Flatten Greenland, and the Atlantic jet stream goes with it
Building off previous research showing the Atlantic jet stream hovers between three preferred latitudes, researchers found the topography of Greenland is responsible for its northernmost position.
The relationship many people have with the Atlantic jet stream involves its impact on flight times between North America and Europe. But this fast-flowing air current also changes weather on both sides of the jet stream – especially when coupled with activity in the polar vortex. Weather regimes over the North Atlantic receive a lot of attention, as they provide the prospect of enhanced predictability of weather events over Europe, including extreme events. But the origins of these atmospheric circulation patterns remain fundamentally unresolved amongst researchers.
In her quest to understand how activity above 20,000 feet impacts life on the ground, atmospheric scientist Aditi Sheshadri takes the approach of changing elements in the existing environment until the crucial building blocks reveal themselves. What happens when mountains are flattened, added or moved? Building off previous research showing the Atlantic jet stream hovers between three different “preferred” latitudes, Sheshadri tested what controls those positions. For the bottom two positions, no matter how researchers changed the landscape and environment, the jet stream hovered over the same two latitudes. But for its northernmost position, Sheshadri and her team found the jet stream was controlled by one land mass: Greenland. When they flattened the island in models, the northern position disappeared.
“I was actually kind of disappointed because I thought that it was going to be something a bit more complicated than that – something involving a bit of complex flow dynamics – but it’s just Greenland,” said Sheshadri, an assistant professor of Earth system science at Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth). The finding is an important factor that should be incorporated in climate models, especially since the majority of Greenland is composed of ice that is slowly melting as the temperatures increase, she said. Stanford Earth spoke with Sheshadri about her latest findings, published Nov. 11 in Geophysical Research Letters.
Why are you interested in the Atlantic jet stream?
The polar vortex is this swirling massive air in the stratosphere, which is about 10 kilometers above the surface of the Earth, and I think quite a bit about how the polar vortex impacts the jet stream. If the polar vortex is active – there are times when it splits up, there are times when it gets displaced off of the pole – then what happens to the jet stream? The conventional wisdom was that the jet stream just sort of shifts over the Atlantic, but given that there seem to be these three preferred positions, does the jet stream actually shift or does it spend different periods of time in these three positions?
Why is the Atlantic jet stream important?
The Atlantic jet stream is important in many ways. For instance, you could imagine that if the jet stream were to move north or south, or speed up, or slow down, you could get from New York to London more quickly, or less quickly. Flight times would change, for instance.
Also, that’s the region in the middle latitudes where storminess maximizes – so the jet stream also signifies where the storms are. If you’re interested in how much rainfall is coming to western Europe, you should care about where the jet stream is and how intense it is.
What do we know about the three preferred positions of the Atlantic jet stream? What about other jet streams?
The northernmost position of the jet stream is basically downstream of the tip of Greenland. The central one is where one conventionally thinks of the jet stream, at about 45 degrees North, and the southern one is around 38 degrees North. We focused our analysis in the winter, which is kind of an interesting period in the North Atlantic, because it’s pretty active. And that’s when the vortex in the stratosphere can split or become displaced, as well, and these events seem to impact only the Atlantic jet stream.
There’s a jet stream in the Southern Hemisphere that pretty much goes all the way around the globe. And in the Northern Hemisphere there are two distinct jet streams: one over the Atlantic, one over the Pacific. The Pacific one is a bit more boring than the Atlantic one; it doesn’t exhibit any of these preferred positions. It actually doesn’t vary all that much. But the Atlantic one does, and we really care about it because people live on both sides of it. Understanding something about how much it’s changing and why it looks like it does is really important to human lives, and potentially sub-seasonal prediction.
Tracking a superstorm
Days before the landfall of Hurricane Sandy in 2012, forecasts of its trajectory were still being made. Some computer models showed that a trough in the jet stream would kick the superstorm away from land and out to sea. One of the earliest to project its true course was NASA’s GEOS-5 global atmosphere model. The storm caused 159 deaths and $70 billion in damages on the East Coast of the U.S. (NASA Earth Observatory image by Robert Simmon with data courtesy of the NASA/NOAA GOES Project Science team)
What did you learn about Greenland?
It turns out the northern peak in Greenland is what is known as a tip jet. People in, for instance, aeronautics know all about tip jets. There’s this flow that impinges sort of the edge of Greenland and there’s a downstream acceleration of flow. So what was thought of as a regime – a preferred position where the jet wants to be – is just a consequence of Greenland.
So we flattened Greenland and we got rid of it. We picked up Greenland and moved it north, and the tip jet moved north. And all of those things convinced us that this northern position is just a physical consequence of Greenland being where it is.
Who would use this information?
Seasonal to sub-seasonal forecasting people would be interested in this – and anyone who’s trying to prepare for floods or snowstorms. Particularly in bad winters, knowing what latitudes the jet stream is preferentially in would be valuable information. And then climate models, of course. Because most climate models don’t get these three positions.
The obvious question there is: Why don’t the models get them? Because they all have Greenland. And the answer seems to be that they do have Greenland, but they don’t model strong enough winds at the latitude of Greenland, and so they don’t get this peak. I think anyone who’s developing a climate model would like to know that.
Sheshadri is also a center fellow, by courtesy, at the Stanford Woods Institute for the Environment. Study co-authors are affiliated with the Barcelona Supercomputing Center and the University of Washington.