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JET STREAMS + CLIMATE CHANGE
A Science
Backgrounder
A new way of looking at climate change and weather
The jet streams are bands of wind that ring the planet and affect the weather we all experience. The jet streams have go:en a great deal of media and scien;fic a:en;on lately, especially surrounding recent weather extremes and their possible connec;ons to global warming.
Changes to the jet streams have been implicated in a number of recent extreme weather events, including record heat in Alaska, cold in Britain, flooding in Alberta and more. Not surprisingly, the science behind the jet stream is oFen misunderstood.
The four jet streams’ paths around the world. Source: NOAA
This backgrounder summarizes the state of the science in three parts: observed changes in the jet stream; connec;ons to global warming; and connec;ons with recent extreme weather events.
Much of the science in this backgrounder is quite new, and scien;sts acknowledge that much more research is needed before we can fully understand the rela;onship between global warming, the jet stream and extreme weather. Scien;sts may not agree on every point, so this backgrounder is meant as a star;ng point rather than the final word.
That said, the research done thus far points toward a more complete picture of how global warming has already changed our climate and what we might expect in the future. Some scien;sts even say it may result in the upending of previously accepted ideas about the mechanisms behind the rela;onship between global warming and extreme weather.
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What’s happening to the jet streams?
Shi@ing toward the poles:
-­‐ In the Northern Hemisphere, the polar jet stream is shiFing northward, also described as “poleward” movement (although the shiF may differ by season).
-­‐ In the Southern Hemisphere, the polar jet stream is shiFing southward too, but this shiF may be par;ally counteracted by the restora;on of the Ozone hole, which pushes the opposite way.
-­‐ These shiFs are consistent with IPCC climate model simula;ons, which predict a poleward shiF of the jet streams as the planet warms.
Slowing and weakening:
-­‐ In the Northern Hemisphere the west-­‐to-­‐east wind flow in the polar jet is slowing and weakening.
-­‐ In the Southern Hemisphere the sub-­‐tropical jet has weakened, whereas the polar jet has strengthened.
Weakening jet streams tend to have larger north-­‐south waves. When these waves get larger, they move more slowly from west to east, which can cause weather systems to also move more slowly and even become stuck in place. This in turn increases the likelihood of disasters caused by persistent weather condiGons, including droughts, heat waves, floods, long cold spells, and heavy snows.
Becoming wavier and more meandering:
-­‐ The large north-­‐south waves in the jet stream (Rossby waves) are occurring more frequently and may be increasing in amplitude.
Larger waves can cause cool air to be pushed toward the equator when the waves dip to low laGtudes, and warm air to be pushed toward the poles when the waves swing back. For areas on the ground below these waves, that translates into wild and unseasonable temperature extremes, someGmes called “weather whiplash.”
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How could global warming be causing these jet stream changes?
Reduced temperature difference between the Equator and poles:
-­‐ The en;re globe is warming, but Arc;c temperatures are warming at a faster rate than the rest of the planet. This means that the difference between the Northern Hemisphere’s warmest and coolest temperatures is shrinking.
-­‐ As the Associated Press described Jennifer Francis’s research: “It all starts with the difference between cold temperatures in the Arc;c and warmer temperatures in the mid-­‐la;tudes … The bigger the temperature difference, the stronger the jet stream, the faster it moves and the straighter it flows.” So, the reduced temperature difference may be causing the slower and more meandering jet stream.
Increased alGtude of the tropopause (boundary between troposphere and stratosphere, about 6 to 11 miles from Earth’s surface):
-­‐ Increased greenhouse gases cause the troposphere (lowest layer of the atmosphere) to warm and expand, while they cause the upper atmosphere to cool and shrink. Physics dictates this would raise the al;tude of the tropopause. Measurements exist showing this occurring, although the data are limited and require more research.
-­‐ IPCC models suggest that a rising tropopause could contribute to observed geographic shiFs in the jet streams.
The jet stream exhibiGng strong west-­‐to-­‐east “zonal flow” above, and a scenario showing a large Rossby wave over North America below, called “meridional flow.” Source: Barry and Chorley, “Atmosphere, Weather, and Climate.”
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How have the changes in the jet stream impacted extreme weather? Here are just a few recent examples:
June 2013 flooding in Alberta and Central Europe:
-­‐ Slowing of jet stream waves can prolong the dura;on of extreme events. -­‐ The jet stream has been iden;fied as a factor causing recent floods in Alberta and Central Europe. In these cases, the wet weather systems became trapped between slow-­‐moving jet stream waves and dumped much of their water in one place, which overwhelmed drainage systems.
May/June 2013 record-­‐breaking Alaska weather whiplash:
-­‐ When the jet stream veered south, it brought cool air from the Arc;c, causing record lows in mid-­‐May. Then the jet stream veered north, bringing warm air from the subtropics, causing record highs in June. -­‐ Scien;sts have suggested that these more frequent large waves in the jet stream are linked to global warming.
Spring 2013 cold snap in Britain and Northern Europe:
-­‐ March 2013 marked an unusually cold spring in Britain and much of Europe. This anomaly was connected to changes in the jet stream that scien;sts suggest may have been exacerbated by a warming Arc;c. A large, southward-­‐dipping Rossby wave in the jet stream allowed cold Arc;c air to cover areas it normally would not reach and stay for a long ;me.
-­‐ The Met Office (the U.K.’s na;onal weather service) issued a statement suppor;ng this theory and providing more background. -­‐ The cold snap is not part of a global cooling trend.
2012 (and ongoing) drought in the southwestern United States:
-­‐ Paleoclimate studies show that a we:er Southwest is associated with a cooler Northern Hemisphere and a southward shiF in the jet stream. We are currently experiencing a warmer Northern Hemisphere and a northward shiF in the jet stream, so increased Southwestern drought is consistent with previous pa:erns.
-­‐ Although study author Mar;n Hoerling denied a connec;on to global warming, his NOAA study explicitly linked the 2012 drought to the unusually northward posi;on of the jet stream. Other studies, including the draF Na;onal Climate Assessment, have linked drought in the Southwest and Great Plains to global warming. -­‐ Weaker west-­‐to-­‐east winds in the jet stream, with larger waves, also means that dry weather systems will s;ck around longer when they do appear, making droughts more likely and more damaging.
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Literature and Experts:
Arblaster et al. (2011). “Future climate change in the Southern Hemisphere: Compe;ng effects of ozone and greenhouse gases.” Geophysical Research Le0ers.
Archer and Caldeira (2008). “Historical trends in the jet streams.” Geophysical Research Le0ers.
Asmerom et al. (2010). “Variable winter moisture in the southwestern United States linked to rapid glacial climate shiFs.” Nature Geoscience.
Francis and Vavrus (2012). “Evidence Linking Arc;c Amplifica;on to Extreme Weather in Mid-­‐
La;tudes.” Geophysical Research Le0ers.
Hall et al. (2011). “Tropopause height at 78° N 16° E: average seasonal varia;on 2007–2010.” Atmospheric Chemistry and Physics.
Hoerling et al. (2013). “An Interpreta;on of the Origins of the 2012 Central Great Plains Drought.” NOAA Drought Task Force NarraAve Team.
Khandu et al. (2011). “GNSS remote sensing of the Australian tropopause.” ClimaAc Change.
Lorenz and DeWeaver (2007). “Tropopause height and zonal wind response to global warming in the IPCC scenario integra;ons.” Journal of Geophysical Research. Overland and Wang (2009). “Large-­‐scale atmospheric circula;on changes are associated with the recent loss of Arc;c sea ice.” Tellus A.
Perlwitz (2011). “Tug-­‐of-­‐war on the jet stream.” Nature Climate Change.
ScienGsts most acGvely publishing and speaking out on the jet stream’s connecGon to global warming and extreme weather include:
Jennifer Francis, Research Professor at the Ins;tute of Marine and Coastal Sciences, Rutgers University
James Overland, Senior Scien;st at NOAA’s Pacific Marine Environmental Laboratory and Affiliate Professor in the Department of Atmospheric Sciences, University of Washington
Judah Cohen, Research Affiliate at Massachuse:s Ins;tute of Technology's Parsons Lab and Director of Seasonal Forecas;ng at Atmospheric and Environmental Research
Stephen Vavrus, Senior Scien;st, Center for Clima;c Research, Nelson Ins;tute for Environmental Studies, University of Wisconsin-­‐Madison
James Screen, NERC Research Fellow at the University of Exeter