Climate Change & The Jet Stream

The jet stream. It’s what drives our weather patterns, transporting air masses and creating clashing zones for storm formation. This is the time of year when the jet stream in the Northern Hemisphere makes its seasonal southward shift, sparking extreme swings in the weather – such as an EF-4 in Iowa, record rainfall in Texas, and four feet of snow in South Dakota. Scientists have only recently begun to detect changes in the jet stream that may be tied to global warming. Unraveling the complexities of this emerging scientific research is critical to understanding where our weather is headed.

As the globe continues to warm, it is already having an effect on the jet stream and corresponding weather patterns, according to the latest U.N. IPCC climate report, which states: “It is likely that circulation features have moved poleward since the 1970s, involving a widening of the tropical belt, a poleward shift of storm tracks and jet streams, and a contraction of the northern polar vortex. Evidence is more robust for the Northern Hemisphere.” The research that goes into this statement comes from multiple lines of evidence – from analyses of the expansion of the tropical Hadley Cell to satellite measured outgoing radiation, radiosonde observations, and weather pattern reanalyses. But just as certainty builds for a poleward shifting jet, there still remain questions about whether the jet is amplifying and promoting more blocking patterns.

Many of you are already familiar with the work of Dr. Jennifer Francis and Steven Vavrus since she recently spoke at a few weather events, including the AMS Broadcast Conference in Nashville. Their report finds that as the Arctic warms faster than the tropics, a lessening of the temperature gradient between the equator and the North Pole slows the jet stream. As the jet stream slows, it supports a "wavier,” more frequently amplifying jet that increases the probability of extreme weather events, known as Arctic amplification.

However, not all research supports Arctic amplification and its impacts on mid-latitude weather patterns. For example, Screen and Simmonds (2013) tried to link the two through planetary wave patterns and did not find any clear trend. And recently, Barnes (2013)found no significant increase in the frequency of blocking events over North America and the North Atlantic, indicating that severe mid-latitude storms cannot simply be understood through Arctic amplification alone. This research does not mean that Francis and Vavrus’ hypothesis is wrong, it simply means that the atmosphere is complex and more research is needed.

Ivana Cvijanovic, a post-doctoral researcher at the Carnegie School for Science, Stanford is also researching the implications of Arctic climate change on mid-latitude weather patterns. In an email exchange, she wrote: “A number of studies indicate sea ice loss as a likely cause of some of the recent weather extremes (Honda et al., 2009; Petoukhov and Semenov 2010; Liu et al., 2012), but it appears we still lack an ‘ultimate proof’ that sea ice loss will be affecting extreme weather events in the future.” One of the difficulties in identifying the right mechanism to link Arctic warming with mid-latitude weather patterns is the lack of longer data sets. Satellite data goes back to the late 1970s, but it’s really only in the last ten to fifteen years that we have experienced such an increased rate of loss - hitting multiple record lows. So studies that are based on observational data, present findings that are not yet robust enough to confirm the statistical significance of one factor over another (see for example work by Hopsch et al. 2012).

The relationship between climate change and the jet stream is a complex one, with much still to learn. But this is what science is all about. Establishing a hypothesis, then researching to prove or disprove it. In the end, we will all benefit from a better understanding of our weather patterns and climate system.