A simple statistical approach for establishing dynamical linkages between specific atmospheric circulation patterns and spatially compounding persistent extremes and impacts

During the last years, the statistical analysis of compound extremes has gained increasing interest among the scientific community due to the multiple threats posed by such events to society, economy, and the environment. In many situations, this analysis is based on bivariate extreme value theory and measures provided by this framework. Such methods may however not properly address two relevant aspects: the non-zero duration of extreme events (which can be rather persistent, e.g. in the case of droughts or heatwaves, heavily violating the independence assumption of classical extreme value theory) and the fact that not all events of practical relevance can actually be described as cases falling into the tails of the continuous distribution of some observable of interest.

A versatile approach addressing the non-extremeness aspect is event coincidence analysis (ECA), which quantifies the empirical frequency of co-occurring events of arbitrary types and allows its comparison with the values for certain random null models like independent Poisson processes with prescribed event rates. While standard ECA builds upon the concept of temporal point processes and hence may be criticized for not applying to persistent events, a new methodological variant called interval coverage analysis (InCA) provides a straightforward generalization specifically addressing co-occurrence properties of persistent events. To highlight the broad range of potential applications of ECA and InCA in the context of compound event studies, we study two examples of co-occurrences between specific atmospheric circulation configurations and different types of surface extremes.

Example 1 highlights the instantaneous as well as time-lagged co-occurrence between boreal summer Northern hemispheric jet stream configurations with two distinct zonal wind maxima (“double jet”) and atmospheric heat waves. The presented results demonstrate that double jet conditions over certain sectors are closely linked with a statistically significant enhancement or suppression of heatwave activity in distinct regions, resembling the spatial patterns of atmospheric wave trains. These patterns provide a useful starting point for further targeted research to reveal the underlying atmospheric circulation mechanisms and their association with other spatially compounding extreme events and impacts.

Example 2 subsequently addresses the co-occurrence of subtropical ridges and atmospheric blockings with precipitation patterns in the Southern hemisphere. The obtained results indicate that the presence of ridges in specific sectors is commonly accompanied by a suppression of precipitation within these sectors, while surrounding regions may exhibit characteristic spatial clusters of significantly elevated probability of precipitation.

This work has been partially supported via the JPI Climate/JPI Oceans NextG-Climate Science project ROADMAP and the bilateral German-Portuguese science exchange project EXCECIF (jointly funded by DAAD and FCT).