Future changes in circulation types in the SMHI Large Ensemble

Recently SMHI has completed and published 50-member ensembles for each of the Tier-1 and Tier-2 future scenarios of ScenarioMIP, using the EC-Earth3 model (SMHI-LENS, Wyser et al. 2021). Monthly and daily output from these simulations are freely available on the ESGF and can serve as a base for assessing the uncertainty of climate projections in a single model, changes in the likelihood, magnitude and duration of extremes, changes in the probability for passing tipping points, or changes in the frequency of occurrence of compound events. To our knowledge SMHI-LENS is the only single-model large ensemble that includes all ScenarioMIP scenarios.

As an application of SMHI-LENS we present results from an evaluation of changes in large-scale circulation types (CTs) over the Scandinavian domain between the present climate and two future periods in the different scenarios. For the classification in 10 CTs we are using the Simulated Annealing and Diversified Randomization (SANDRA) method applied to daily sea level pressure fields where the spatial means have been removed (Hansen and Belusic 2021). Most of the 10 CTs occur predominantly in a specific season and can hence be referred to as summer or winter CTs. We find that the frequency of the CTs does not change significantly towards the middle of the 21st century, but that most significant CT frequency changes happen towards the end of the century during summer. The magnitude of the frequency changes is found to be proportional to the warming in the different scenarios. Our results further suggest that the distinction between summer and winter season in terms of CTs becomes more pronounced in the future climate.

Each CT has its specific effect on other variables such as temperature and precipitation, meaning that a specific CT can, for example, be associated with lower-than-normal temperatures or less-than-normal precipitation. In our study, we also investigate how this effect changes in the different future scenarios. For both temperature and precipitation, the spatial extent of the effect change is considerably larger at the end of the century compared to the change at the mid-century, but the average magnitude of the change is similar in both periods. For temperature, the effect change is strongest in the winter half-year for almost all of the 10 CTs.

Ref: Hansen, F. and D. Belušić. "Tailoring circulation type classification outcomes." International Journal of Climatology (2021).