As extreme weather events become more frequent, scientists are searching for clues in the chaotic atmosphere, such as El Niño-Southern Oscillation (ENSO), sea ice, the polar vortex, and tropical convection, to improve forecasts. These factors are key to predicting weather and climate, and their anomalies often relate to extreme events. But when examining a specific event, do we risk oversimplifying causality by confusing general patterns with specific roles?

In summer 2021, the Pacific Northwest faced a record heatwave. British Columbia reached nearly 50°C (about 20°C above normal), causing wildfires and casualties. About a week earlier, a strong rainband appeared from South China to Japan, prompting scientists to ask: Could this distant monsoon have helped trigger the North American heatwave?

To answer the question, a research team from the Institute of Atmospheric Physics of the Chinese Academy of Sciences, along with international collaborators, has conducted detailed study. Their findings were recently published in Geophysical Research Letters.

Based on statistical analysis and numerical experiments using historical climate data, the researchers found that Asian summer monsoon activity similar to that observed in late June 2021 typically induces a cooling effect over the Pacific Northwest, thus lowering the likelihood of a heatwave. However, an analysis of the actual 2021 conditions and tests using operational forecast models revealed that this specific monsoon activity instead generated a warming effect, amplifying the heatwave's intensity.

The study identifies the key factor as the unusual background atmospheric circulation that year. In June 2021, the Pacific jet stream was abnormally strong and shifted farther north. Acting as a more efficient "atmospheric waveguide," it directed Rossby wave energy (excited by the monsoon) toward North America. This ultimately led to the formation of an unusually stable blocking high-pressure system over the region. When the researchers adjusted the basic flow field in an ideal model to match the actual 2021 state, the monsoon's impact on North American climate reversed: it shifted from the typical "cyclonic cooling" to "anticyclonic warming."

The study further notes that the spatial pattern of the Asian monsoon activity in late June 2021 was highly complex, differing from the typical anomalous patterns observed in previous years. It contained multiple active and suppressed convective centers simultaneously. The researchers noted that inferring the monsoon's impact on the heatwave based solely on a single "classic" monsoon pattern or a single convective center would likely lead to misjudgment of the actual physical processes.

When discussing the relationship between large-scale climate drivers and extreme events, the researchers emphasized, it is critical to clearly distinguish between two questions: "Can it?" (i.e., Does the driver have the potential to influence under average conditions?) and "Did it?" (i.e., Did it actually play a role in the specific event?).