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In 2018, an atmospheric block caused drought that dried out the Rhine River in Germany.

CHRISTOPHE GATEAU/DPA/ALAMY LIVE NEWS

Why does the weather stall? New theories explain enigmatic ‘blocks’ in the jet stream

It was the summer of 2003 in Europe, and, for a while, it seemed as if Earth’s weather system had broken down. For weeks a huge mass of air stalled over the continent, slowly subsiding and suppressing cloud formation, leaving day after day of brilliantly clear skies. The mercury rose, and a record-breaking heat wave gripped countries including France and Germany, causing 70,000 deaths. Then, as abruptly as it set in, the persistent atmospheric block eased, and high winds brought relief.

Few weather phenomena are as widely experienced—but poorly understood—as an atmospheric block. When a block arises, typically at the western edge of a continent, the jet stream splits, trapping a blob of seemingly static air thousands of kilometers across. Such blocks can last for weeks, and drive heat waves, drought, and winter cold snaps. At the same time, the persistent flows around the edges of a block can route storm after storm to the same spot. A block “has very different impacts in different seasons,” says Tim Woollings, an atmospheric dynamicist at the University of Oxford. “But it’s always quite extreme.” Yet atmospheric scientists have long struggled to understand why blocks occur and last so long, and how they might change in a warming world.

Several new theories are offering answers. A leading idea links blocking to the behavior at high latitudes of the Coriolis force, an effect of Earth’s rotation that can cause the jet stream to meander and contort. The theory, developed by Harvard University atmospheric scientist Lei Wang, is unlikely to be the full picture, but it has a sobering implication. As the world warms, the jet stream is likely to move to higher latitudes, which could lead to even more blocking events.

The new ideas about blocking emerged from the debate over another potential impact of climate change. Researchers led by Jennifer Francis, a climate scientist at the Woods Hole Research Center, have suggested a warming Arctic is leading to more severe winter storms in the midlatitudes. Francis has argued that the encroaching warmth is slowing the jet stream and causing it to form larger blocklike meanders, exacerbating winter weather. But other scientists are skeptical of the mechanism. The controversy has exposed just how much there is to learn about blocking, Wang says. “The lack of theory is really the root cause of a lot of confusion we now have.”

Scientists did have a standard explanation for what sustains blocks. Published in 1983, the idea stems from a close study of a single block in 1976 that led to drought in Europe. Once the block was established, the split jet stream seemed to shear later weather systems apart. The energy of these atmospheric eddies then fed into the slowly rotating trapped airmass, creating a feedback loop that reinforced and sustained the block. It was an elegant idea, says Stephan Pfahl, an atmospheric dynamicist at the Free University of Berlin, but “its connection to more realistic cases has always been a question mark.”

Wang revisited this work. First, he simulated blocking events in a series of simple atmospheric models, hoping to detect the eddy feedback, as it’s known. He was shocked—it didn’t exist. Looking at the original calculations from the 1980s, Wang saw an error: They generated the feedback even when the block was missing. He then used modern weather records to examine some 100 similar blocking events over several decades. He found that eddies played a minimal role, at most, in maintaining them, as he reported this year at the annual meeting of the American Meteorological Society. All signs pointed to one conclusion, he says. “The existence of the feedback that was discovered in that paper is a false alarm.”

No change in the weather

In a “dipole” atmospheric block, the jet stream splits, wrapping around slowly rotating high- and low-pressure air masses and trapping them.

HIGH LOW
MAP: X. LIU/SCIENCE; DATA: U.K. MET OFFICE/OPEN GOVERNMENT LICENSE V1.0

Two years ago, Wang’s former adviser, atmospheric scientist Noboru Nakamura at the University of Chicago, proposed an alternative explanation for blocks. Over the years, he had painstakingly developed an equation to track large meanders in the jet stream. Such bends can eventually break, causing the jet to split. Nakamura found that these meanders seemed to behave like cars on a highway; pile enough together on the jet, and a traffic jam—a block—arises, he argued 2 years ago in Science. Atmospheric scientists are still digesting Nakamura’s proposal. But Wang and others suspect it may be more complicated than needed.

Wang and his co-author, Zhiming Kuang, also at Harvard, have come up with their own theory. While a student at the University of Chicago, Wang had pulled off the lunchroom shelf the dissertation of Tu-cheng Yeh, who studied in Chicago with pioneering geophysicist Carl-Gustaf Rossby in the 1940s. Yeh, who became a famed meteorologist in China, had noted that blocking events are more common and last longer when the jet stream moves to higher latitudes.

That pattern, Yeh proposed, is set by the atmosphere’s inherent dispersiveness—essentially, the noise and chaos in the system. At low latitudes, the atmosphere is too noisy to sustain a long-term pattern. For example, giant undulations in airflows like the jet stream, called Rossby waves, are quickly canceled out by other waves of different amplitudes and frequencies. But at higher latitudes, where the globe gets narrower, the waves generated by the Coriolis force are squeezed into a smaller band. That squeeze makes them more likely to align, which can amplify them, forcing the jet to split and give rise to a block.

Yeh had demonstrated his idea only in a simple, 1D model. But now, Wang and Kuang have found it holds in more complex simulations. The work is still under review, with one part posted on the preprint server arXiv. “But it’s potentially a breakthrough,” says Jian Lu, a climate dynamicist at the Pacific Northwest National Laboratory. “Fundamentally, blocking may be a simpler phenomenon than we previously thought.” In the past, simple explanations couldn’t explain why long-lasting blocks are so frequent. In Wang’s model, the latitude dependence does just that, says Eric DeWeaver, a program manager for climate and large-scale dynamics at the National Science Foundation. “It’s very appealing as a back to basics rethink.”

Wang’s theory shares a flaw with its predecessors, however: They all treat the atmosphere as dry. Over the past 5 years, Pfahl and others have shown that moisture is crucial to sustaining blocks. Indeed, Pfahl found in case studies and models that many blocks persist only if fed air in which water vapor has condensed into clouds and rain, releasing heat that uplifted the air mass. “For me, this is the really new thing,” Woollings says.

The next step will be to see whether the new insights can improve forecasts of weather and climate. Current models project a small decline in the frequency of blocks as the world warms, but scientists have low confidence in that prediction, says Olivia Martius, an atmospheric dynamicist at the University of Bern. “There is no simple answer” for how to improve such predictions. But the new theories could identify thresholds—specific atmospheric conditions—at which blocks are likely to proliferate, Nakamura says. “If you can nail the threshold condition, then you can ask how climate changes that threshold.”

Wang’s theory suggests one answer. Warming is expected to push the polar jet stream in the Northern Hemisphere farther north. And Wang has found that shifting the stream 10° closer to the pole could bring a 10-fold increase in blocks—and the heat waves and droughts they foster. “Northern Europe would experience many more,” Lu says. “Especially Russia. It’s a huge impact.”