(singke) #1
SCIENCE 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



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.





Published by AAAS
Free download pdf