EMILY KWONG, BYLINE: You're listening to SHORT WAVE from NPR.
AARON SCOTT, BYLINE: Hey, SHORT WAVErs. I'm Aaron Scott. And, well...
(SOUNDBITE OF CUTTING DRYWALL)
SCOTT: ...Hear that? Is the sound of contractors in my basement cutting out soggy drywall because our basement flooded over the holidays. While much of the country was dealing with blizzards, we on the West Coast have been dealing with monster rainstorms, freezing and flooding. And one of the big drivers of this wet weather is what is known as atmospheric rivers. First, the northwest got hit and now California.
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JAMES BROWN: New year has brought record rainfall, deadly flooding and high winds to northern California.
JERICKA DUNCAN: In Sacramento, it was the wettest New Year's Eve on record. At least two levees have been overwhelmed.
UNIDENTIFIED PERSON #1: Governor Newsom is declaring a state of emergency tonight as California braces for a huge winter storm.
UNIDENTIFIED PERSON #2: That upcoming storm now has a Bay Area in its sights. And experts say it could be deadly if people don't prepare now.
DANIEL SWAIN: There's already been some flooding, along with wind damage and power outages. But the real concern is what could happen later this week and into next week as the storm sequence continues, and additional very wet atmospheric rivers will progressively increase the flood risk with each passing storm.
SCOTT: This is Daniel Swain. He's a climate scientist with UCLA, the National Center for Atmospheric Research and the Nature Conservancy of California. And he's done a lot of work on atmospheric rivers.
SWAIN: The key difference between atmospheric river-related precipitation extremes and other precipitation extremes is the sheer concentration of water vapor into a pretty narrow corridor. So this isn't a huge, amorphous blob of moisture like you might get in a different context. But this is a very well-defined corridor. I mean, if you look at these from space looking downward at Earth from Earth-orbiting satellites, they literally look like rivers. They're relatively narrow, only a couple hundred miles across, but they can be thousands of miles long.
SCOTT: Wow.
SWAIN: The atmospheric rivers can move hundreds of miles just in a day. And in the case of California right now, for example, this atmospheric river is really booking it.
SCOTT: Today on the show, we wade hip deep into an atmospheric river to ask, what is it? What causes it? And should we expect these rivers in the sky to get worse? I'm Aaron Scott. And you're listening to SHORT WAVE, the daily science podcast from NPR.
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SCOTT: OK, Daniel, let's begin with the basics. What is an atmospheric river?
SWAIN: So an atmospheric river is essentially a corridor of highly concentrated water vapor moving quickly above your head, literally, in many cases, a river in the sky. Atmospheric rivers can transport volumes of water many times that of the Mississippi River.
SCOTT: Wow.
SWAIN: And it's doing so all in the air above your head. But only occasionally, they make landfall and produce flooding. So in in places like California, atmospheric rivers are a staple of the typical wintertime climate there. They're responsible for half of the precipitation, but they're also responsible for nearly all flood damages. So they can be a blessing or a curse, depending on the context. And that's true really anywhere where they're prevalent.
SCOTT: And what keeps them flowing? I mean, they don't have a bank, like a river up in the sky. What kind of keeps them in these river ribbon formations?
SWAIN: Imagine if you had a terrestrial river, the traditional kind, with, you know, earthen banks or something like that and water flowing in the middle. Imagine you just had water flowing from all sides towards the river, from both banks perpendicular to the flow of the river so that all of that water was converging along the river, which is flowing in one direction. And that's because of topography. So rivers essentially flow downhill. Well, in the sky, these rivers are mainly being pushed by prevailing winds, which in the mid-latitudes are generally from west to east. These are known as the westerlies. And they drive a lot of the weather patterns in the regions north of the tropics on Earth but south of the Arctic.
So these atmospheric rivers can be often quite longitudinally elongated, if you will. But if a storm system comes along, it can tilt it more in the latitudinal direction, and that's when you get all of the water that flows essentially from south to north. These are a mechanism by which the climate system evaporates water from the oceans and moves them to other places, higher latitudes or from ocean to land in various parts of the world. So these sort of are the thing that really move most of the water in the atmosphere poleward on Earth.
SCOTT: So I'm in Oregon, where it seems like we probably get an atmospheric river at least once a year. And yet for a lot of my co-workers on the East Coast, this seems like the first time they're hearing about this phenomenon. So can you give us a sense of where atmospheric rivers tend to happen and how common they are?
SWAIN: Well, they're most common, actually, on the west coasts of continents rather than the east coasts. This comes from the fact that in the Earth's mid-latitudes, you have prevailing westerly winds, meaning that the winds blow from west to east rather than from east to west most of the time. So if you have winds blowing from west to east and these kinds of storms form over the oceans, which side of the continents are they most commonly going to affect? It's going to be the west sides of the continents that they impact first. And because most of the moisture gets squeezed out by the mountains on the west coast of North America, for example, these atmospheric rivers don't really have much left to them by the time they get to the east side of the Rockies. In South America, the Andes are an even starker barrier to Pacific moisture making it eastward across the continent of South America. In Western Europe, there isn't a major mountain range right along the coast, and so more of Europe tends to see the effects of atmospheric rivers periodically. So you can kind of see how the constraints of geography dictate where these events most commonly occur. They form over ocean basins and usually make landfall along the west coast of continents in both hemispheres.
SCOTT: Got it. Interesting. So knowing that climate change is making precipitation more extreme, can we expect that it's going to make atmospheric rivers more common and more powerful, as well?
SWAIN: There is a lot of evidence that climate change is dramatically altering atmospheric rivers, especially the intensity and the amount of moisture that atmospheric rivers can hold. So overall, it certainly appears that atmospheric rivers are going to get a lot moister and produce more extreme precipitation in a warming climate. That much is pretty clear. And some of our own research has really focused on that in recent years. If you have long sequences of these atmospheric rivers storms one after the other, day after day and week after week, and those storms on average are getting moister and have a higher ceiling on how much precipitation they can squeeze out of an ever warmer, moister atmosphere, then you're going to see an increased risk of flooding associated with them. So there is a climate change component to the atmospheric river story, even though atmospheric rivers have been around since time immemorial.
SCOTT: What do you see as kind of the future of how we will need to change or adapt to these increasingly wet rivers flowing through our sky?
SWAIN: Well, one of the truisms in - from a climate science perspective is that there are really two types of extreme events that are unambiguously increasing essentially everywhere. And this is going to continue for as long as we continue to experience further warming. And that is heat extremes - so more prolonged and more intense heat waves - and also increasingly intense precipitation extremes - so heavier downpours. And that's mainly because the water vapor holding capacity of the atmosphere increases exponentially. That actually presents major problems because it really means that the ceiling on how intense precipitation events can get is accelerating. So even if the warming itself is relatively steady, the extreme precipitation implications and the potential flood implications are nonlinear and accelerating.
So we may see accelerating increases in hydrologic extremes even as global warming hopefully slows down in the coming decades. In a place like California, we may actually be seeing more droughts, as well as more extreme precipitation. So how do we co-manage those risks? We can't just assume it's going to be wetter all the time because it won't be. It will certainly be wetter some of the time. But what about the rest of the time? How do we deal with the fact that we are going to see and are seeing more extreme droughts, as well as greater flood risk? And that's - you know, that's tricky.
SCOTT: Before we go, I have to ask because I think a lot of us nonscientist types probably look up at the sky and imagine that it's just this, you know, big, kind of smooth soup of air sitting on top of us. But the way you've been describing it, it doesn't sound remotely homogenized. It sounds like it's this writhing, dynamic mass of winds and rivers and eddies and storms. Can you help us see the atmosphere through your eyes?
SWAIN: Well, the atmosphere is a fluid, just as the ocean is a fluid, but the fluid is different. You know, the ocean is mostly water, and the atmosphere is mostly air. But they're both fluids, and they behave often in surprisingly parallel ways. If you talk to solid-Earth geologists, well, the Earth's crust also flows over long enough millions-of-year time periods, as well. But if you're an atmospheric scientist, you get to deal with the fluid motions of the atmosphere on very short timescales. So you can look out the window and watch, you know, clouds evolving minute by minute.
So, you know, sometimes, I've heard meteorologists joke that a lot of people in atmospheric science might have otherwise been geologists, but the timescale was just too long, and they found it kind of boring. You want to be able to go outside and sit for 10 minutes and, you know, watch the system you study evolve over time. I mean, that's literally something, you know, that I do not infrequently because I do find it personally - and as well as professionally - fascinating.
Of course, there are both short and long timescales in the atmosphere. We talk about, you know, the weather from hour to hour and day to day. We also talk about climate over decades, centuries and even millennia. So, you know, you don't have to wait a million years for the geology to change. You can just go outside with the weather, watch the clouds build and collapse, watch the wind currents, you know, wash across the landscape. It's sort of gratifying because you get to see it evolve on your lunch break, for example.
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SCOTT: Thank you so much, Daniel, for taking some time to peer up at the sky with us.
SWAIN: Oh, absolutely. It was a pleasure.
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SCOTT: This episode was produced by Thomas Lu, edited by Gabriel Spitzer and fact-checked by Abe Levine and Rebecca Ramirez. Brendan Crump is our podcast coordinator. Beth Donovan is our senior director of programming. And Anya Grundmann is our senior vice president of programming. I'm Aaron Scott. Thanks for listening to SHORT WAVE from NPR.
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