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 North American heat extremes. We show that the observed winter-spring (December to May) PT frequency and duration have increased significantly over the past 76 years, contributing to amplified extreme heat in the northern Rocky Mountains and surrounding areas of the Hudson Bay. These observed changes are not well represented in the climate simulations analyzed herein. However, our results indicate that the rising greenhouse gas concentrations likely contribute to increased winter-spring PT frequency, which is further modulated by Pacific sea surface temperature (SST) variability. While recent La Niña-like SST trends and potentially anthropogenic aerosols have temporarily dampened this increase, our results suggest that an eventual El Niño-like response to elevated CO2 will reinstate the increasing trend in PT frequency, duration, and downstream amplification of regional extreme heat. The timing and magnitude of this shift remain uncertain, given the complex interplay between climate forcings and internal variability, as well as models' inability to reproduce historical trends. Therefore, 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.