Tuesday, July 4, 2023

What Causes Giant Rogue Waves?

Sea monsters?

From Quanta Magazine's The Joy of Why podcast, June 14:

Once dismissed as myths, monstrous rogue waves that tower over ships and appear without warning are real. Wave-science researcher Ton van den Bremer and Steven Strogatz discuss how rogue waves can form in relatively calm seas and whether their threat can be predicted.


Steven Strogatz (00:03): I’m Steve Strogatz, and this is The Joy of Why, a podcast from Quanta Magazine that takes you into some of the biggest unanswered questions in math and science today. In this episode, we’re going to ask what causes the monster waves in the ocean known as rogue waves. 

(00:20) Throughout human times, the oceans have swirled with all sorts of legends. Think of the Flying Dutchman ghost ship that disappeared during a storm in the 1600s and is now forced to sail the seas forever. Or the reptilian sea serpents that fishermen swear have menacingly popped up from the surface of the water, or the Sirens of Greek mythology who lured sailors to their deaths with their soothing, seductive songs. We know all of these to be myths. But there’s one mystery of the sea that is not a myth, and it can be deadly: rogue waves. These are giant waves that seemingly come out of nowhere. They can slam into ships or hit oil platforms. And because the ocean is so big, with so many factors, they’re really hard to study because they’re difficult to actually witness. 

(01:08) One of the most famous is called the Draupner wave. It struck the Draupner gas pipeline platform in the North Sea in 1995, reaching an astounding maximum height of 25.6 meters, or 84 feet. That’s about the size of a six- to eight-story building. It was the first time a rogue wave was ever measured with instruments. 

(01:30) Capturing a rogue wave in the vast ocean is rare, so we still know relatively little about them. But scientists like Ton van den Bremer are trying to change that. Dr. van den Bremer uses wave pools and modeling to study rogue waves at Delft University of Technology in the Netherlands. He’s an associate professor of civil engineering and geosciences, and also a senior research fellow at the department of engineering science at the University of Oxford. Ton, thanks so much for joining us today to talk about rogue waves.

Ton van den Bremer (02:01): Thank you. It’s a pleasure.

Strogatz (02:02): Well, I’m really looking forward to this. It’s a fascinating subject. Let us start with just the basic issue of characterizing them. What makes a rogue wave rogue? Like, how is it different from the ordinary ocean waves that we see at the beach? Or tell us a little more about how big they are. How fast they can travel.

van den Bremer (02:19): Usually, you have a lot of waves, right? So you can compare one to the next. And that’s indeed what you do. So you look at basically a characterization of what we call the sea state. So this is an average of how tall the waves are. And then rogue waves are defined to be much, much greater than what is the average wave in that sea state. And so we actually very specifically say that a rogue wave is defined as being two times a quantity called a “significant wave height.” And significant wave height is basically a measure of how big the waves are at that point in time. And if your wave exceeds two times that, we say, “Oh, that is now a rogue wave.”

Strogatz (02:55): So that’s interesting. It’s a kind of outlier relative to whatever…

van den Bremer (02:59): You could think of it maybe as something, as an abnormal wave, both in a mathematical, precise sense — we have the normal distribution — but also in a sort of common parlance, right? We have the normal distribution of waves, and this is something that’s very far away from normal. So much bigger than what you would expect. 

(03:14) But of course, you have to be careful with that because what you expect depends on how long you wait. So waves, they come and go, right? They occur all the time. So that means if you wait for long enough, you always have a rogue wave. It’s just a question about how long are you willing to wait. And rogue waves are basically waves you have to wait for a long period of time.

Strogatz (03:34): What was that phrase you used earlier? The characteristic wave height?

van den Bremer (03:37): We call this the significant wave height. In any type of statistical description, you have the standard deviation, right? It’s a width of — if I have a sample, I’d like to describe that sample, say, of the height of a human being, right? I have a sense of what’s the mean height, but I also have a sense of the variance of that population. And in a sense of the variance, or the variation of the surface, is the significant wave height, how large the waves typically are. It’s called a significant wave height.

Strogatz (04:03): Why was it so impossible for scientists in the olden days to believe the sailors’ tales about rogue waves?

van den Bremer (04:08): What you have to imagine is that, you know, we’ve been studying the sky and, indeed, outer space for some time, and it’s been relatively easy. But the ocean is much harder. We can only study this from ships, or from wave buoys or from the coast. And if you study things from the coast, you know, you only learn things about the immediate vicinity. So everything coming from, basically, from the deep, deep ocean, the surface of the deep ocean, has come from ships. So that has been sailors’ tales....