2025-11-05 中国科学院（CAS）
＜関連情報＞
- https://english.cas.cn/newsroom/research_news/earth/202511/t20251106_1096464.shtml
- https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025GL117205
2021年太平洋岸北西部の熱波に対するアジア夏季モンスーンの影響：起こり得るのか？実際に起こったのか？ Impact of Asian Summer Monsoon on the 2021 Pacific Northwest Heatwave: Can It? Did It?
Peiqiang Xu, James A. Screen, Lin Wang, Tim Woollings, Hanjie Fan, Matthew Patterson, Zizhen Dong
Geophysical Research Letters Published: 19 September 2025
DOI:https://doi.org/10.1029/2025GL117205
Abstract
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.
Plain Language Summary
The Pacific Northwest (PNW) experienced a record-breaking heatwave during the summer of 2021, resulting in significant adverse effects on both human society and ecosystems. A heavy rainfall band was observed stretching from south China to south of Japan 1 week prior to the heatwave, fueling the debate over whether the monsoon activity contributed to this event. Our study found that while the monsoon activity typically has a cooling effect on the PNW’s climate, in this particular year, it had a warming effect and thus contributed to this specific heatwave. This unusual warming effect was driven by a stronger and more northward-shifted Pacific jet stream, which altered the extratropical response to the monsoon, resulting in an anticyclonic pattern over the PNW instead of the typical cyclonic response seen under average climatic conditions. Therefore, it is important to distinguish between the general question of whether monsoon can influence such events on average, and the specific question of whether it did in any specific case. We argue that when discussing the influence of large-scale climate drivers on extremes, it is crucial to clearly state whether the focus is on the general potential for influence or on the specific role in a particular event.
Key Points
- Anomalous Asian monsoon activity like that observed in late June 2021 can exert a cooling effect over the Pacific Northwest on average
- However, in the case of 2021, anomalous monsoon activity had a warming influence, contributing to the extreme Pacific Northwest heatwave
- Confusing the “can it” and “did it” questions could lead to contradictions on how the monsoon or other large-scale drivers affect extremes
