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

Abstract Projected changes in summer heatwave variability over North America (NA) are examined using a multimodel 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 (2051–2100) period, 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-North America (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 the 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. Significance Statement Understanding how heatwaves and their associated dynamic and thermodynamic processes will evolve is essential for climate impact assessments. This study identifies distinct modes of summer heatwave variability over North America and reveals shifts in their drivers by 2100. While large-scale and regional dynamic processes that shape heatwaves weaken, thermodynamic influences—particularly changes in moisture availability and longwave radiation—become more dominant in the future. These findings highlight a fundamental shift in the physical mechanisms controlling heatwave variability, emphasizing the increasing role of surface radiation heating. By evaluating multiple climate models and internal variability, this study provides robust evidence of future changes in heatwave variability and its drivers, offering insights to improve projections of extreme events.