The Jet Stream Gets Weird / Jeff Masters

The harsh weather that has prevailed for the past four years in North America, both in winter and summer, may become a matter of routine

From November 2013 to January 2014, the jet stream in North America and Europe took on an extreme form that persisted over time. This global river of eastward winds in the upper atmosphere curved further south than usual and passed over the eastern US, allowing cold air from the infamous "polar vortex" that normally swirls over the Arctic to rush south and freeze the eastern two-thirds of the US. B. Ice cover on North America's Great Lakes has reached the second-largest recorded area ever, and two crippling snow and ice storms shut down Atlanta for several days.

At the same time, a stubborn barometric ridge swept over California and brought the warmest winter on record. The warmth may have been pleasant, but the accompanying drought, which was the worst since records began in the late 19th century, caused billions of dollars in agricultural losses.
The meandering of the jet stream also hit Europe, where a series of intense storms added several billion dollars to the total damage. England and Wales recorded the wettest winter since at least 1766. while most of the other parts of Europe were immersed in extraordinary warmth. Norway saw unprecedented forest fires in January, and organizers of the Winter Olympics in Sochi, Russia, battled the bare ski slopes. In May, a large rainstorm flooded almost a third of the territory of Bosnia.
Usually the jet stream resembles a band of air blowing along the middle latitudes. In TV weather forecasts it is often seen to have gentle bends to the south and north, similar to a sine wave on an oscilloscope. These curves are called "planetary waves" or "Rossby Waves". Inside Rossby waves are smaller curves, called short waves, which usually cross the USA across its width in three to five days, and are responsible for most of the day-to-day weather changes.
But in the winter of 2013-2014, Rossby waves intensified and were characterized by enormous dimensions and steep curves, like an EKG chart out of its mind. This wind pattern also passed over the earth much more slowly than usual, and sometimes remained in place for weeks and caused unusual weather that lasted for unusually long periods. A study conducted by Shi-Yu (Simon) Wong of the University of Utah in May 2014 found that the jet stream pattern over North America this winter was the most extreme on record.
Was this extreme change in the jet stream an unusual anomaly? Apparently not, because it seems to be happening more and more. In 2010, Russia suffered the worst heat wave in its recorded history, which killed more than 55,000 people. At the same time, heavy rains flooded Pakistan in a natural disaster that caused the greatest financial damage in its history. In 2011 Oklahoma was hit by the hottest summer ever known to any state in the USA. The drought that hit the US in 2012 was the most extensive since the 30s.
The twists and turns of the jet stream during all these unusual events had a common feature, claims an article published in April 2013 by scientists from the Potsdam Institute for the Study of Climate Effects in Germany, led by Vladimir Petukhov. The waves, which normally move east, "stopped in place and became very strong," wrote two of the authors of the article on the blog that deals with their research. There were cases when the coils remained motionless for days and even months. The scientists also showed that the anomalous formations were twice as common in the summers of 2001 to 2012 than they were in the 22 years prior to 2001.
And as Bob Dylan once sang: "You don't need a forecaster to know where the wind blows," there is no doubt that something is happening to the jet stream, and it is not difficult to guess the reason. The basic state of our climate has changed dramatically in the last 150 years, and this change is beginning to show in the behavior of the jet stream. For example, the concentration of heat-trapping carbon dioxide in the atmosphere has increased by more than 40%, mainly due to the burning of coal, oil and natural gas. The area of ​​the ice sheet around the North Pole in summer has decreased by almost 50% since 1900, and this affects the flow of heat in the air and sea. The amount of solar energy that is returned from the surface of the Earth has changed greatly because we have changed the face of more than half of its surface with construction, field crops and pasture. Large clouds of soot and pollution, some of which reflect sunlight and some of which absorb it, are emitted from power plants, vehicles, buildings and factories. A huge hole in the ozone layer is disrupting the order of winds in the high air above Antarctica.
Humans have dealt a severe blow to the climate system, and the laws of physics dictate that the fundamental weather patterns of the planet will change as a result. And it is true that Wong and his colleagues conclude that the configuration of the jet stream probably would not have distorted so much if it were not for the global warming caused by humans.
What is dangerous is that the climate does not behave linearly. A slight increase in the global temperature could cause a sharp transition to a new regime of weather that is completely different from the one that prevailed before. Climate scientists are vigorously debating whether the climate in general, and the jet stream in particular, have crossed some threshold and moved into a new long-term regime. They also deal with a controversial theory put forward by researchers from the Potsdam Institute and others, according to which the changes in the jet stream originate in large part from events occurring in the fastest-warming region on Earth: the Arctic.
If the jet stream does transition to a new state, it is bad news for humanity. An article published by James Screen of the University of Exeter in England and Ian Simmonds of the University of Melbourne in Australia in the August 2014 issue of the journal Nature Climate Change even lists the possible outcomes. If the tidal fluctuations in the jet stream "increase in response to anthropogenic [man-made] climate change, as some researchers have suggested," they write, "our results show that this will increase the likelihood of heat waves in western North America and central Asia, cold waves in eastern North America, droughts in central North America, Europe and Central Asia, and extreme rains in Western Asia."
The new routine will take place in terrible summer forms in the Midwest of the USA. Series of snowstorms like "snowmageddon" that paralyzed Washington, DC in 2010, will hit the eastern US with greater frequency. And all over the world, food prices will rise due to the severe, long-term droughts that will affect Central North America, Europe and Central Asia.
natural variability
Climate change affects the jet stream indirectly: it acts on the major forces in the atmosphere that ultimately shape the shape of the jet stream. The nine to 14 kilometer high perpetual river of winds hugs the earth in its northern and southern halves and acts as a carrier and directs along its length the low pressure systems that carry the precipitation. The current usually has two branches: a polar jet that acts as a buffer between the cold air near the poles and the warmer air closer to the equator, and a subtropical, milder jet that is deployed closer to the equator. From here on, wherever the combination "jet stream" appears, it refers to the polar jet, which is more important.
The latitudes where this jet flows vary slightly according to the season: in the winter the jet usually moves over the center of the USA and in the summer over the border of the USA and Canada. But the flow is chaotic and always has large Rossby waves. In the Northern Hemisphere, as the jet stream curves northward as a barometric plateau, warm air flows from south to north. As the jet curves south, like a depression, cold air flows south.
The jet stream is created by three large cells of swirling air movement intertwined in both hemispheres of the Earth [see box on previous pages]. The flow chambers do participate in the shaping of the jet stream, but there are additional forces in the heights that distort it. The atmosphere literally oscillates like a string under the influence of the sun's energy, the shape of the continents, the currents of the oceans and their location, the mountain ranges and the amount of heat-trapping greenhouse gases and light-reflecting dust particles in the air. Just as a guitar resonates with a different sound according to the specified string, so changes in these components cause the atmosphere to oscillate in different sounds called long-term patterns (teleconnection). These natural reverberations can affect the shape of the jet stream, making it difficult to determine whether its recent behavior is a sign of permanent change.
In the Northern Hemisphere, the most important long-term patterns are El Niño, or the "Southern Oscillation" and the "Arctic Oscillation". The Southern Oscillation is a cyclical fluctuation in the atmospheric pressures of the tropical region that lasts three to eight years. The cycle has two peaks: in the first peak, known as El Niño, warmer than usual seawater moves to the eastern Pacific Ocean, and in the opposite peak, known as La Niña, colder than usual water moves. During El Nino, the jet stream over the eastern Pacific usually curves south, and during La Niña it curves north. The Arctic Oscillation is caused by weekly fluctuations in the pressure differences at sea level between the Arctic region and between the middle latitudes. When the pressure differences are small, the jet stream winds tend to weaken, allowing for large bends. Small pressure differences are typical for winter and as a result cold air flows far south to the eastern US, western Europe and eastern Asia.
Revelation in California
The long-term patterns in the atmosphere are intertwined. They can cancel or enhance each other. Changing the basic state of the atmosphere that creates these patterns, may change them in a way that causes the jet stream to be distorted. I thought about this possibility in 2011 in view of a strange phenomenon: an extreme jet stream stopped in its place when it was accompanied by a weak to moderate La Niña that lasted only part of the year. At that time, theories detailing how such a situation could arise had not yet been published. But in December of that year, at the "United States Geophysical Union" conference, the largest global gathering of climate scientists, held in San Francisco, Jennifer Francis, an atmospheric scientist from Rutgers University, presented new and very intriguing findings related to the event. At one point she said: "The question is not whether the shrinking of the ice sheet [in the Arctic] affects the major atmospheric circulations..., but how is it possible that it does not?" Francis mentioned that the Arctic region is warming at a rate two or three times faster than other regions in the northern hemisphere of the planet, a phenomenon known as "Arctic amplification", and declared that this phenomenon could significantly disrupt the flow of the jet stream in the northern hemisphere of the planet.
This statement makes a lot of sense. One of the main reasons for the Arctic increase in autumn and winter is the decrease in the amount of sea ice. The Arctic Ocean has lost a huge amount of ice in recent years due to melting and due to winds acting to reduce it. In September 2012, 49% of the ice sheet was lost compared to its average area between 1979 and 2000, an area equal to 43% of the area of ​​the USA without Alaska. When the floating ice melts, it exposes water, which is darker than the white ice and therefore absorbs more solar energy. And because of that the ocean and the atmosphere are warming, increasing the warming and the melting of the ice in a destructive vicious cycle.
The exposed water releases the heat it has stored in the fall and winter, which leads to a heavy disturbance, lasting months, in the basic state of the arctic atmosphere. Anomalous Arctic intensification is also occurring in the summer lately, because the snow-covered areas are shrinking. Global warming is bringing spring forward at the rate of about three days per decade, and the snow is melting and exposing the dark soil beneath earlier. The soil absorbs heat and dries out, and precedes the beginning of the continental warming season.
The Arctic amplification, which results from the shrinking of the ice sheet and the snow-covered areas in the spring, and other factors, have considerably reduced the temperature differences between the mid-latitudes in the north of the Earth and the North Pole. The narrowing of the difference can greatly affect the jet stream. If the difference is small, less energy passes between two of the large vortex cells in the atmosphere, and the jet stream winds slow down. Francis and Stephan Weavers of the University of Wisconsin-Madison found a 10% decrease in wind speed in the high brome in autumn over North America and the North Atlantic since 1979, consistent with a reduction in the temperature difference.
The relatively slow airflow allows the jet stream to meander in large curves, and Francis has documented a marked increase in the size of the arctic jet's meanders since 2000 in both summer and winter. The large bends usually allow warm air to flow much further north than usual to areas closer to the pole on one side of the jet stream, and cold air to move farther south on the other side. Such a pattern characterized the cold wave that hit the eastern US in January 2014, the well-publicized "invasion" of the polar vortex, and at the same time the record heat and drought in California. The math shows that a slower jet stream can also slow down the progress of the short waves to the east, leaving in its place the unusual weather caused by the large bends for a longer time. It is also possible that these meanders are more prone to a complete stop and the creation of "barriers" that prevent the undulating movement of the jet stream, similar to the way in which recurring eddies create "dead" spots in a river where there is no flow.
Controversy as to the role of the Arctic region
The study linking the Arctic amplification and the jet stream insanity caused an uproar among climate scientists. More than 2013 scientists came to a workshop on this topic, convened by the US National Research Council in 50 at the University of Maryland, and there was a vigorous debate. Although many of the experts in the field agree that the jet stream is apparently undergoing a change, many dispute that the relatively short period of about 15 years, during which the Arctic amplification is strongly expressed, is enough to link the two phenomena.
Some experts even doubt the hypothesis due to energy considerations: since the large-volume flow of the jet stream carries a lot of energy, a lot of energy is also needed to change the flow. The amount of heat energy added to the Arctic region due to Arctic amplification is ten times smaller than the energy of those changes that were studied in the natural changes caused by El Niño, or the Southern Oscillation, in the jet stream, claims Kevin A. Trenberth from the US National Center for Atmospheric Research. He co-authored an article published in August 2014 in the online edition of the journal Nature Climate Change, showing that the large energy changes that have occurred naturally in the tropical Pacific in recent years due to a long-term pattern known as the "Pacific Decadal Oscillation" may have caused the waves The exception we have seen in the jet stream recently. And yet, the article also concludes and says that the nature of the changes that this fluctuation has known in the last ten years perhaps indicates that climate change is causing changes in the range of natural changes itself.
Turnbreth was one of five leading climate scientists who published a critique of Francis's study in the journal Science in February 2014. The study linking Arctic warming to increased jet stream undulations "deserves a fair hearing," they write. But in conclusion, in their eyes "the theoretical arguments on which it is based leave room for doubt".
Some scientists even doubt the fact that the waves in the jet stream are increasing. In a 2013 paper, Skrein and Simmonds measured the bends of the jet stream using a different standard than Francis and found few statistically significant changes in the size of the bends, although they did find a weak overall increase. But critics have not offered many other explanations for the extreme events in the behavior of the jet stream. One idea, published in the August 2014 issue of the Proceedings of the National Academy of Sciences (PNAS) by Die Comeau of Potsdam and his colleagues, suggests that a narrowing of the temperature difference between the poles and the midlands is enough to increase the bends in the jet stream and lock them in place, at least in the summer.
too late to wait
The scientists' opinions are still divided regarding the explanations, but the weather data cannot be ignored. In 2011 and 2012 alone, there were several weather events that equaled and even surpassed some of the most famous and destructive weather events in US history: the "Super Tornado" storm outbreak in 1974, the heat and drought event of 1936 known as the "Dust Bowl" , and the great Mississippi floods of 1927. It is quite possible that the behavior of the jet stream recently indicates the crossing of a threshold and a transition to a new, more energetic climate.
As the planet warms, the rising temperatures will trigger heat waves and in more severe forms in places where barometric ridges will reach and will be carried along in the curves of the jet stream. Stronger storms with more precipitation will hit places where the jet will curve south toward the equator and will bring with it depressions of low pressure along with the increased evaporation from the oceans that will introduce more moisture into the atmosphere. If the jet stream continues to be characterized by slow and large waves, the severe weather conditions they cause will worsen. And this extreme weather will stay in place for a long time and sow death and destruction. If the theories presented by Francis and her colleagues are correct, the old climate will not return unless we find a way to grow ice around the North Pole. Since the amount of carbon dioxide trapped as heat in the atmosphere continues to grow at a rate of about half a percent per year, no scientist studying the Arctic ice sheet foresees its long-term recovery.
Drought is the greatest danger of all, as it affects the two things most necessary for our survival: water and food. If a particularly wavy pattern of the jet stream causes abnormal barometric levels to be stuck for an entire summer over the grain growing areas in Russia and the USA, there will not be enough rain needed for these crops. In such forms may cause sharp increases in food prices that will lead to hunger and violent conflicts in many parts of the world. The great heat wave and drought that hit Russia in 2010 was caused by a large and impenetrable barometric ridge of high pressure that "settled" above it. The ridge pushed the low pressure systems that normally bring rain to the fields of Russia towards Pakistan and caused terrible floods there. The drought and heat wave were the deadliest and most expensive natural disaster in Russian history. They forced it to reduce its wheat exports, which raised world grain prices and fueled the outbreak of the "Arab Spring" that overthrew several governments in 2011.
Clearly, the world cannot wait for scientists to fully understand why the climate is changing. According to the Intergovernmental Panel on Climate Change (IPCC), we must act quickly, with full force and worldwide to prevent global warming from crossing the dangerous two-degree threshold. By 2050, the use of energy sources such as solar, wind and nuclear, which emit almost no carbon dioxide, if at all, must be tripled, technologies for the capture and burial of carbon dioxide must be implemented and greenhouse gas emissions must be reduced by 40% to 70% from its level in 2010 . The economic price of this move may be incredibly low, entailing a reduction of only 0.06% per year in the global growth rate, the IPCC states. But if we wait until 2030, the necessary actions will be much more expensive and there may be no escape from crossing the threshold.
In 2030 it is also expected that the ice in the North Pole will disappear completely in the summer, according to some leading climate scientists. If the changes in the Arctic are really causing the wild behavior of the jet stream, the disappearance of the entire Arctic summer ice sheet by 2030 will cause even greater shocks. But even if the Arctic region is not involved, this is worrisome because then the changes in the jet stream occur due to an unknown mechanism, and we have no idea how the jet stream will respond to continued climate change. My prediction for the next 15 years is therefore: expect unprecedented events.
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About the author
Jeff Masters is responsible for the field of meteorology at the Weather Underground organization and was one of its founders in 1995. He specializes in predicting extreme weather conditions. He also writes WunderBlog, one of the most popular weather blogs on the web.
in brief
Severe weather events have occurred in the past four years as the jet stream has become radically distorted.
Unusually abnormal weather prevailed for extended periods because these bends in the jet stream were "stuck" in place.
Some scientists claim that the main reason for the strange behavior of the jet stream is the shrinking of the ice sheet in the Arctic region, although other experts believe otherwise.
Either way, the extreme deviations in the path of the jet stream are expected to cause more severe droughts, floods, heat waves and freezing waves in many parts of the world.
More on the subject
Linkages between Arctic Warming and Mid-Latitude Weather Patterns: Summary of a Workshop. Katie Thomas et al. National Academies Press, 2014. www.nap.edu/catalog.php?record_id=18727
Jeff Masters' blog: www.wunderground.com/blog/JeffMasters/show.html
US National Climate Assessment: http://nca2014.globalchange.gov
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