The flow-on effect of Arctic warming

Climate Central

By showing that Arctic climate change is no longer just a problem for the polar bear, a new study may finally dispel the view that what happens in the Arctic, stays in the Arctic.

The study, by Jennifer Francis of Rutgers University and Stephen Vavrus of the University of Wisconsin-Madison, ties rapid Arctic climate change to high-impact, extreme weather events in the US and Europe.

The study shows that by changing the temperature balance between the Arctic and mid-latitudes, rapid Arctic warming is altering the course of the jet stream, which steers weather systems from west to east around the hemisphere. The Arctic has been warming about twice as fast as the rest of the Northern Hemisphere, due to a combination of human emissions of greenhouse gases and unique feedbacks built into the Arctic climate system.

The jet stream, the study says, is becoming ‘wavier', with steeper troughs and higher ridges. Weather systems are progressing more slowly, raising the chances for long-duration extreme events, like droughts, floods, and heat waves. 

“[The] tendency for weather to hang around longer is going to favour extreme weather conditions that are related to persistent weather patterns,” said Francis, the study's lead author.

One does not have to look hard to find an example of an extreme event that resulted from a huge, slow-moving swing in the jet stream. It was a stuck or ‘blocking weather pattern' – with a massive dome of high pressure parked across the eastern US for more than a week – that led to the remarkable March heat wave that sent temperatures in the Midwest and Northeast soaring into the 80s (Fahrenheit). In some locations, temperatures spiked to more than 40 degrees Fahrenheit above average for that time of year.

The strong area of high pressure shunted the jet stream far north into Canada. At one point during the heat wave, a jetliner flying at 30,000 feet could've hitched a ride on the jet stream from Texas straight north to Hudson Bay, Canada. In the US, more than 14,000 warm-weather records (record-warm daytime highs and record-warm overnight lows) were set or tied during the month of March, compared to about 700 cold records. 

According to the study, Arctic climate change may increase the odds that such high-impact, blocking weather patterns will occur. The study cites examples of other patterns that led to extreme events that also may bear Arctic fingerprints, including the 2011 Texas drought and heat wave, which cost the state's agricultural sector a staggering $US7.62 billion – making it the most expensive one-year drought in that state's history.

In addition, the study also mentions jet stream configurations that led to heavy snows in the Northeast and Europe during recent winters. Such events are also “consistent” with the study's findings, according to the paper.

The reasons why the Arctic is heating up so quickly, a phenomenon known as “Arctic amplification,” has to do with factors that are unique to the Arctic environment, involving feedbacks between sea ice, snow, water vapour, and clouds. As the area warms in response to manmade greenhouse gases, melting ice and snow allow exposed land and water to absorb more of the Sun's heat, which melts more ice and snow, and so on. A relatively small amount of initial warming can be greatly magnified in the Far North. 

The temperature contrast between the frigid Arctic and the milder mid-latitudes is what drives the powerful jet stream winds, which are so important for determining day-to-day weather conditions.

In addition to making the jet stream have more pronounced north/south swings, the reduced temperature gradient between northern and southern areas is causing the westerly component of upper-level winds to slow, especially during the fall when extra heating in the Arctic is exceptionally strong.

Path of the jet stream on March 21, 2012. Credit: weatherunderground.

The westerly component of upper-level winds during the fall has weakened by about 14 per cent since 1979, the study found. 

A slight slowdown in the jet stream may not sound like a big deal. After all, jet stream winds have been clocked at upwards of 200 mph. But it turns out that slowing of the jet stream influences its shape and the motion of individual storm systems. 

Weaker westerly winds causes the big north/south swings in the jet stream to move more slowly from west to east, making weather conditions in a given location more persistent than they used to be. “That means that whatever weather you're experiencing now is going to tend to hang around longer because the passage of those waves is really what causes the weather to change,” Francis said.

The study contains a stark warning about future weather patterns, given projections showing that Arctic climate change is likely to accelerate in coming years. “As the Arctic sea ice cover continues to disappear and the snow cover melts ever earlier over vast regions of Eurasia and North America, it is expected that large-scale circulation patterns throughout the northern hemisphere will become increasingly influenced by Arctic amplification,” the study reports.

In other words, rapid Arctic warming is expected to exert a growing influence on the weather far beyond the Arctic Circle, for many years to come.

This article was originally published on Climate Central – www.climatecentral.org. Reproduced with permission.