The Warm West, Cool East U.S. Temperature Divide
While California and the western U.S. have experienced record drought and high temperatures in recent years, many areas in the eastern U.S. have experienced extreme or even record-breaking cold temperatures. So why do the West and East experience such different temperatures, even as the global average temperature continues to rise? The answer has to do with how the heat getting added to the climate system is distributed and affecting regional weather patterns.
In the U.S., the prolonged cold spells in the East, drought in California, and record heat in the Northwest and Alaska are all connected to the atmospheric polar jet stream. There is a growing body of scientific evidence suggesting that rising temperatures in the Arctic may be linked to the strong and persistent jet-stream pattern that has led—three winters in a row—to this warm West and cool East divide.
Why are we seeing the “warm West and cool East” divide?
Here the jet stream assumes the persistent pattern of recent years, with a ridge bringing hot and dry conditions to the west, and a trough bringing cold, wet conditions to the East.
Source: The Conversation
The polar jet stream in the Northern Hemisphere is a fast-flowing and narrow current of air that circles the mid to high-latitudes. In recent years, the polar jet stream has slowed, its winds have weakened, and its north-south waves, known as Rossby waves (which bring Arctic air to the U.S.) may be getting bigger and occurring more frequently. For three consecutive winters, starting in December 2012, the jet stream has had significant north-south Rossby waves. In the East, the jet stream has been dipping unusually far south, creating a low “trough” that brings down cool Arctic air. Meanwhile, in the West, high pressure conditions over the northeast Pacific have confronted cool Arctic air from the sunken jet stream, creating a “ridge” of high atmospheric pressure causing the jet stream to divert towards the North. As a result, rain and snow that would normally fall on the West coast is instead re-routed to Alaska and as far north as the Arctic Circle. The exceptional persistence and unusual recurrence of this pattern has led scientists to nickname the ridge and trough as the “Ridiculously Resilient Ridge” and the “Terribly Tenacious Trough.”
How might climate change be linked to the divide?
There is strong agreement in the scientific community that the most immediate cause of the drought in California—and western heat more broadly—is the "Ridiculously Resilient Ridge.” The RRR, as it’s called for short, is a product of unusual jetstream behavior that scientists are linking to many complex factors including Arctic warming and warmer atmospheric conditions over the northeast Pacific.
A recent study led by Noah Diffenbaugh investigates how climate change might be playing a role in the California drought and extreme west coast heat by analyzing atmospheric conditions over the Pacific. In the study, Diffenbaugh and his team find that the region of high atmospheric pressure over the Pacific Ocean responsible for diverting the jet stream is much more likely to form in the presence of modern greenhouse gas concentrations.
As for the remarkable cold in the East, Arctic researchers have recently begun to study the effects of rapid warming in the Arctic and their possible link to jet stream behavior and mid-latitude weather conditions. The Arctic Ocean has experienced rapid sea ice loss due to global warming at a rate of 11 percent per decade since 1979. This has created an expanse of newly available ocean water to absorb the Sun’s energy. The extra energy has created conditions that are conducive to even more warmth, creating a positive feedback effect known as Arctic amplification. With changes in cloud cover, more atmospheric water vapor, and more heat creeping in from warmer latitudes below, the Arctic has been warming at a rate twice as fast as lower latitudes.
In 2012, climate scientists Jennifer Francis and Stephen Vavrus proposed that Arctic amplification driven by sea ice loss could significantly affect mid-latitude weather by decreasing the temperature gradient between mid-latitudes and the Arctic, causing the jet stream’s winds to slowdown and waves to meander. A 2015 paper by Francis and Vavrus tests this theory further and finds that as the Arctic warms faster than the rest of the planet, the frequency of extreme weather events caused by persistent jet stream patterns will increase.
Scientists are identifying additional mechanisms that may be contributing to the unusual mid-latitude weather. All sea ice studies agree that the loss of sea ice warms the Arctic atmosphere and increases moisture content, but scientists have identified different ways that the atmosphere may be responding to the loss of ice and resulting temperature increase. For example, a team led by Judah Cohen has proposed a chain of events where less Arctic sea ice and more open water cause the atmosphere to heat, which leads to more snow cover in Siberia in October, cooling the atmosphere and forcing the meandering jet stream pattern we are currently seeing.
There are a few steps to get from increased snow in Siberia to colder winters in the eastern United States. Cohen’s theory holds that when there is increased October snow in Siberia, cold air builds over the expanse of snow, strengthening the pressure system known as a Siberian high. The high weakens the winds that circle the North Pole, allowing the cold Polar Vortex air to leak into the lower latitudes.
While the scientific community is still investigating whether sea ice loss and Arctic amplification are the ultimate causes of the jet-stream pattern responsible for the divide between the warm West and cool East, the link between a slow, wavy jet-stream and extreme U.S. weather is well-established, and there is evidence supporting Francis and Vavrus’s theory that the decreased temperature gradient between mid-latitudes and the Arctic is slowing the jet stream waves.
How unusual are the opposing temperature extremes in the U.S.?
Average global temperatures in 2014 were record hot, but in the U.S. there was a record-setting split between regions experiencing unusually hot and cold temperatures. Almost one-fourth (23.2 percent) of the U.S. experienced temperatures “much above normal,” while 18.6 percent saw temperatures “much below normal.” NOAA’s definition of “temperature extreme” is when regions experience temperatures at the top or bottom 10 percent of the historical range of recorded values. Since the U.S. temperature record began in 1895, only a handful of years have a pattern similar to 2014, where more than 10 percent of the country was experiencing extreme warmth while a similarly large or larger area experienced extreme coolness. Among these years, 2014 experienced an unprecedented range of simultaneous, opposed temperature extremes. As the jet stream continues to stick in a position where the eastern U.S. experiences Arctic conditions and the western U.S. gets little relief from extreme heat and drought, people are starting to discuss the extent to which climate change may be at work.
How does this fit into the bigger picture?
The unusual jet stream behavior has created a situation where the eastern U.S. is by far the coldest place in the world compared to average temperatures, leading some to argue that it might be more appropriate to say “global weirding” than warming. From the U.S. perspective, this may be true: the U.S. is experiencing a weird divide between extreme heat in the West and cold in the East. But it’s also important to remember that even as U.S. winter temperatures get unusually cold, overall temperatures in the U.S. are actually unusually warm. While select regions may experience cooler temperatures trends as the global temperature ratchets up, long-term trends and projections clearly indicate continued warming in all regions of the U.S. including the east, and globally.
Temperature difference from normal February 19, 2015 at which time the jetstream assumed the persistent ridge pattern over the west and trough over the east, bringing extreme heat to the former and cold to the latter.
Source: Washington Post
Judah Cohen et al. “Recent Arctic amplification and extreme mid-latitude weather,” Nature Geoscience. August 17, 2014.
Jennifer Francis, “A melting Arctic and weird weather: the plot thickens,” The Conversation. February 18, 2015.
Carolyn Gramling, “Arctic impact,” Science Magazine. February 20, 2015.
Dr. Judah Cohen: “The seven years between 2007 and 2013 have exhibited the lowest minimum sea-ice extents recorded in September since satellite observations began, with an all-time record low in 2007 followed by another in 2012, when sea-ice extent fell below 4 million km2 for the first time in the observational record. Several of these seven winters following the low sea-ice minima have been unusually cold across the Northern Hemisphere extratropical landmasses. The recent winter of 2013–2014 was characterized by record cold and widespread snowstorms across the eastern United States and Canada with the most intense cold-air outbreak in decades associated with the weakening of the polar vortex. The persistent and harsh cold resulted in all-time record cold winters around the Great Lakes of the United States since record keeping began in the 1870s.”Dr. Jennifer Francis: “[My team’s] new work, published [in January 2015]...uses a variety of new metrics to show that the jet stream is becoming wavier and that rapid Arctic warming is playing a role. If these results are confirmed, then we’ll see our weather patterns become more persistent. In other words, Ridiculously Resilient Ridges and Terribly Tenacious Troughs may become the norm, along with the weather woes they cause.”