Impact of Asian summer monsoon on the 2021 Pacific Northwest heatwave: can It? did It?

This study examines the role of Asian summer monsoon in the unprecedented 2021 Pacific Northwest (PNW) heatwave. We address this by separating it into two relevant but independent questions: Can monsoon activity observed prior to the event impact PNW climate, and did it specifically impact the 2021 PNW heatwave? Based on observational diagnostics, numerical experiments, and subseasonal-to-seasonal (S2S) forecasts, our results consistently indicate although the monsoon activity can exert a cooling effect on the PNW, on average, it had a warming influence in the specific case of 2021 and thus contributed to the heatwave that summer. The contrasting answers to the “can it” and “did it” questions highlight how background flow and specific forcing pattern during the event can modulate—or even reverse—the expected impact. We advocate future work exploring the link between large-scale climatic drivers and extremes should be undertaken in an event-specific context to better understand these relationships.