Externally forced circulation changes amplify mid-latitude regional heat extremes in climate model nudged-circulation experiments

Large-scale atmospheric circulation changes have strongly modulated the frequency and intensity of regional heat extremes in the NH mid-latitudes over recent decades. But whether these changes are due to external forcing or internal variability remains unclear. This challenge arises from the intertwined nature of thermodynamic and dynamic responses to climate forcing. We present an atmospheric circulation-constrained framework using the CESM2 model to quantify externally forced dynamic and thermodynamic contributions to seasonal surface temperature and heat extremes. We show that, depending on the region, a substantial portion of circulation-driven changes in boreal summer average temperature and heat extremes is externally forced, with distinct regional patterns. Forced circulation changes have amplified summer heat extremes by up to 1°C since 1979 in several northern mid-latitude regions, including the Pacific Northwest, Central Europe, South Siberia, and North China/Mongolia. These signals coincide with persistent high-pressure anomalies and enhanced mid-tropospheric ridging, consistent with increased atmospheric blocking. In contrast, forced circulation changes have contributed to regional cooling in parts of the northeastern United States, central Africa, and East Asia. With a physically consistent way to constrain feedback and isolate externally forced circulation signals from internal variability, this new framework offers critical insights into reducing uncertainties in the attribution and projection of regional climate extremes.