Impact of Asian Summer Monsoon on the 2021 Pacific Northwest Heatwave: Can It? Did It?

Journal article

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.

Authors: Xu, P; Screen, JA; Wang, L; Woollings, T; Fan, H

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Wiley
• Journal: Geophysical Research Letters
• Volume: 52
• Issue: 18
• Article number: e2025GL117205
• Publication date: 2025-09-19
• Acceptance date: 2025-09-04
• DOI: 10.1029/2025gl117205
• EISSN: 1944-8007
• ISSN: 0094-8276
• Language: English
• Keywords: monsoon; teleconnection; climatic driver; heatwave; subseasonal‐to‐seasonal