Atmospheric Rivers Bring Wet Weather To Arizona

By Lauren Gilger
Published: Tuesday, December 10, 2019 - 2:50pm
Updated: Wednesday, December 11, 2019 - 8:03am

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Atmospheric river Bodega Bay
A storm darkens the sky at the mouth of the Russian River, north of Bodega Bay, California. The storm was driven largely by an "atmospheric river" over California.

LAUREN GILGER: Here's a phrase you might have heard lately: atmospheric rivers. This weather phenomenon is back this winter in the West, and we're seeing the results right here in Arizona. But what exactly are they? Well, according to our next guest, it's a lot like it sounds: rivers in the sky. Here is University of Arizona Professor Christopher Castro.

CHRISTOPHER CASTRO: So, atmospheric rivers are these very narrow and low jets of moist air that occasionally get entrained with our winter storms here that affect the West Coast. And because they're very narrow, they produce lots of rain, but it's typically over very narrow areas where they impinge on the coast.

GILGER: So what is causing this? You get a sense from reading about these that they may be related to climate change or that there may be some recent research in how they are increasing as related to climate change. What does that say?

CASTRO: So a basic principle with climate change is with the atmosphere getting warmer, it can potentially hold more water vapor. And so now when these atmospheric rivers are tending to occur, maybe their frequency doesn't change or even maybe there might be less frequent. But when they do occur, they're tending to be a little bit more intense than they used to be. And this is what, as best we know, the current research suggests.

GILGER: How dangerous then can they be? I mean, I know we've seen quite a bit of flooding, flash flooding here in Arizona this winter. Is that related?

CASTRO: It is peripherally. So if these winter storms can tap into these atmospheric rivers, then you can get really big flooding events here in Arizona. And that's happened on occasion. So our biggest winter storms are typically associated with these these taps of subtropical to tropical moisture.

GILGER: Is this related to this El Niño weather pattern we're having this year, in 2019, or not?

CASTRO: So in an El Niño year, we tend to get greater than average rainfall during the winter season. And this is because of greater frequency of winter storms that tend to track through the Southwest. And so, because you're having this greater frequency of winter storms and there's more ability to tap into these atmospheric rivers in those types of years.

GILGER: We've spoken with several meteorologists this year on The Show in Phoenix who predicted, leading up to this winter, that we would have a wet winter after particularly a disappointing monsoon season in the state. So this isn't entirely unexpected, right?

CASTRO: No. And statistically, when we have a dry and delayed monsoon, as we certainly did this year, it tends to be paired with winters that are above normal in terms of precipitation. And that seems to be what's happening now. November was really wet.

GILGER: So as we sort of see these more extreme versions of this, can we predict when we will see another winter like this in the future? Is this going to be kind of a new normal?

CASTRO: I would be hesitant to say it's the new normal. But what I can say is current research suggests that there is becoming these greater swings between the extremes and climate. So you have these dry versus wet years. And the contrast between those dry and wet years is, on the West Coast, just seeming to become more extreme than it used to be. It's this phrase called a climate whiplash effect.

GILGER: What do we need to do, I guess, as as these things become more intense, as we see atmospheric rivers dropping more rain in more specified areas, do we need to be more prepared?

CASTRO: Yes, we do. And I think this is a question of when we get these type of events, are they exceeding the limits of our infrastructural design that are going to cause societal impacts like flooding? So this is something that we in the research community need to work with the planners and engineers as to what the impacts of these future storms might be. And that's consistent with what we're currently doing here at the University of Arizona, working with partners throughout Arizona and beyond.

GILGER: All right. That's Christopher Castro with the University of Arizona. Christopher, thank you so much for coming on.

CASTRO: Thank you.

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