Prominent Theory of Atmospheric Waves May Not Explain Extreme Weather Patterns

Across much of the northern hemisphere, extreme weather events like heat waves and heavy precipitation have increased in frequency and severity over the last several decades. A new study from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) shows that one proposed partial explanation, so-called “quasiresonant amplification of quasistationary Rossby waves,” may not be capable of explaining any of this increase in severe weather events. 

The study, published in Science Advances and co-authored by Assistant Professor of Environmental Science and Engineering Marianna Linz and research associate Todd Mooring, calls for greater caution in how certain types of atmospheric dynamics are interpreted. 

Rossby waves are large-scale wiggles in the jet stream — the fast-flowing air in the upper atmosphere that acts as a boundary between cold polar air and warm tropical air. If a Rossby wave stalls in the right place, it can contribute to an extreme weather event, like a heat wave. The Harvard team set out to test a specific theory of how these large, stationary waves arise.

Linz and Mooring examined “quasiresonant amplification of quasistationary Rossby waves,” also known as QRA theory, a mechanism often used to explain persistent jet-stream patterns linked to heat waves and floods. In recent years, this theory has been described in both scientific papers and media stories as a direct cause of some specific extreme weather events.

QRA theory predicts that when the average winds in the mid‑latitudes take on particular configurations, certain wave patterns become strongly amplified, making extreme events more likely. To test this, Mooring used a simplified atmospheric model that captures realistic fluid dynamics but strips away complicating factors such as moisture and detailed representations of heating and cooling from sunlight and infrared radiation.

Mooring designed the model so that flow states deemed favorable to quasi-resonant amplification of jet stream waves would occur frequently, allowing robust statistics. The team then compared the amplitudes of waves during periods that QRA theory says should be favorable, versus unfavorable, for strong, stationary waves.

“The result was the opposite of what you would expect,” Mooring said. “In conditions that are supposed to favor these large-amplitude waves, we actually get smaller waves.”

Because the test was carried out in a model specifically chosen to be simpler than the real atmosphere, yet more realistic than the highly idealized equations on which the amplification theory is based, the authors argue that the failure is significant.

It’s true that climate change is likely making some kinds of extreme weather events worse and more frequent, the researchers emphasize. But they caution against using one particular theory of a specific jet-stream behavior to conclude that simultaneous heat waves will necessarily occur across many regions. “These are fundamentally complex phenomena, and clear-declarative statements around these predictions may well be inaccurate,” Linz said.

“Resonant Rossby wave mechanism for extreme weather performs poorly in simple model test” was supported by the National Science Foundation CAREER award No. 2239242 and the Harvard University Dean’s Competitive Fund for Promising Scholarship.