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Researchers link extreme weather events to global warming in new study

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Scott Sutherland
Meteorologist, theweathernetwork.com

Monday, August 18, 2014, 9:20 AM - Unseasonably cool through central Canada, with downpours and flooding taking their toll on several regions. Warmer and drier weather spreading up into the western prairies, southern B.C. and the mountains, and even up through the northern territories. It seems like summer has foregone the more usual consistent heat, in favour of performing a 'copy and paste' routine from this past winter - dropping in the same kind of patterns that gave us January's polar vortex slip and the subsequent persistent conditions that made early months of this year so unusual. Although the comparatively extreme cold snaps (winter or summer) from these patterns is causing some to question whether global warming is really happening, a new study is showing evidence that these patterns are a direct result of global warming.

When a weather forecast mentions the 'jet stream', it's referring to the meandering 'river' of powerful winds that stretches around the globe, and acts as the southern boundary of the north polar vortex. You can see the patterns this forms in the video animation above. The reason why the jet stream plays such a featured role in forecasts is because it has a big impact on our day-to-day weather. It may be over 9 kilometres above the ground, but the pattern it forms at any time has a big influence on our temperature, wind and precipitation patterns. It can even influence how long a particular kind of weather - frigid and snowy, hot and sticky, stormy, etc - stays over any particular area. 

The pattern of the 'planetary waves' that are created in the flow of the jet stream are largely dependent on the difference in temperatures between the equator and the north pole. If the temperature difference is large (hot at the equator and cold at the pole), the polar vortex is relatively strong and the waves are fairly small, as shown below (from early in the animation).

In this case, the waves tend to move along at a fairly good pace, and thus the weather conditions change fairly quickly as well, and no particular area gets trapped under extreme conditions for long periods of time. However, if the temperature difference between equator and pole is smaller (hot equator and cool or warm polar region), the polar vortex weakens, waves can become quite large, dipping far to the south and stretching far to the north - like in the snapshot below - and the overall movement of the waves slows down at the same time.

"Behind this, there is a subtle resonance mechanism that traps waves in the mid-latitudes and amplifies them strongly," said Stefan Rahmstorf, a researcher at the Potsdam Institute for Climate Impact Research who co-authored a new study investigating these waves and how they become trapped in these static patterns. According to the study, one of the key conditions for the patterns to become stuck is the formation of a 'double-jet' - seen at times throughout the animation, when a second, weaker band of winds (in blues and greens) appears closer to the pole, while the main jet (greens through reds) is still flowing through the mid-latitudes (over Canada and the U.S.). You can see this double jet in the image above, and it shows up again just at the end of the animation as well.

The animation also shows the long-lived nature of some of these patterns, as one develops at about 12 seconds in, and lasts until the 19 or 20 second mark. With each second of the animation representing 1 day, that's at least a week of persistent weather conditions. This could mean a week-long heat wave for anyone living in the central part of North America (in the above image), and an equally-long period of chilly cold or extreme storms for those along the eastern part of Canada and the United States. We saw this sort of situation develop over the winter, with the initial slip of the polar vortex in early January, and then a pattern of persistent dips in the jet stream over the weeks after. We're also seeing a similar pattern during this summer, which is why it's been so cool across eastern Canada and so warm and dry through the west.

Lately, global warming has been implicated in the fact that we're seeing more of these extreme patterns in the jet stream, largely due to the unprecedented warming that's been seen in the Arctic in recent years. This is due to a process called Arctic Amplification, where less sea ice in the Arctic Ocean to reflect sunlight back into space means more heat being absorbed into the ocean waters. This not only melts more ice, causing something of a cascade effect, but it also means that it takes longer for the Arctic to cool down in the winter, so there's even more open ocean for longer. This causes the overall temperature difference between the equator and the pole to be smaller for longer periods of time, and we get these persistent large meanders in the jet stream. While the process of Arctic Amplification is well known, and linking it to the processes of global warming is easy enough, it's still difficult to point to these events and definitively say in each case 'That was caused by global warming.'

Phil Plait, of Slate's Bad Astronomy blog, uses an excellent analogy in a recent article: "Think of it as playing craps with ever-so-slightly loaded dice. You can’t be sure that snake eyes you threw was due to the dice being weighted, but over time you’ll see a lot more of them than you’d expect, statistically, from fair dice."

"We’re throwing an awful lot of meteorological snake eyes lately," he added.

Incidences of 'planetary wave resonances' over time.

According to Rahmstorf, "Evidence for actual changes in planetary wave activity was so far not clear. But by knowing what patterns to look for, we have now found strong evidence for an increase in these resonance events."

The study conducted by Rahmstorf and his colleagues showed the link from weather records. As the graph to the right shows, there has been a fairly consistent 1-2 incidences of these 'planetary wave resonances' in the past, even going back to just the early 2000s. Now, in the past decade or so, the number has been increasing - to more than double what it was before.

"We argue that the changes in the zonal mean temperature profile, associated with rapid warming in the Arctic, have created favorable conditions for double jet formation and hence resonant flow regimes," the researchers concluded in the study. "This study thus adds to the growing body of evidence that rapid changes in the Arctic affect the large-scale circulation and thereby extreme weather in the midlatitudes."

The full study is available online, in the journal Proceedings of the National Academy of Sciences (PNAS) - click here.

Images and video courtesy of NASA Visualizations, Graph courtesy Potsdam Institute for Climate Impact Research

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