Dynamics of pan-Atlantic winter compound extremes in ERA5 and CMIP6 models.

Recent work has provided robust evidence for the systematic co-occurrence of wintertime cold spells in North America and wet and windy extremes in Europe, which we term compound pan-Atlantic extremes. Both cold spells and wet and windy extremes are individually highly impactful, and their concurrence further amplifies their effects for actors with international exposure who are vulnerable to correlated losses. This study aims to investigate further the atmospheric processes associated with compound pan-Atlantic cold, wet and windy extremes and how these processes are represented in CMIP6 models.

On aggregate, cold spells in different parts of North America statistically co-occur with wind and precipitation extremes in specific European regions. However, North American cold spells can arise from multiple dynamical pathways, altering the location and timing of the associated European extremes for individual cold spells. Here, we use ERA5 reanalysis data (1940-2014) to identify North American wintertime cold spells in three different regions as well as the co-occurring wet and windy extremes in northern and southern Europe. We relate these compound pan-Atlantic extreme occurrences to dynamical pathways based on Pacific and Atlantic weather regimes. We then compare the CMIP6 historical simulation model data (1940-2014) with the ERA5 results. First, we discuss the performance of the models in capturing the weather regime frequencies and dynamical pathways to cold spells. Second, we review the ability of the models to replicate the spatial and temporal pattern of pan-Atlantic extremes for the three cold spell regions.
The results of this study contribute to the evaluation of the model fidelity in reproducing pan-Atlantic compound extremes and the associated circulation, with direct implications for the assessment of climate projections.