Jennifer Francis On Melting Arctic, Jet Stream Impacts & The Fate Of Ice
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e360: What are the immediate causes of this exceptional year of warm weather in the Arctic and this particularly warm autumn?
Francis: There are a number of factors that are involved here. One goes back to what I was describing earlier, where when we lose a lot of ice, it tends to hold that heat into the fall and delay the formation of the freeze-up time. Another reason this year is the tendency for the jet stream — and the jet stream is intricately connected to the Arctic in many ways — to take a very large swing north and south. When we get one of these big northward swings, like we had at the very beginning of 2016, it made a lot of headlines when it went above freezing at the North Pole. We've been seeing these northward swings over and over again this year, and particularly this fall.
When we get those big northward swings, we tend to see a lot of extra heat and moisture from the lower latitudes transported up into the Arctic. The moisture is one of the big keys to this whole story. Ocean temperatures all around the Northern Hemisphere are running well above normal. When the jet stream is able to take one of these big northward swings, it's bringing even more heat and moisture northward than it would have, say, 30 years ago. That moisture adds to the extra water evaporating from the surface up there. We're seeing record amounts of water vapor in the Arctic this year. This is a really important part of the story.
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e360: I’d like to ask you about your ideas on the impact of declining Arctic sea ice on the jet stream.
Francis: It was only late in 2011 when I started to look into how the sea ice disappearing so rapidly within the Arctic might have an effect on the atmosphere, not just in the Arctic, but far away, all the way down to even Florida, for example. We've known for a long time that the difference in temperature between the Arctic and areas farther south — of course the Arctic being cold, and the mid-latitudes being much warmer —is really the fuel behind what drives the jet stream. The jet stream is literally a river of wind that flows where the jets fly. The jet stream is really the reason we have shifting weather patterns down here in mid-latitudes. Anything that affects the jet stream is going to affect weather patterns. What we're seeing, though, is that because the Arctic is warming so much faster than anywhere else, this difference in temperature between the north and the south is getting smaller. That, in turn, means that the jet stream is getting weak — this is something that we are able to measure.
When the jet stream gets weaker, it tends to be more susceptible to any obstacles in its path. If it runs into a mountain range, for example, or an area of very warm temperatures over the ocean or over the land, and it's more easily deflected from its straight west-to-east flow around the Northern Hemisphere. We tend to see these bigger swings northward and southward when the Arctic is far warmer than it should be. The reason those big waves in the jet stream are important is because it's really the waves that create the weather patterns that we feel on the surface. Certain parts of the wave create high pressure and nice weather, and then other parts of the wave are responsible for storms. When the wave is big, they also tend to remain in one place for longer periods of time. It makes our weather patterns and our weather regimes hang around a lot longer.
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http://e360.yale.edu/feature/unusually_warm_arctic_climate_turmoil_jennifer_francis/3060/