Probing the Unfathomable: Ensemble Boosting for Physical Climate Storylines of Unseen Extremes

Recent unprecedented extremes such as the 2023 New Year’s winter warm spell in Europe, the 2022 summer heat and dryness in China and Europe, the 2021 Pacific Northwest heatwave, the 2021 floods in northwestern Germany and the 2020 Siberian heat anomaly broke previous observed record intensities by large margins. Based on the observations up to the year before the event, some of these record-shattering extremes were inconceivable. Could the scientific community have better quantified the potential for such unseen events based on the current generation of climate models?

Here, we demonstrate how a new ensemble boosting approach can be used to generate physically coherent storylines of such unseen events. In ensemble boosting a fully-coupled free-running climate model (CESM2) is used to develop physical storylines of very rare extremes. To this end, the most extreme events in an initial-condition large ensemble for the near future are re-initialized with slightly perturbed atmospheric initial conditions to efficiently generate events with magnitudes unprecedented in the observational record.

We demonstrate that, with this approach, CESM2 can simulate events that e.g. substantially exceed the magnitude of the 2021 Pacific Northwest (PNW) heatwave anomaly. Even though the most extreme ensemble members were only selected based on the local temperature anomaly over the PNW region, they show a very similar temporal evolution and spatial pattern as the observed event. The associated 500hPa geopotential height anomaly is remarkably similar to the observed event with a strong anticyclone over PNW that is part of a wave pattern extending over much of the Northern Hemisphere. We further show that in some of the storylines pre-conditioning dry soils strongly contributed to the heatwave intensity, whereas in others, heatwaves of similar magnitude occur even at average land surface conditions.

We further develop storylines for heatwaves in the Greater Paris and Chicago regions of much greater intensities than ever observed. Particularly the US Midwest, where no intensification of heatwaves has been observed in recent decades, has not experienced anything close to the heatwave intensities possible in the coming years.

Finally, we demonstrate examples how ensemble boosting can also be used to generate storylines for multi-year drought events and large-scale heavy precipitation extremes and compare the findings to other storyline approaches. Event storylines based on ensemble boosting can be used in impact studies that require physical consistency across variables, e.g., for the evaluation of humid heatwaves or compound events, for assessing wildfire risks or for ecosystem modelling.