What is the Kelvin wave on the West Coast?

Andrew Leising (0:00)

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Flora Lichtman (0:06)

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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.

Andrew Leising (0:26)

Kelvin waves, kind of the latest buzzy word.

SPEAKER_2 (0:29)

You've probably heard a lot about them. We're worried about them coming our way with an enu. They're invisible underwater waves and could raise sea levels along the California coastline by up to a foot.

Andrew Leising (0:42)

So what does that exactly mean?

Flora Lichtman (0:44)

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 (1:04)

Hi, Flora.

Flora Lichtman (1:06)

So, I mean, I feel like ocean physics is really having a moment right now. Is this Kelvin's wave news fun or annoying?

Andrew Leising (1:14)

It is. It is. 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 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 Nino, but it can happen in other times and other places too. But so it's actually kind of exciting to see this sort of breakthrough to the popular press. It's pretty cool.

Flora Lichtman (1:39)

Wasn't on your bingo card, it sounds like.

Andrew Leising (1:41)

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

Flora Lichtman (1:46)

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 (1:52)

Sure.

Flora Lichtman (1:52)

Give me a visual. Where is this wave? How fast is it moving and where is it headed?

Andrew Leising (1:59)

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 Nino affects what goes on 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 Nino 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. Okay.

It actually almost piles up. You can think of it as piling up like a mountain of water. And what happens when an El Nino starts is that it disrupts the normal sort of atmospheric pattern. That's the main thing in El Nino 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 east. And that sloshing is essentially what causes this wave. But the wave isn't like what most people think of as the wave at the surface. Okay, 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. Okay, 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 sort of 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, right? 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 (4:03)

Okay, 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 (4:11)

Ah, because just like everything, when you heat something up, it expands, okay? 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.