How is climate change affecting winter storms in the US?
With extreme winter weather breaking out across the United States this week, a question in many people’s minds is—how is climate change affecting winter storms? I had the good fortune to chat about this with world-renowned expert, Dr. Jennifer Francis, Senior Scientist with the Woodwell Climate Research Center.
Rachel: Thank you so much for speaking with us. Can you start with how climate change is generally affecting winter weather in the US?
Dr. Francis: On average, winters are getting warmer, and we are breaking way fewer cold records than warm records in the US and around the world. That’s just the average, and this is a very intuitive, logical connection as the globe continues to warm.
Climate change is also indirectly affecting winter storms, making these extreme events happen more often. This is less intuitive and a very active area of research, but we’re starting to understand how these extreme events are happening more often because of climate change.
For example, it’s causing storms to have more fuel to work with in the form of water vapor and heat, more moisture, and as a result, these storms are dumping more precipitation.
Rachel: Can you explain in more detail how climate change is affecting winter storms in the US?
Dr. Francis: There are a lot of factors playing a role. Some are direct, and some are much less direct—these are hot areas of research at the moment.
Focusing on the big bomb cyclones (big storms like Winter Storm Elliott, which hit much of the US late last year)—these storms are being affected in several direct ways.
The warming ocean is especially important. First, as we warm the ocean, this is creating a bigger temperature contrast with temperatures over the land when Arctic air plunges southward. The waters off the Atlantic have warmed several degrees, and the storms feed off the temperature contrast.
Another factor is there is more evaporation from the land and ocean as the globe warms, so there is more water vapor in the atmosphere. When that vapor condenses into cloud droplets, it releases a lot of heat into atmosphere, and that’s something that storms use as fuel—that’s called latent heat.
The other thing is that that water vapor provides more moisture for storms, which results in more frequent heavy precipitation events, which has been clearly observed. This heavy precipitation in turn increases the intensity of the impacts of winter storms.
There are also less direct influences—including how climate change is affecting the jet stream, which is responsible for creating almost all of our weather. This is what my research is focused on.
We know that the Arctic is warming four times faster than the globe as a whole, making the north/south temperature difference smaller. And the north/south temperature difference is the main factor that creates the jet stream.
The jet stream is this river of fast, west-to-east wind about seven miles high where jets fly. As the westerly jet-stream winds weaken, we see it take bigger north/south swings, or, becoming wavier. We expect these wavy jet stream patterns to occur more often with a weaker jet stream.
That’s what happened with Winter Storm Elliott. The Arctic warming is probably one of the factors contributing to it. And this also connects to climate change.
There is also an ocean heat wave in the Pacific that’s been there for several months. When we get these, the jet stream tends to bulge northward over the area of abnormally warm water. Think of the jet stream like a jump rope—if you flick it north, it has a downstream effect.
If you can imagine a big wave bulging northward over Alaska and then taking a big southward dip—that’s what’s allowing all that cold, Arctic air to escape the far north and plunge southward. The ocean heat wave favors northward bulges. It’s not a given, but it favors it.
One of these effects is to have these dips in the jet stream penetrate further south. What makes these cold spells so devasting is, when they penetrate so far south, they affect communities that aren’t equipped to deal with frigid temperatures and howling winds. This is what happened two years ago in Texas. We saw it again with Winter Storm Elliott.
Rachel: What about the polar vortex?
Dr. Francis: The term “polar vortex” has been greatly misused and misunderstood. The polar vortex does not affect the surface directly. It does not bring that cold air directly to you. The polar vortex is way up high in the stratosphere (about 30 miles up), over the North Pole, and only in the winter. It normally spins around in a circular path centered over the North Pole. Every once in a while, it becomes disrupted, meaning it can become elongated into a bean shape or even split into different circulations. When it’s disrupted, we tend to see extreme winter weather events.
Another way to think of the polar vortex is as a spinning top. What we’re seeing very clearly is that the region northwest of Russia—known as the Barents and Kara Seas—is one of the Arctic regions where things are warming the fastest and where sea ice is disappearing the fastest.
The heat accumulated in the open water here during summer gets released back to the atmosphere in the fall, creating a heat bubble, which generally sits right under the strong winds of the polar vortex. If the heat bubble is strong enough and lasts long enough, it can poke that spinning top, causing it to wobble, possibly leading to a disruption of polar vortex. Because of human emissions, the area is warming rapidly, and evidence suggests that polar vortex disruptions are occurring more often as a result.
The polar vortex can have an influence on the jet stream and make the southward dips penetrate farther south. This can make cold outbreaks last longer, too. It’s not correct to say, “the polar vortex is coming.” What’s happening is that disruptions in the polar vortex (which again, is way up high in the stratosphere) can send wave energy downward to the jet stream, making it wavier, and allowing Arctic air to penetrate farther south.
We think the polar vortex is being affected by climate change, but it is still a very active area of research.
There’s a lot going on. Any one or combination of these factors can be playing a role at any time with respect to an extreme winter storm. It’s hard to disentangle which factors are playing the largest roles. In the case of Winter Storm Elliott, we saw all of these factors in place and possibly contributed to this highly disruptive event.
Rachel: Is there anything else you’d like to share with readers?
Dr. Francis: There is perhaps a silver lining to this. These extreme and disruptive events are piquing people’s interests. It’s providing an opportunity to educate people about how climate change is affecting them and hurting individuals and communities right now.
When you have people’s attention, it’s a chance to bring in climate science. This is critically important to think about and plan for, as places like Texas and Louisiana are going to see these events more often.