Science 6.03.2020

SCIENCE sciencemag.org 6 MARCH 2020 • VOL 367 ISSUE 6482 1063

Francis, a climate scientist at the Woods Hole

Research Center, have suggested a warm-

ing Arctic is leading to more severe winter

storms in the midlatitudes (Science, 18 April

2014, p. 250). Francis has argued that the en-

croaching warmth is slowing the jet stream

and causing it to form larger blocklike mean-

ders, 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 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 ed-

dies then fed into the slowly rotating trapped

airmass, creating a feedback loop that re-

inforced and sustained the block. It was an

elegant idea, says Stephan Pfahl, an atmo-

spheric 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 simu-

lated blocking events in a series of simple at-

mospheric models, hoping to detect the eddy

feedback, as it’s known. He was shocked—it

didn’t exist. Looking at the original calcula-

tions 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 simi-

lar 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 Ameri-

can Meteorological Society. All signs pointed

to one conclusion, he says. “The existence of

the feedback that was discovered in that pa-

per is a false alarm.”

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 Univer-

sity 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 sys-

tem. 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 dif-

ferent amplitudes and frequencies. But at

higher latitudes, where the globe gets nar-

rower, 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 simu-

lations. 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. “Fundamen-

tally, blocking may be a simpler phenom-

enon 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 dynam-

ics at the National Science Foundation. “It’s

very appealing as a back to basics rethink.”

Wang’s theory shares a flaw with its pre-

decessors, however: They all treat the atmo-

sphere as dry. Over the past 5 years, Pfahl

and others have shown that moisture is cru-

cial 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, releas-

ing 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 dynami-

cist 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 far-

ther north. And Wang has found that shift-

ing 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.” j

HIGH

LOW

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.

In 2018, an atmospheric block caused drought that

dried out the Rhine River in Germany.

CREDITS: (PHOTO) CHRISTOPHE GATEAU/DPA/ALAMY LIVE NEWS; (MAP) X. LIU/

SCIENCE

;

(DATA) U.K. MET OFFICE/OPEN GOVERNMENT LICENSE V1.

Published by AAAS