Observed and Modeled Amplification of the Frequency, Duration, and Extreme Heat Impacts of the Pacific Trough Regime

The large‐scale atmospheric circulation is a key driver for regional climate extremes, yet its response to anthropogenic forcing remains uncertain. The Pacific trough (PT) regime is a persistent circulation pattern modulating temperature, precipitation, and fires over North America. We show that the observed boreal winter‐spring (December to May) PT frequency and duration have increased significantly over the past 76 years, contributing to amplified extreme anomalous heat over western and central Canada. These observed changes are not well represented in the climate simulations analyzed herein. However, our results indicate that rising greenhouse gas concentrations likely contribute to increased winter‐spring PT frequency, which is further modulated by sea surface temperatures (SSTs). While the recent La Niña‐like and negative Pacific Decadal Oscillation‐like SST trends have dampened this increase, our results suggest that if an eventual emergence of the modeled El Niño‐like response to elevated were to occur in reality, it would reinstate the increase in PT frequency, duration, and downstream amplification of regional extreme heat. However, the occurrence, timing, and magnitude of this shift remain uncertain, given the complex, interlaced role of external forcings and internal variability in modulating historical trends, as well as models' inability to reproduce them. Additionally, modeling decisions regarding future trajectories for anthropogenic emissions, including aerosols and greenhouse gases, play a critical role in projecting future changes in PT frequency. Our findings underscore the need for a better understanding and modeled representation of long‐term changes in the atmospheric circulation to inform climate adaptation and risk assessment.