Regional Trends in Northern Hemisphere Summer Extratropical Cyclone Activity and Their Link to Precipitation and Temperature Trends

Abstract In recent years a robust weakening of the storm tracks has been identified in reanalysis data. This trend has been linked to coinciding changes in surface temperature extremes and precipitation over land. Here we use a surface‐based metric to quantify regional trends in the boreal summertime low‐level extratropical cyclone activity (ECA) and link this to surface impacts. We compare trends over the satellite‐era across a single model large ensemble (SPEAR‐MED Large Ensemble) and the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations, including full and single forcing simulations and prescribed sea‐surface temperature simulations. The models successfully capture the hemispheric‐wide ECA decrease and regional trends over the Pacific, North America, and Europe seen in reanalysis. However, this masks a model‐observation discrepancy over the North Atlantic, which is anti‐correlated with mean circulation trends. We then re‐examine previous links between extratropical cyclones and increasing daily maximum surface temperatures and shifts in precipitation in historical trends. ECA trends cannot account for the magnitudes of daily maximum surface temperature trends over North America or Europe but can explain a large fraction of land precipitation trends over the North Atlantic. This suggests other drivers, such as greenhouse gas radiative forcing, are more important in strengthening daily maximum surface temperature trends. In contrast, models struggle to capture the North Atlantic ECA‐related trends, primarily because they underestimate ECA trends. This can explain model‐observation discrepancies in total precipitation change over land in the West Atlantic, but not the East Atlantic.