What is the Kelvin wave on the West Coast?

[THEME MUSIC] FLORA LICHTMAN: Hey. It’s Flora Lichtman, and you’re listening to Science Friday. There is a strange ocean phenomenon creeping up on the coast of California.

[AUDIO PLAYBACK]

– Kelvin waves, kind of the latest buzzy word.

– You’ve probably heard a lot about them. We’re worried about them coming our way with El Niño.

– They’re invisible underwater waves and could raise sea levels along the California coastline by up to a foot.

– So what does that exactly mean?

[AUDIO PLAYBACK]

FLORA LICHTMAN: Yeah. What exactly does all this mean? Here to give us the Kelvin waves lowdown– the who, what, where, when, and how concerned we should be– is my guest, Dr. Andrew Leising, an oceanographer who studies marine heat waves at NOAA Southwest Fisheries Science Center in San Diego, California. Welcome to Science Friday, Andrew.

ANDREW LEISING: Hi, Flora.

FLORA LICHTMAN: So I feel like ocean physics is really having a moment right now. Is this Kelvin wave news fun or annoying?

ANDREW LEISING: Actually, it’s hilarious because it’s the kind of thing that us oceanographers have known about for a long time, but it’s the kind of thing that in general, we’ve kind of shied away from talking about, because it gets a little bit technical. It’s something that happens at every time there’s an El Niño, but it can happen at other times in other places too. So it’s actually kind of exciting to see this sort of breakthrough to the popular press. It’s pretty cool.

FLORA LICHTMAN: Wasn’t on your bingo card, sounds like.

ANDREW LEISING: No, absolutely not. Nobody’s bingo card said we’re going to talk about Kelvin waves for the next six months.

FLORA LICHTMAN: We’ll have to come back to why it’s having a moment now. But first of all, just take me under the sea.

ANDREW LEISING: Sure.

FLORA LICHTMAN: Give me a visual. Like, where is this wave, how fast is it moving, and where is it headed?

ANDREW LEISING: So where is the wave? It starts at the far-western end of the equator in the Pacific Ocean. And this is the main way that El Niño affects what goes on the US West Coast and actually through a lot of the South American coast too. So what happens is, essentially, during a not El Niño time, the normal times, the wind is blowing from the east towards the west right at the equator.

It’s pushing all this warm water towards the west. And it actually almost piles up. You can think of it as piling up like a mountain of water. And what happens when an El Niño starts is that it disrupts the normal sort of atmospheric pattern. That’s the main thing an El Niño is. It’s an atmospheric disturbance. So it disrupts that wind. And when the wind stops blowing towards the west, it actually changes the currents a little bit right at that surface.

When that wind stops pushing that water, it sloshes back towards the Middle East. And that sloshing is essentially what causes this wave. But the wave isn’t like what most people think of as a wave at the surface. This wave, it’s a type of wave known as an internal wave. And we call it an internal wave because it’s deep in the ocean. It’s not the surface. It’s down deep. And what’s really cool that most people don’t think about when they think about waves is how waves transmit energy. They don’t transmit matter.

So these internal waves, they’re not necessarily moving a lot of water. They’re moving a little bit of water at the surface back towards the east along the equator. But what they’re really doing is, they’re sending this internal wave that thickens the normal surface layer of warm water. And most people can understand that you’re at the surface. It’s warm. If you dive down deep, it gets cold. And so what happens is, this wave, it’s not necessarily it’s moving a lot of water from the west towards the east. It’s thickening that layer at the surface.

FLORA LICHTMAN: OK. So it’s thickening this warm water patch. And then why does that amount to more water– why does that amount to higher sea level?

ANDREW LEISING: Ah, because just like everything, when you heat something up, it expands. And so if you’re increasing the amount of warm water, that layer of warm water, if it gets thicker at the surface, the entire column of water– you can think from the surface all the way down to the deep– that whole thing is expanding, and it sits, basically, taller than it used to.

FLORA LICHTMAN: And that’s why sea level goes up.

ANDREW LEISING: Exactly, exactly. And so the cool thing is that this wave is so big that, essentially, it’s called a planetary gravity wave in a sense, because it’s so massive. And Lord Kelvin described this back in 1900s. That’s why it’s called the Kelvin wave. And it perfectly travels from the west to the east. And then it gets really neat because then it breaks apart and travels up the coast to us.

FLORA LICHTMAN: OK. So it’s very helpful. I feel like I have a much better sense of it now. So this wave is a symptom of El Niño.

ANDREW LEISING: That’s right. Yep.

FLORA LICHTMAN: How common are they?

ANDREW LEISING: OK. So what happens is that even though we declared it El Niño on June 11, these atmospheric disturbances that can cause these pulses that form the Kelvin waves, those can happen at any time. And, in fact, one did happen starting even in February this year. So long before we declared it was an El Niño officially, these Kelvin waves were happening.

And so what happens, though, is as the season goes on, we’ll get more and more of them. And because the water itself is warmer as we go towards summer, the pulses themselves are stronger and will transmit more warm water, and the wave itself can be larger and increase the depth of that warm layer. So you typically have– during an El Niño, you’ll have maybe five or six of these. over the course of several months.

FLORA LICHTMAN: Five or so I.

ANDREW LEISING: Yeah. And the first one has probably just arrived, like, right this moment or last week.

FLORA LICHTMAN: Oh, really?

ANDREW LEISING: Yes.

FLORA LICHTMAN: OK. So five or six during an El Niño cycle– obviously, this isn’t our first El Niño rodeo. Why are we just now talking about it? Why has the Kelvin wave burst onto the scene?

ANDREW LEISING: Well, I think it was because a good colleague of ours, Dr. Dillon Amaya, happened to be chatting with someone on the news, and he brought it up. And people were like, well, what’s that? We’ve never heard that term before. So he was brave enough to bring that up. Like I said, normally, when people ask me about El Niño, the past big one, I would not bring it up, normally, to the media, because it’s a complicated thing to describe. It takes a long time to describe.

And I think just somehow, this year, the media is just very excited about it being this, quote unquote, “super El Niño,” even though there’s no such thing as a super El Niño. It’s a very strong El Niño. But I think people have just gotten enchanted by this idea that this is a really strong event, and there’s just all this going on. And we’re seeing a lot of observations of unique and interesting things. And so people latched onto that term and said, hey, what is this?

FLORA LICHTMAN: The headlines have made it sound scary, but how scary is it? Or what should we be concerned about here, if anything?

ANDREW LEISING: It’s something that happens with every El Niño, and it can happen other times in other places too. I think in some sense, we’ve maybe passed the worst point of it, because there’s a bunch of features that go into the total sea surface heights, not just this Kelvin wave coming through, but it’s the total overall water temperatures. Actually, right about now, we’re actually over the cusp of the yearly sort of seasonal cycle of when is the temperature warmest in the water.

So the warmest-ever temperatures we usually get along the US West Coast is at the end of September. So we’re past the hump there. And so overall, even if the water level is higher than it normally would be because of these Kelvin waves yet to come, the overall temperature of the water is cooling at this point, except that– here’s the caveat. So the overall levels are coming down. We’re going to have more Kelvin waves that will keep raising it up.

The only thing to really worry about I think is coming in the next month or two, when we have storms. So now we typically, with El Niño, will have quite strong storms, and those create this storm fetch that can raise it more. So we could have a case where we’ve got warmer-than-normal waters, we’ve got the Kelvin wave making it higher than normal, and we could have some strong storms. At that point, we could still have some high sea levels.

FLORA LICHTMAN: It’s an added risk to a menu of risks. Yeah.

ANDREW LEISING: It’s an added risk. And, again, like this year, that was following on the heels of a very strong marine heat wave that was already there, warming up the waters.

FLORA LICHTMAN: Are there any after-effects? What does the Kelvin wave leave in its wake?

ANDREW LEISING: Ah. OK, interesting. So actually, after it passes, it’ll raise the sea level height and thicken that layer. The biggest after-effect is really, because we’ve thickened that surface layer of warm water– so not only is it warm, and that can be a problem for some animals. But by thickening that layer, it decreases the ability of upwelling to happen.

And now, upwelling is a really important process along the US West Coast. Upwelling, just simply put, is, when the winds blow a certain direction, it can pull actual cold water up from to deep right at the coast, and that cold water has nutrients in it that fuel our ecosystem. And so one of the biggest after-effects of these Kelvin waves is that it kind of tamps down on that upwelling and makes the upwelling less sort of biologically effective, we say.

And so that’s a big after-effect. The other thing that’s kind of interesting is that like all waves, these waves can rebound a little bit. So sometimes after this, the wave can pass, and thickened layer, there can actually be an upwelling part of that wave itself that comes through and can raise and can make the layer thin again. But usually, it’s a little bit more one sided. It’s usually more deepening than it is raising it. But those are the two after-effects of the Kelvin waves.

FLORA LICHTMAN: And the heat waves?

ANDREW LEISING: So the heat waves are a whole separate thing. So heat waves could be caused by the Kelvin waves, but you could also have marine heat waves in the surface caused by other atmospheric things. So essentially, we had, in California, long before the El Niño sent the Kelvin waves up our way, we had, ever since last December, abnormally warm surface waters in most of Southern California and all the way, actually, up to San Francisco.

If you remember, in April, we shattered a bunch of records for the April ocean temperatures. And so that was a very strong marine heat wave, not related to El Niño. El Niño hadn’t even sent us pulses yet or anything. So there was a strong heat wave there already. And so that’s a little bit of a problem, because it means that we’re heading into this El Niño season, where the water already being much warmer than normal.

So it gives you the possibility of raising the sea level even higher. And it’s kind of similar to what we saw in 2015. So same thing happened in 2015. We had an event called The Blob, which was a massive marine heat wave. It put marine heat waves on the map. And basically, ever since then, that’s a big part of my job, is studying marine heatwaves. So that really put things on the map for us. And that was actually a somewhat similar event to this year. Yeah.

FLORA LICHTMAN: In the 10 seconds I have left, how long does the Kelvin wave stick around, and when will we be saying goodbye and looking for those after-effects?

ANDREW LEISING: Oh. Well, the Kelvin waves themselves will continue as long as El Niño does. So we’ll keep getting these pulses maybe once a month, all the way until March or April.

FLORA LICHTMAN: Dr. Andrew Leising is a research oceanographer at the National Oceanic and Atmospheric Administration Southwest Fisheries Science Center in San Diego, California. Thanks for joining us.

ANDREW LEISING: Sure.

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[THEME MUSIC]

FLORA LICHTMAN: This episode was produced by Imaan Moin. And if you have any questions or comments about this or any other science story, we love hearing from you. 877-4-SCIFRI is our number. Give us a call. Thanks for listening. I’m Flora Lichtman.

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