Here’s the press tip from PNAS on our new article demonstrating an increase over the past 70+ years in the occurrence of quasi-resonant amplification (“QRA”) of planetary waves in the Northern Hemisphere that is tied to persistent summer weather extremes.

A study finds effects from anthropogenic climate warming can perturb planetary waves and raise the likelihood of summer extreme temperature events. Planetary waves are large-scale atmospheric circulation disturbances which greatly impact  weather and climate. Amplification of these planetary waves due to factors including human-caused climate warming, can favor the types of summer weather extremes observed in recent decades. Michael Mann and colleagues examined the potential drivers and implications of planetary wave amplification with respect to extreme summer temperature events. Using temperature and atmospheric reanalysis data from 1950 to 2024, the authors determined that the frequency of planetary wave resonance events tripled since 1950. This increased frequency correlates with changes in the climate conditions that favor planetary wave amplification, including Arctic warming and thermal gradients between land and sea. Planetary wave events were also found to increase in frequency following strong El Niño events, the authors write, suggesting that the El Niño phenomenon—and changes therein due to climate change—are an important source of uncertainty in projecting future changes in planetary wave resonance events.. According to the authors, current climate models do not accurately represent the effects of human-caused warming on planetary wave resonance events, suggesting that modeling underestimates the potential for future extremes in summer temperatures in the Northern Hemisphere.

And here is Q&A with Penn’s press office that we did about the article:

1. What are planetary wave resonance events and why are they significant?

These are events, typically during summer but sometimes early fall or late summer, during which the jet stream gets locked into a very wavy, stationary pattern, with deep high and low pressure centers that remain stuck in place, leading to extended periods of hot dry or alternatively, very wet weather. Many of the most deadly and devastating extreme weather events  summer weather extremes (heat waves, droughts, wildfire outbreaks, floods). The mid-July 2021 Pacific Northwest “Heat Dome” event is one of the best examples (we had a paper on this early last year). Other very prominent examples are the 2003 European heatwaves, the simultaneous occurrence of the 2010 Russian heatwave and Pakistan flooding and the 2016 Alberta wildfires.

2. How has the frequency of planetary wave resonance events changed over the past half-century?

Our study is the first to demonstrate an historical trend in the phenomenon. We estimate that there has been a near tripling since the 1950s, from an average of one resonance event per summer to about three (and many recent summers have seen as much as six resonance events). (see Figure below from article)

3. Does this new study build on previous findings? If so, how were did your prior work inform steps taken in this new paper?  

Indeed it does. Our past studies have shown that models predict an increase in the phenomenon, but this is the first study to demonstrate an historical increase, taking advantage of the more reliable atmospheric “reanalysis”” observations that are available today. 

4. What are the primary climate factors contributing to the increased frequency of these events?

We show that the increase in the occurrence of QRA events  is tied to the polar amplification of warming and increased land/ocean thermal contrast, both of which can be tied to the pattern of human-caused warming.

5. How has human activity affected natural processes? 

Human activity has altered natural processes both by warming the planet and by reshaping the atmosphere’s large-scale circulation patterns. The burning of fossil fuels has released vast amounts of carbon dioxide and other greenhouse gases into the atmosphere, driving global temperatures higher. But the impact doesn’t stop there. Human-induced warming is also modulating the jet stream and other circulation systems, making extreme weather events more frequent, persistent, and severe.

6. How does the El Niño/Southern Oscillation (ENSO) influence planetary wave resonance events?

During ENSO events, shifts in heat and convection across the tropical Pacific ripple outward, altering the structure and strength of the jet stream. These changes can create atmospheric conditions that either favor or suppress the trapping and amplification of planetary waves. ENSO acts more like a background conductor, subtly influencing the broader atmospheric state in which planetary wave resonance may occur. The relationship is complex and not always immediate. In fact, our findings suggest that peak QRA years often follow the mature phase of strong El Niño events, pointing to a lagged influence that isn’t yet fully understood.

7. Are current climate models accurately predicting the future increase in these extreme weather events?

Part of why this is so significant is that the current generation climate models do not do a good job in capturing this phenomenon (as we’ve shown in past work), and this means that the models are likely underestimating the potential for increase in these sorts of damaging and deadly summer weather extremes.

8. How do scientists detect and measure planetary wave resonance events?

We detect planetary wave resonance by examining atmospheric conditions that favor the trapping of certain large-scale waves in the jet stream, creating the potential for those waves to resonate and become amplified. To confirm a resonance event, we use spectral analysis—a technique that breaks down the atmosphere’s wave structure into distinct components. This allows us to isolate the relevant waves (typically those that circle the globe six to eight times) and track their strength and duration over time. 

9. What is the role of wave amplitude versus phase conditions in the observed increase of QRA events?

The observed increase in QRA events reflects the alignment of both wave amplitude and wave phase conditions. For a QRA event to occur, high-amplitude waves must also be in just the right phase, meaning they are positioned in the atmosphere in a way that allows them to become trapped and resonate. Without that precise alignment, even strongly amplified waves may simply move along and fail to lock into place. In essence, it takes both strength and phase to trigger these rare but high-impact resonance events.

10. For future research, what are you and your team looking to explore?

We would like to explore the ability of more recent, higher-resolution climate models to better capture the underlying physical mechanisms of QRA. 

By testing these advanced models, we are also better positioned to develop a more comprehensive mechanistic understanding of how ENSO interacts with QRA—an important and still evolving area of research.