2021 North American Heatwave Fueled by Climate-Linked Nonlinear Interactions Between Common Drivers

Extreme heat conditions in the Pacific Northwest US and Southwestern Canada in summer 2021 were of unprecedented severity. Constituting a 5-sigma anomaly, the heatwave affected millions, likely led to thousands of excess deaths, and promoted wildfires that decreased air quality throughout the continent. Even as global warming causes an increase in the severity and frequency of heatwaves both locally and globally, this events magnitude went beyond what many would have considered plausible under current climate conditions. It is thus important to attribute such an exceptional event to specific physical drivers to gain understanding about its underlying mechanisms. A particularly pressing question is whether any changing variability of atmospheric dynamics or land-atmosphere interaction is implicated in amplifying current and future heat extremes, as such processes going beyond simple thermodynamics may be underestimated in state-of-the-art climate models. Using ERA5 reanalysis, we find that slow- and fast-moving components of the atmospheric circulation interacted during this event, contributing to extreme geopotential height anomalies. In addition, we identify below-average soil moisture levels as a critical driver further amplifying heat conditions. Overall we find that nonlinear land-atmosphere interactions contributed 3K to the peak heat anomaly of 10K (regional mean). This is supported by a model experiment demonstrating that soil moisture interaction may increase the likelihood of a monthly-scale temperature anomaly of the observed strength by O(10)x. From a climate perspective, we find that ongoing trends in soil moisture, geopotential height, and near-surface temperature, which favor the observed nonlinearities, have already made an event of this heatwaves peak magnitude O(100)x more likely in 20002020 than in 19791999 (changing from a once-in-O(10,000)-years to a once-in-O(100)-years event). Our results therefore suggest an important role of atmospheric dynamics and nonlinear land-atmosphere interactions in driving this exceptional heat extreme, promoted by a long-term warming trend due to anthropogenic climate change that will continue to increase the likelihood of such extremes under continued emissions.