AS DEPUTY director of the Japan Esperanto Society, it was clear what language Wasaburo Ooishi would choose to publish his discovery in. Unfortunately, it meant hardly anyone noticed.
In the mid-1920s Ooishi, a meteorologist in his day job, was releasing research balloons near Mount Fuji when he saw something odd. Once the balloons had climbed high into the atmosphere above the clouds, they suddenly hurtled out eastwards over the Pacific. Persistent high-level winds, often stronger than a hurricane, were blowing from west to east over Japan.
Other people had observed something similar in Europe, but Ooishi was the first to put two and two together and pinpoint the existence of a permanent, narrow tunnel of wind circling Earth at mid-latitudes, travelling at 100 to 400 kilometres per hour.
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Gradually, knowledge of the jet stream circulated around the globe, too – albeit by unconventional means (see “Fu-Go no go“). Today, surfing the jet stream is commonplace: slipstreaming on it eastbound can slice up to an hour off a flight across the Atlantic. And as we have learned more about the jet stream, it has become clear that it is no rarefied curiosity. Its speed and path is the invisible hand guiding most weather systems on the continents below. When it falters, extremes of all sorts can result, from freeze-ups to droughts, heatwaves and catastrophic floods.
That makes it all the more worrying that, just lately, the jet stream has seemed to be changing. But is it really? And, if it is, how and why?
See more in our gallery: “Jet extremes: When winds turn the weather wild“
Earth’s atmosphere actually has several different jet streams at different latitudes. The strongest are the polar jet streams, one each in the northern and southern hemispheres. A few hundred kilometres across, these polar jet streams carve a sinuous path at the top of the troposphere, the lowest layer of Earth’s atmosphere. They lie anywhere between 7 and 12 kilometres up, at latitudes generally between 50 and 70 degrees, although with periodic excursions beyond.
Their origin is simple enough. Where cold, dense polar air meets warmer, lighter air from near the equator, winds rush in to equalise the pressure difference. Earth’s west-east rotation diverts these winds from what would otherwise be a north-south trajectory to one travelling east. In the southern hemisphere, the polar jet encircles the Antarctic, mostly over the Southern Ocean. In the northern hemisphere, it passes over North America, Europe and Japan – some of the most densely populated places on Earth.
And we feel its drag on the ground. The fronts and low-pressure systems familiar in our weather forecasts are the jet stream’s earthbound manifestations, as its high winds pull the air below around the planet (see diagram). Most of Europe’s weather rides in under the jet stream from the Atlantic and most of the western US’s weather comes from the Pacific in a similar manner. If you are north of the jet stream – and so beneath air from the poles – it will be cold. If you are to the south, it will be warm. If you are under the jet stream’s path, as it sucks moist air upwards water will condense, and fronts will bring changeable, rainy weather.
But the jet stream does not follow a straight line. It meanders like a river on a flat floodplain, sometimes moving north, sometimes south. Most often, such meanders are triggered by the stretching and squashing of the air as the jet stream passes over mountains. Known as Rossby waves, they travel slowly east, typically taking a week to cross North America, for instance.
Occasionally, they get stuck, generally when the jet stream slows as a result of random fluctuations in the temperature difference between polar and non-polar air. That brings “blocking highs” within the loops of the malingering meanders. Bits of the globe get stuck under a vast tongue either of hot, dry air stretching north from the tropics in summer, or of ice-cold air reaching south from the Arctic in winter.
A couple of days of hot or cold, wet or dry doesn’t matter much to most people. But a couple of weeks can matter a great deal. The long heatwave in Europe in 2003 – a classic piece of sticky weather, in all senses – killed an estimated 70,000 people. Meteorologists now blame the dust bowl in the US Midwest in the 1930s on a faltering jet stream that tracked south, diverting the usual rains and triggering drought in the blocking zone.
Arctic heatwave
The jet stream has been particularly weak during several recent northern winters, meandering erratically and bringing polar air plunging as far south as Florida – where chilled iguanas fell out of the trees – and delivering long cold winters in western Europe that, for some, made a mockery of the idea of global warming. The massive summer heatwave of 2010 that sparked forest fires across Russia also arose because an exceptionally long-lasting blocking high brought hot dry air up from Africa for weeks on end. At the same time, further east, another long loop in the jet stream pushed wet air down from the north towards the Himalayas, where it interacted with the Asian monsoon and delivered huge floods down the Indus river. At one stage, a fifth of Pakistan was under water.
“The jet stream brought polar air south to Florida where chilled iguanas fell out of the trees”
A changeable jet stream and intermittent blocking highs have always been part of weather in the middle latitudes, but some researchers see a worrying trend. In 2008, Cristina Archer, now at the University of Delaware in Newark, and Ken Caldeira of Stanford University in California analysed jet-stream data from 1979 to 2001 and found In late 2012, James Overland of the US National Oceanic and Atmospheric Administration reported that the jet stream had . Also last year, of Rutgers University in New Brunswick, New Jersey, found that since the 1990s its average speed in autumn has fallen by 14 per cent over North America and the North Atlantic, .
At a US Senate hearing in July this year, Francis went so far as to suggest that a weakened jet stream had caused tropical storm Sandy to take the unusual path that devastated parts of Manhattan last year. She was speaking days after Anchorage, Alaska, and Norilsk in Siberia reported temperatures more akin to the Mediterranean – all blamed on blocking highs funnelling heat north.
Meanwhile, the southern hemisphere’s polar jet has also gone walkabout, drifting poleward but strengthening in recent decades. There is strong evidence that both are due to the ozone hole in the stratosphere above. This southern drift , because this area is now less often under the southern polar jet stream.
In the northern hemisphere, however, Francis thinks something odd is afoot that has nothing to do with ozone holes. In March last year, together with Stephen Vavrus of the University of Wisconsin-Madison, she published her idea that . The Arctic is heating up two to three times faster than most of the rest of the planet, as white snow and ice that reflect solar energy back into space are disappearing, to be replaced by dark, energy-absorbing ocean and land. This “Arctic amplification” means that the temperature difference between the Arctic and lower latitudes is diminishing. Since this difference creates the jet stream, it will weaken too. “The dynamics are complicated, of course, but what we are seeing is the effect of Arctic amplification on the jet stream. I am convinced of it,” she says.
By no means everyone is persuaded by Francis’s argument, however. at the University of Reading, UK, for example, doesn’t discount Arctic effects, but thinks changes in the stratosphere caused by low solar activity might also be playing a part in slowing the jet stream. “The jury is out,” he says. “I wouldn’t rule out either factor, or indeed that they are working together.” And the apparent role of stratospheric ozone loss in the acceleration and poleward shift of the southern hemisphere jet shows that stratospheric influences can also be strong on the jet streams beneath.
Poles apart
Perhaps the strongest backing for Francis’s idea about the northern jet stream comes from the fact that it weakens and wavers most obviously in autumn, right after the September peak of seasonal ice loss in the Arctic Ocean when the north-south temperature gradient is at its smallest.
Many researchers remain cautious about drawing definitive conclusions, however. Modelling studies do show that if all the Arctic sea ice disappears at the end of summer, the probability of blocking episodes increases, making colder spells more likely the following winter, says Julien Cattiaux of the French National Centre for Meteorological Research in Toulouse. “But recent blocking episodes are single exceptional events, and their rarity prevents us from drawing any conclusions on long-term changes.”
of Colorado State University in Fort Collins is more critical. She argues that the slower and wavier jet stream described by Francis was in fact “an artefact of the methodology” used to measure the Rossby waves. She has reanalysed Francis’s numbers, and argues that poleward movement of the northern polar jet stream corrupted the data and that the weather was not sticking any more than it used to. “We find no significant increase in blocking highs in any season,” she says.
The dispute has got personal, with Francis publicly accusing Barnes of . One leading figure in the field refused to comment on the work when contacted by New Scientist for fear of being seen to take sides. Atmospheric physicist of Imperial College London strikes a more emollient note. “I don’t think Barnes is saying this [greater wave activity] doesn’t happen, just that Francis hasn’t properly established it,” she says.
What this brouhaha does establish is how difficult it is to say with great confidence what the future holds for the jet stream. Most of the big computer models developed to predict climate change – including a new analysis of different models developed for the latest assessment from the Intergovernmental Panel on Climate Change, published online last month – actually predict faster polar jet streams, and fewer blocking highs in winter, though more in summer, says Cattiaux. This would mean more summer heatwaves but fewer instances of persistent extreme winter cold. The lengthy review process for the IPCC report meant it was not able to include an analysis of Francis’s recent work. It says simply that ““.
The future of the weather for billions of people depends on who is right – in ways big and small. Sirpa Häkkinen of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, has argued that in the far North Atlantic. Currents there are directly driven by surface wind patterns, which are themselves driven by the jet stream. These include the overturning circulation, sometimes called the Global Conveyor, which maintains the Gulf Stream that keeps north-western Europe unusually warm for its latitude.
“The most important point about the jet stream is how chaotic it is on all time scales, from day-to-day changes to year-on-year and even decadal variability,” says Cattiaux. The growing fear is that, although we cannot exactly predict how things will change, big changes are coming. Such sudden and visceral changes to our day-to-day weather are more likely to bring home the reality of climate change than any gradual changes in average temperature. In some of the most heavily populated parts of the planet, we could be in for a bumpier ride than even climate modellers predict.
Fu-Go no go
Wasaburo Ooishi’s pioneering research into the jet stream in the 1920s was “essentially ignored” in the West because he published his research in Esperanto (see main story). So says John Lewis of the US government’s National Severe Storms Laboratory in Reno, Nevada, who has researched the affair.
The turnaround came during the second world war when, now in conflict with the US, Japan hatched a plan to surprise Uncle Sam by using the wind to express-deliver bombs. Hydrogen balloons rode the jet stream from Japan, carrying incendiary devices that were timed to drop on arrival over land. Guided by Ooishi’s wind charts, 9000 balloon bombs, called Fu-Go, were unleashed from Japan between November 1944 and April 1945.
Luckily for the US, Japan’s meteorologists got the timing wrong. The jet stream was a little weaker than Ooishi had calculated, and the balloons took 96 hours on average to cross the Pacific, rather than the estimated 65 hours, says Lewis. All but about 300 of them dropped their bombs harmlessly into the Pacific Ocean. One that did make it hit a power line, blacking out the Hanford nuclear weapons plant in Washington, which was then preparing the atomic bombs destined for Hiroshima and Nagasaki. Another Fu-Go bomb landed on a Sunday school picnic in Oregon, killing six people – the only combat casualties on the US mainland during the entire war. That made the West finally wake up to the jet stream’s power. Balloon bombs spoke louder than Esperanto.
This article appeared in print under the headline “Jet extreme”
