A Methodological Framework to Isolate Forced Dynamic Responses in Heat Extremes Using Nudged Climate Simulations

Regional trends in heat extremes are significantly influenced by large-scale atmospheric circulation changes across the Northern Hemisphere, with circulation-induced changes contributing up to one-third of the observed warming in regions like Western Europe. Understanding whether these trends are driven by external forcing or internal variability is key for improving model evaluation, trend detection, attribution, and reducing uncertainties in future climate projections. Here, we present a novel methodological framework to isolate the forced dynamic components of heat extremes. We nudge tropospheric winds in CESM2 pi-control simulations towards transient climate conditions, which provides an estimate of the forced thermodynamic component. By subtracting these thermodynamic contributions from the large ensemble mean, we effectively isolate the forced dynamic contributions to heat extremes. Our results reveal distinct regional patterns. Previously identified heatwave hotspots such as the Pacific Northwest, Central Europe, South Siberia, and North China/Mongolia exhibit substantial warming of up to 1°C since 1950, which is attributable to forced circulation changes. In contrast, forced circulation chnages induces cooling of up to 1°C in regions such as the northeastern United States, parts of Asia, and central Africa. We have rigorously tested the sensitivity of our approach through various experiments and nudging strategies. This framework provides a valuable tool for disentangling forced thermodynamic and dynamic signals from internal variability, offering critical insights to reduce uncertainties in future climate projections.