Effects of Subseasonal Variation in the East Asian Monsoon System on the Summertime Heat Wave in Western North America in 2021.

In the early summer (late June–early July) of 2021, a prolonged and extremely high temperature event was observed in the Northwest United States and Southwest Canada. In this study, a connection was found between the subseasonal variations of the East Asian monsoon system and the anomalous heat dome over western North America (WNA). Around 2–3 pentads prior to the period of maximum heat, a northward‐propagating rain belt and abnormally intensified jet stream in East Asia excited a Rossby wave train, causing the high‐pressure anomaly over WNA that was responsible for the occurrence of this heat wave in 2021. Multivariate singular value decomposition analysis results using historical data and a series of numerical experiments further supported the finding of this East Asia–western North America teleconnection. The results provide precursor signals for better monitoring and predicting heat waves over WNA. Plain Language Summary: A persistent and extremely high temperature event was observed in several states in the Northwest United States and Southwest Canada in summer 2021. This severe heat wave resulted in the hottest early‐summer season in western North America (WNA) since 1979 and huge social and economic losses. Although the heat dome (high‐pressure anomaly) over WNA was the factor responsible for this mega heat wave, the cause of this high‐pressure anomaly was unclear. The present study reveals the importance of an atmospheric wave train emanating from East Asia toward North America in inducing the heat dome over WNA. The generation of the wave train is attributable to a northward‐propagating rain belt and abnormally enhanced jet stream over East Asia from 10 to 15 days before the peak of this heat wave. Thus, monitoring the East Asian summer monsoon and associated wave train activity may help in predicting heat waves in WNA, and therefore in knowing when to put measures in place to mitigate against their effects. Key Points: An extreme heat wave occurred in western North America (WNA) in the early summer of 2021, causing huge socioeconomic losses in several big citiesThe subseasonal activity of East Asian summer monsoon contributed to the occurrence of the heat wave via excitation of a Rossby wave trainThe East Asia‐North America teleconnection can provide precursor signals for forecasting heat waves in WNA [ABSTRACT FROM AUTHOR]