In recent years, as extreme weather events have occurred with increasing frequency, scientists have been searching within the chaotic atmospheric system for clues that can enhance forecasting capabilities—factors such as ENSO, sea ice, the stratospheric polar vortex, and tropical convective activity. These factors provide critical basis for weather and climate predictions across different time scales. Anomalies in these factors are often closely related to the occurrence of extreme weather events. However, when tracing the causes of a specific extreme event, are we prone to falling into the trap of equating "general patterns" with "specific roles," thereby oversimplifying the true complex causality?

In the summer of 2021, the Pacific Northwest experienced one of the most intense heatwaves ever recorded. In Canada's British Columbia, at a latitude higher than Mohe, China, temperatures soared to nearly 50°C, approximately 20°C higher than the seasonal average, triggering massive wildfires and severe casualties. A notable phenomenon was the appearance of an anomalously strong rainband extending from South China to Japan in the Asian monsoon region about a week before the heatwave. This sparked widespread discussion in the scientific community: was this distant monsoon activity one of the "drivers behind the scenes" for the North American heatwave?

Dr. Xu Peiqiang and Dr. Wang Lin from the Monsoon System Research Center at the Institute of Atmospheric Physics, Chinese Academy of Sciences, collaborated with scholars from the University of Exeter, University of Oxford, University of St Andrews, Sun Yat-sen University, and other institutions, provided a detailed answer to this controversial question in their latest study published in Geophysical Research Letters. The study found that, based on statistical analysis and numerical experiments using the historical climate state, Asian summer monsoon activity similar to that in late June 2021 would typically induce a cooling effect over the Pacific Northwest, thus reducing the probability of a heatwave. However, analysis of the actual conditions in 2021 and tests using operational forecast models showed that this particular monsoon activity instead produced a warming effect, exacerbating the intensity of the heatwave.

Why did this seemingly contradictory result occur? The study points out that the key lies in the special background atmospheric circulation of that year. In June 2021, the Pacific jet stream was anomalously strong and positioned further north, acting as a more efficient "atmospheric waveguide" that directed the Rossby wave energy excited by the monsoon towards North America, ultimately forming an anomalously stable blocking high-pressure system over the region. When researchers set the basic flow field in an ideal model to the actual state of 2021, the monsoon's impact on North American climate indeed reversed from the usual "cyclonic cooling" to "anticyclonic warming."

As heatwaves intensified globally in July 2025, regions from East Asia to Europe felt the effects. In Exeter, UK, record-breaking temperatures turned grassy areas a dry beige. (Image by Xu Peiqiang)

The study further pointed out that the spatial pattern of the Asian monsoon activity in late June 2021 was very complex, not resembling the typical anomalous pattern of previous years. It simultaneously contained multiple active and suppressed convective centers within it. If one were to infer its impact on the heatwave based solely on a single "classic" monsoon pattern or a single convective center, it would easily lead to misjudgment of the actual physical process.

As a visiting scholar at the University of Exeter, Dr. Xu experienced the record-breaking heat of July 2025 firsthand. The severe conditions, unusual for the UK's typically mild summers, gave him direct insight into the extreme events he researches. He highlighted a critical nuance in his field: "When discussing the relationship between large-scale climate drivers and extreme events, it is crucial to clearly distinguish between the questions of 'Can it?' (i.e., whether it has the potential to influence under average conditions) and 'Did it?' (i.e., whether it actually played a role in the specific event). Future attribution studies for extreme events should pay more attention to the specific circulation background of the event and the actual spatial pattern of the forcing factors."

Paper Information:

Xu, P., Screen, J. A., Wang, L., Woollings, T., Fan, H., Patterson, M., & Dong, Z. (2025). Impact of Asian summer monsoon on the 2021 Pacific Northwest heatwave: Can it? Did it? Geophysical Research Letters, 52, e2025GL117205. https://doi.org/10.1029/2025GL117205