Did Global Warming Cause the Polar Vortex?
A slowing Jet Stream and the Future of Weather
The Headlines:
Wavier jet stream ‘may drive weather shift’ — BBC
Tired Of Endless Winter? Blame Changing Jet Stream — TIME
Changes in Jet Streams Might Lead to Harsher Winters, Researchers Say — Nature World News
The Science:
1. What’s a Jet Stream? Jet Streams are fast flowing air currents that flow across the planet at high altitude. The strongest and most well-known are the Polar Jets, which are the subject of this story. The northern hemisphere’s Polar Jet flows west to east around the world, usually across northern N. America, Europe and Asia. Sometimes the Jet’s circular route develops a kink and sinks down to flow across more southerly regions, bringing with it cold and unstable air that would usually be contained in the arctic.
The “Polar Vortex” weather that gripped much of the eastern half of the United States through early 2014 was an extreme example of a kink in the jet stream, bringing frigid arctic air far further south than usual. In addition, this southern dip lasted much longer than usual before retreating north to its usual, circular path, locking in extreme cold above its southern meander until it finally retreated.
2. Why would the Jet Stream be slowing down? The Jet Stream’s strong currents are powered by the difference in air pressure between the atmosphere in cold polar regions and warm areas nearer to the Earth’s equator. Just like the fizz of gas when you open a bottle of soda, air rushes from the high pressure areas that form around the equator to fill lower pressure areas that form in the cold polar regions, powering this fast-flowing air current.
Some scientists have suggested that, as the arctic warms faster than the rest of the planet, this pressure difference becomes smaller, and as a result, the Polar Jet is weaker and slower than usual. Just like a slow-moving river takes a more meandering and leisurely path than a fast-flowing stream, they suggest that this would cause more, and longer-lasting kinks in the Jet Stream’s path — this would cause more cold air to sink further to the south (and vice-versa; some polar regions, such as Alaska, have seen extremely unusual patterns of warmer air bringing unseasonable heat to the far north), and also that these kinks in the Jet Stream’s path would become more persistent, locking in these unusual weather patterns for longer until the Jet Stream’s southern diversion finally breaks down.
3. What evidence is there to support the idea that the Jet Stream is slowing? This idea was first introduced by Jennifer Francis of Rutgers University and Stephen Vavrus of the University of Wisconsin-Madison in a March 2012 research paper in the journal Geophysical Research Letters, Evidence linking Arctic amplification to extreme weather in mid-latitudes. Their study looked at the strength of high altitude arctic winds the drive the Jet stream, and showed that these winds had recently slowed significantly. This correlates with the recent observations that the Jet Stream has taken unusually large and long-lived diversions from its normal path, suggesting that the two observations might be related.
4. What do other scientists think of this model? There is universal consensus among climate scientists that the Earth is warming up on a global scale, and that this warming is more pronounced in arctic regions than around the warmer tropics. As for this particular study, many scientists agree that the model appears feasible, but there is considerable debate as to the strength of this phenomenon, and how it relates to the warming of the arctic. Jet Streams are highly complex and are affected by many independent factors, including the rotation of the Earth, and heating both from outside (i.e. the Sun) and inside the Earth’s atmosphere. Therefore, while the model these scientists have suggested seems likely, we cannot entirely rule out other factors or causes for the recent extreme behaviors of the Jet Stream. In order to fully differentiate between the subtle effects of these many factors, it will be necessary to look at these data for much longer time periods than are currently available.
What Does This Mean for Me?:
Whether the changes in the Polar Jets are caused by amplified heating in the polar regions, or more complex phenomena that scientists have not completely understood yet, it is clear that the Polar Jets have slowed in recent years, and that the extreme kinks in its path have diverted it far further from its usual circular route around the pole. This, in turn, has led to the persistent frigid cold in the eastern US, drought and heat in the western US, and likely the large number of extreme storms seen across Europe.
If the model suggested by Francis and Varvus is correct, you can expect to experience more intense and longer periods of extreme cold, or extreme heat — depending on which side of the Jet Stream you are currently stuck on. As the weatherman might say, “the weather you have is the weather you’re keeping” for weeks or months at a time. If this pattern continues, or even intensifies, we are likely to see more extreme periods of both regional drought and flooding, and more extended periods of unseasonably cold or warm weather.
On an individual level, this means preparing better to cope with more extreme weather events by stocking up cold weather food and supplies in winter, and being ready to cope with extreme heat, or flooding. As a society, it means understanding better the delicate balance in nature that allows life on Earth to thrive, and making choices that minimize the impact on the Billions of people who have no choice but to live where they live regardless of what weather events are coming their way.
