Future Changes in North American Summer Heatwave Variability and Associated Dynamic and Thermodynamic Processes

Projected changes in summer heatwave variability over North America (NA) are examined using a multi-model ensemble of 32 Global Climate Models (GCMs), including three models with 30-member initial-condition ensembles, under the Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5) scenario. Applying the common Empirical Orthogonal Function (EOF) approach, two distinct modes of heatwave variability are identified for a historical (1961–2010) and future period (2051–2100), collectively explaining ∼30% of variability in each period. Spatial patterns of these modes differ between the periods, with significant regional changes detected regarding the variability across GCMs. Prominent North Pacific-to-NA wave trains, linked to the distinct modes, are evident in tropospheric geopotential and mean sea level pressure anomalies. These wave trains significantly weaken in future projections, reflecting a decline in large-scale dynamic processes. However, enhanced thermodynamic influences on these modes become apparent over the North Pacific and NA in the future. At regional scale over NA, dynamically-driven changes in cloud cover and shortwave downward radiation weaken, while thermodynamically-driven changes in moisture availability and longwave downward radiation strengthen, reshaping energy balance and heat redistribution associated with heatwave variability. Consequently, surface radiation heating, the sum of shortwave and longwave radiation, emerges as the most influential variable for explaining changes in the distinct modes of heatwave variability between the periods. Internal variability, assessed using initial-condition ensembles, is comparable to model structure uncertainty in both large-scale and regional dynamic and thermodynamic processes that drive heatwave variability across the periods.