent developments in compound event research highlight the importance of sufficiently sampling internal variability to robustly capture the risks associated with extreme values of multivariate extremes, which requires even larger ensemble sizes than conventional univariate extremes (Bevacqua et al., 2023; Burger et al., 2022). The availability of SMILEs from multiple models further allows us to better quantify and differentiate sources of uncertainty in climate projections, especially uncertainties arising from internal variability and those from model differences (Deser et al., 2020; Hawkins & Sutton, 2009, 2011; Lehner et al., 2020).…”

End-of-century heat and drought stress approaching Europe swiftly

“…MPI-GE is the largest existing initial-condition ensemble of a comprehensive, fully coupled Earth System Model currently available. This large ensemble size is crucial for robustly sampling and assessing changes in low-probability univariate events, and it is even more important for multivariate compound events and temporally successive extremes 42 . In addition to its large ensemble size, compared to other large ensembles MPI-GE also offers one of the most adequate representations of the historical internal variability and forced changes in observed temperatures 43 and precipitation 44 .…”