Wacky Weather, a Warmer Arctic, and a Slower Jet Stream – Is There a Link?

, climate scientist | July 31, 2013, 9:21 am EDT
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The flow of the jet stream is somewhat like a river – at times, a straight rushing ravine and at other times, a slow twisting meander. It’s almost like the atmosphere is alternately dancing the quickstep or doing a slow waltz. Why does the jet stream change its behavior and how might a warming climate be affecting this central influence on our weather in the U.S.? Let’s take a close look at what’s going on in the atmosphere up above us every day.

Why planes fly faster from west to east – it’s the wind!

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A Summer of Extremes: Confronting the Realities of Climate Change

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In mid-latitudes in the northern hemisphere – over most of the lower 48 states – the dominant jet stream is the Polar jet stream. It separates cold air to the north from warm air to the south, and lies five to seven miles above the Earth’s surface with winds reaching more than 200 miles per hour.

With a tail wind like that, it’s no wonder flights from west to east are faster. Flying with the jet stream can decrease fuel consumption and shorten flying times considerably. In fact, pilots seek out the jet stream to either use it or avoid it. Jet streams can be thousands of miles long and hundreds of miles wide, but we usually focus on thin bands of faster-flowing air that influence local weather patterns.

Air flows downhill from the tropics to the poles and then turns east

How does the jet stream form? Well, most of our weather occurs in the very lowest layer of the atmosphere called the “troposphere” where the air is turbulent and mixes rapidly (the Greek word “tropo” means turning). The upper boundary of this part of the atmosphere is higher at the equator than at the poles, simply because warmer air at the equator expands and takes up more room. As a result, air flows downhill from the puffed up warm air at the equator to the compressed cold air at the poles.

In the northern hemisphere as this air flows from equator to pole it is turned to the right because of Earth’s rotation. This resulting west to east flow is known as a jet stream. You can see more thorough explanations of the physics from NOAA’s National Weather Service and from Skeptical Science.

A slowed jet stream meanders like a lazy river

When the Polar jet stream slows down, the wave patterns known as planetary or Rossby waves start to meander and widen, just like a river does when it slows down in its lower reaches. The meanders form large lobes that bring warmer weather much farther north and colder weather much farther south than usual. Low pressure systems form in the southern end of the troughs – these cyclonic or inward-flowing masses of air result in convective storms, heavy rainfall events, and flooding. High pressure systems form in the northern end of the ridges – these anti-cyclonic or outward-flowing masses of air bring hot, dry weather.

The slowing of the west to east flow of the jet stream produces large meandering lobes that can stall, resulting in long periods of unchanging weather.  Source: Skeptical Science

The slowing of the west to east flow of the jet stream produces large meandering lobes that can stall, resulting in long periods of unchanging weather. Source: Skeptical Science

The jet stream becomes stuck, leading to extreme weather

When the jet stream slows down our weather tends to become “stuck” in either of these modes, resulting in long periods of the same patterns of weather that leads to extremes. We saw the consequences of this in June of this year when wildfires raged in the western half of the U.S. while the eastern seaboard was drenched in rain. And in October 2012 Hurricane Sandy took an unusual path through New Jersey when the large loops of the jet stream were stuck, as explained in a blog here.

A recent paper suggests the stalling of these planetary waves could have caused the U.S. heat wave of 2011, the floods in Russia and Pakistan in 2010, and the European heat wave of 2003. It’s not clear yet whether these stalled weather patterns are becoming more frequent and we won’t know until we have longer data sets. However, scientists are currently trying to figure out the most likely cause of a slowdown of the jet stream.

The uncharacteristic north-westerly path of Hurricane Sandy was influenced by a blocking ridge that developed from an unusual jet stream pattern. Source: NASA

The uncharacteristic north-westerly path of Hurricane Sandy was influenced by a blocking ridge that developed from an unusual jet stream pattern. Source: NASA

A stalled jet stream in June this year resulted in intense dry weather in the west and downpours on the eastern seaboard.

A stalled jet stream in June this year resulted in intense dry weather in the west and downpours on the eastern seaboard. Source: NOAA National Climate Data Center

The Arctic “amplification” — a possible explanation for a stalling jet stream?

In recent years the Arctic has warmed at an alarming rate (as was predicted by models) due to strong reinforcing feedbacks in the climate system. With the Arctic warming at twice the pace of the rest of the planet, the temperature contrast between the equator and the poles has decreased. The downhill run of air from equator to pole is not as steep as it was, diminishing the strength of the jet stream and resulting in the large meandering patterns.

Jennifer Francis, a Research Professor at Rutgers University, and Jeff Masters from the Weather Underground explain why this amplification of warming in the Arctic might be an explanation for a weakening jet stream. With her colleague Stephen Vavrus, Francis has also published a paper on the topic. There are other explanations that also consider changes to the heat content of the Arctic ocean as well as the role of increased water vapor transport in the atmosphere, as discussed in an American Geophysical Union blog with Kevin Trenberth. Scientists are still working on the details, but plausible explanations of this “wacky weather” are starting to emerge.

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  • Melanie – Thanks for the links. I loved your “Welcome to the Pliocene” message for the 400 ppm event. And I spent some time finding out the history and purpose of the UCS organization of which your blog is a part. Here’s my concern: Your work is excellent, but it joins a vast and overwhelming overload of climate information and interpretation that one encounters when trying to enter and get up to speed on this issue. Methinks what is most needed now are trustworthy “information aggregators”, each speaking to a particular population or subset of teachers and other interested parties delving into climate change online.

    Because I myself have a youtube channel and have made 3 climate videos (one of which is clearly an information aggregating climate video), I have recently taken it on to create a trustworthy and visually inviting information aggregation webpage expressly for the best educational videos on climate change. You can check it out by googling this exact title, “Science-Rich Educational Videos on Climate Change”. (You should see a picture of Jennifer Francis right away). Six of the climate scientists featured on that page have told me they highly approve of what I am doing with their work and the aggregation.

    My plan is to try to keep up on climate science myself, and thus to keep updating that page. I am also trying to encourage climate scientists and their tech staff to begin doing what I always do (which you can see on that webpage) — creating a table of contents with hotlinked time codes for all the longer science videos.

    So my question to you: Who is doing a capable job of threading through the exponentially greater volume of text, news, press release, blog material online and sifting through it all in a way and posting the results in a visually attractive way for folks who want to enter (or just keep up on) popularly accessible climate science in those formats?

  • Richard

    Thanks for the video and the clear explanation as to why weather events are getting more extreme. I have heard the latter quite frequently in recent years. Now I have a better understanding as to the importance of the warming of the Arctic as the reason for it.

    How about in the Southern Hemisphere?

    • Great question, Richard. The northern hemisphere has been warming at a greater rate than the southern hemisphere since the early 1980s. The main reason for this is the presence of a much vaster ocean in the southern hemisphere that takes up heat. As the earth’s atmosphere warms from increased CO2 levels, much of the heat is transferred into the oceans – which partly explains the dramatic decrease of Arctic sea ice. Because the southern hemisphere is largely ocean and not land, a much greater amount has been transferred from the atmosphere and stored in the deep layers of the ocean. As a result, there has been a smaller rate of change in the atmospheric temperature in the southern hemisphere. A recent paper in the Journal of Climate (Friedman et al., 2013) suggests this difference in warming between hemispheres could significantly alter tropical precipitation patterns.

  • Thank you especially for the explanation of why the troposphere upper boundary is higher in the tropics than in the arctic. Please know that a longer video of Jennifer Francis putting the jet stream and Arctic sea-ice loss in the context of extreme weather is available on youtube at “Climate Change and Extreme Weather: Prof. Jennifer Francis (2013)”. I am concerned that, since scientists are beginning to attach a degree of CO2-rise causation to a variety of extreme events, and because at this early stage the science is still “unsettled”, the media and the public lose track of the underlying foundation of understanding that was the primary focus of climate change understanding in previous years: that a rise in CO2 will alter the climate. That point is no longer subject to any serious scientific dispute. But when we get into the details of how and when, and then also the secondary consequences, the whole issue breaks wide open again and the climate deniers race in to spin doubt.

    For this reason, I think it would be wise to ensure that, at least in this blog, you pay more attention to recent fossil discoveries (pollen in Siberian Pliocene lake sediments) that empirically link a 400 ppm atmosphere with vastly warmer land conditions in the Arctic. Accordingly, I created a video that highlighted a formal university video taken this year of a presentation by Julie-Brigham Grette, lead author of the 9 May 2013 paper published in Science, “Pliocene Warmth, Polar Amplification, and Stepped Pleistocene Cooling Recorded in NE Arctic Russia”. (I created an image-enhanced video of one of her talks, titled “Hot Climate Women Scientists in Cool Places.” Note: Both Jennifer Francis and Julie Brigham-Grette approve of the videos I made and posted.