Friday, 6 January 2017

Feedback loops between record Arctic temperatures and the jet stream

The Arctic Is Getting Crazy
Feedback loops between record Arctic temperatures and the jet stream may be altering our weather

5 January, 2017

In the past year the climate in the Arctic has at times bordered on the absurd. Temperatures were 30 to 50 degrees Fahrenheit above average in some places during the recent Christmas week. Through November the area of ice-covered ocean in the region reached a record low in seven of 11 months—an unprecedented stretch. More important, perhaps, the difference between Arctic temperatures and those across the midlatitudes of North America, Europe and Asia during 2016 was the smallest ever seen.

That narrowing gap is important to note because it seems to be driving extreme weather in the midlatitudes, from heat waves and droughts to heavy snowfalls. Why is the Arctic so crazy lately, and how strong is the connection to bad weather to its south, where so many people live? Scientific American asked Jennifer Francis, who is a research professor at the Institute of Marine and Coastal Sciences at Rutgers University and has investigated Arctic climate change and its links to weather worldwide since 1994.

[An edited transcript of the interview follows.]

How unusual is the ongoing string of Arctic climate records?

The records are astounding because there are so many of them. The extra warming that is happening up in the Arctic—the “Arctic amplification”—has been the greatest we’ve ever seen. We’ve also seen the lowest sea-ice thickness, and we’ve seen the greatest amount of water vapor in the atmosphere. That one doesn’t usually make headlines but it should; that water vapor comes from more evaporation because there is more exposed, open ocean. Also, a lot more water vapor is being transported northward by big swings in the jet steam. That’s important because water vapor is a greenhouse gas just like carbon dioxide and methane. It traps heat in the atmosphere. That vapor also condenses as droplets we know as clouds, which themselves trap more heat. The vapor is a big part of the amplification story—a big reason the Arctic is warming faster than anywhere else.

Does the extra vapor contribute to any sort of feedback loop—conditions that tend to feed upon themselves?

We’re starting to think so. It is directly part of a feedback, in that more loss of sea ice causes more evaporation, which traps more heat, which melts more ice—one of the vicious cycles. But another vicious cycle that may be emerging is that when the Arctic is very warm, we think that is leading to the jet stream taking wavier paths—big northward swings and southward dips. When the jet stream does that, it transports more heat and moisture up into the Arctic, which heats the Arctic more, which make the jet stream even wavier—another vicious cycle related to disappearance of sea ice. During Christmas the North Pole was above freezing—which is crazy for that time of year—and it was related to one of the big swings in the jet stream.

Very recently scientists have begun to more directly link climate change patterns to extreme weather events, which they have typically been reluctant to do. Are the links becoming clearer?

Well, first, warmer temperatures worldwide are adding to heat waves. And more water vapor worldwide is related to the atmosphere being warmer—we have about 7 percent more water vapor in the atmosphere now than we did in the 1950s, which is directly linked to the increase in heavy precipitation events. Drought is also pretty directly related to a warmer atmosphere.

Arctic amplification—[the faster rise of Arctic temperatures than midlatitude temperatures]—may be the most controversial factor. What we think is happening is that amplification is favoring these very wavy patterns in the jet stream. When those waves get large, we tend to see very persistent weather patterns across midlatitudes. The waves tend to move very slowly, and the waves are what create weather we experience. Different parts of those waves tend to favor very stormy patterns, very dry patterns or warm versus cold. So in your neck of the woods, weather conditions are going to hang around longer.

It’s still difficult to unravel the persistent drought in California—what fraction of that is due to general global warming versus more persistent jet stream patterns because the Arctic is warming fast. But that’s where the research is happening right now.

So can we link the California drought to Arctic conditions or the jet stream?

This plays into how natural fluctuations in the climate system in the midlatitudes and the tropics are perhaps being intensified by Arctic warming. A good example—and some studies make this pretty clear—is when sea-surface temperature patterns in the Pacific Ocean tend to put one of these northward swings, or ridges, in the jet stream in particular locations. In the last few years the patterns have tended to put a ridge near the west coast of North America. That’s a natural thing. Along with that, though, we’ve had very little sea ice on the Pacific side of the Arctic. There’s been a lot of warm air over that region—Alaska has had a lot of record temperatures and a lot of rain. What we think happens is that when there is a ridge forming in a location where Arctic warming can intensify it, that makes the ridge strong and builds it even farther northward. It creates an even bigger wave in the jet stream. You get a stronger ridge over western North America and a stronger southward dip that is farther toward eastern North America.

How would that contribute to the California drought? Does it move the rain northward?

Ridges are dry, clear weather. This has been the so-called ridiculously resilient ridge—it has been in just the right place to make California really dry. It’s sent the storms up into Seattle, the Pacific Northwest and Alaska.

Can we attribute other extreme weather to the Arctic, for example the excessive snowfall and cold in Siberia?

The 20- to 30-degree warmer anomalies in the Arctic have been matched by 20- to 30-degree colder anomalies in Siberia. This mechanism has been very well documented. The warm Arctic, particularly the region north of Scandinavia—the Barents and Kara seas—is one of the places where sea ice has been lost the most. It has been persistently low this year. That tends to bulge the jet stream northward, which creates a dip to the east that has helped usher more Arctic air down into Siberia, and also caused earlier snowfall there. Once you get snow on the land, it insulates the land and makes the weather even colder. Having snow earlier, and more of it, is one of these vicious cycles that makes it even colder—and the cold helps to induce the jet stream’s southward dip there.

What about the polar vortex, which is said to be “meandering” more lately?

This gets weedier. Several trends in the last year and half suggest that when the jet stream gets wavy in the wintertime and the Arctic is really warm, we tend to see the stratospheric [or polar] vortex became less circular—more meandering, even split into two. This has happened a lot in the last few years. When the polar vortex gets disrupted, it tends to affect the jet stream late in the winter. There seems to be a long-term memory in the system, starting with low sea ice and warm temperatures in the fall in the Arctic, then this vortex behavior into February and even March.

Was there any connection to the heavy 2016 floods in Louisiana?

The flooding storm system was very slow-moving, but we’ve seen that before. Whether this is connected to the Arctic is tenuous.

Scientists have been reluctant to attribute specific weather events to climate change. Is that changing?

I think the reluctance initially was that this was a new hypothesis. And I think some of the people who have studied the tropics have pretty much maintained that the tropics controls weather patterns all over the world. So then comes a new kid on the block saying, “Oh, it’s not just the tropics, it’s the Arctic, too!” There was some reluctance to accept that this region on the top of the world could impact something as huge as the jet stream. But there have been many dozens of studies since 2012 that have supported the general idea. They’re also finding that it is complicated—different mechanisms happen during different seasons in different places—but it’s all starting to become clearer. There has been enough research now that the hypothesis is transitioning to a theory.

Is it fair to say the changing Arctic behaviors line up with what scientists expected?

Some events do not fit what was expected 10 years ago or even five years ago, but they do fit this newer hypothesis that links the Arctic with the jet stream. There is a clear indication that ENSO [the El Niño–Southern Oscillation, or El Niño/La Niña cycle] is not the only game in town anymore.

And are the Arctic records happening sooner than expected?

It does seem as though things are unfolding faster than most people expected. The ice is certainty disappearing faster than we expected. Even five years ago most models were projecting that we’d see a summer without sea ice probably by end of the century. Now the estimates or more like 2030 or 2040, and even that might be too far into the future. The whole ream of records that has been broken in the Arctic in the past year seems to indicate that we’re seeing things unfold faster. And that's happening all across the Earth’s climate system

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