Tides and Currents: The Motion of the Ocean

Transcript

HOST: This is the NOAA Ocean Podcast. I'm Troy Kitch. Today, we're going talk about how the ocean moves.

I was thinking I should call this episode, everything you wanted to know about tides and currents but were afraid to ask, but I guess that would be overstating it a bit.It's not everything you want to know. But you're going to hear a lot that  you didn't know about the ocean's tides and currents in this wide-ranging chat with NOAA oceanographer Greg Dusek. We sat down recently to talk about a bunch of different topics: high tide flooding, how the rotation of the Earth, wind, and the shape of the land affects tide levels and currents, the Gulf Stream ... there’s a lot to take in here and it’s a lot of fun- we hope you enjoy this conversation.

GREG DUSEK: My name is Greg Dusek. I'm a physical oceanographer and senior scientist with the National Ocean Service in the office of the Center for Operational Oceanographic Products and Services, which is the tides and currents office at NOAA. And I'm a physical oceanographer, so I study the motion of the ocean, how the ocean moves from things like waves, and tides, and currents. You know, when I tell people I'm an oceanographer, I think a lot of people assume oceanographers study the critters: the whales and the dolphins and things that live in the ocean. And I have to always explain, "No, no, I'm a physical oceanographer. I study the physics. How water moves."

HOST: Do you get some blank stares when you say that?

GREG DUSEK: Yeah, usually, they're like, "What's a physical oceanographer? I don't understand." Yes. (Laughs) The motion of the ocean, I think that's the easiest way to break it down.

HOST: So let's start big picture, with high tide flooding. What is it? And how do you know when it's going to happen?

GREG DUSEK: It's flooding that's most frequent at high tide and most significant at high tide. I'll give you an example of high tide flooding. I was at the boat show in Annapolis, Maryland, back in October. And we show up at the boat show and there's water everywhere. Water in the streets. Probably about a foot of water in the streets, for no apparent reason. There wasn't a storm. There was a little bit of wind, but nothing really significant, and people next to me were like, "Why is this water here? It didn't rain that much. I don't understand what's going on."

HOST: They probably expect you to have the answer to that.

GREG DUSEK: Yeah right, well, I was just kind of listening in curiously to see what people thought the reason was. But it was one of the higher tides that Annapolis saw, and there were some winds in Chesapeake Bay that moved the water around, so it was a little bit higher than you normally would expect, and that combined with the high tides, you have just enough water level to cause flooding. And I think the ground was a little bit saturated from some rain. And so that type of flooding where there's no storm, there's nothing obvious, but it's a high tide and there might be some other effect moving the water to be a little bit higher than normal, and then we start seeing some low level flooding, water in the streets, that's typically what we think of as high tide flooding.

HOST: And this is happening more and more now, right?

GREG DUSEK: Yes. I think since the 1960s we've seen an increase in the number of high tide flooding days per year up over 500 percent at most places, like between 500 and 1,000 percent since the '60s. So it's increasing rapidly. Back in the '20s — so we have water level gauges across the U.S. that have been established since the 1920s or even earlier in some cases. So we have one in Charleston, South Carolina, that's been there since 1921. And in 1928 there was the Lake Okeechobee hurricane, it was one of the more significant hurricanes in U.S. history. It was a Category 4 when it struck Florida. Thousands of people lost their lives, so it was a tragic hurricane, but by the time it made it's way up to Charleston, it was still a Category 1. You know, the water from storm surge was pretty high, caused a little bit of flooding, had some damage to boats and buildings near shore. And so we observed that water level with our gauge, and at the time it was the highest recorded water level at that gauge from when it was installed in 1921 to 1934. So for a 14 year period it was the highest observed water level. Fast forward to now. We exceed that water level like ten times a year in Charleston and just from high tides, maybe a little bit of winds and high tides and we're hitting the same water level that we used to see in a once-in-a-decade storm. So it's changing quite a bit from sea level rise.

HOST: And that's something that people are just going to have to get used to see happening more and more, right?

GREG DUSEK: Yeah, that's right. So going forward, assuming even relatively low levels of sea level rise for the range of projections that are possible, we're going to keep seeing more and more high tide flooding to the eventual point where in some places it could be happening almost daily.

HOST: And how do you predict when that's going to happen?

GREG DUSEK: The only good part of the fact that high tide flooding is caused by the tide is that the tides are very predictable. And so we have a good idea by using tide predictions of when the highest tides are over the course of a year. And we can look out several years into the future and know when we're going to have the highest tides. So just knowing that, and knowing at what water level a place is going to start flooding, we can get pretty close to knowing when we're going to see these high tide flooding days. But then we add in to that other effects of water level that are predictable, like the seasonal effects. So a good example would be, as the water warms up, it expands, and you get higher water levels. So typically in the late summer, early fall, we'll see higher water levels than in early spring, because the water's warmer. And that is very regular, so we're able to predict that and include that in our predictions. And there are several other seasonal type of things like that, that change water levels regularly. So we account for that. So we combine those two things. We combine the information we know about sea level rise and we can get pretty close to knowing when we're likely to get water levels that are close to being flooding. Now, usually you need some other part to put it over the top. So you need a little bit of wind, maybe. Some other kind of weather forcing or something that typically is going to end up causing flooding. But we're getting to the point to where that's less and less needed. So in places like Charleston, you get flooding from the tide alone, without any other forcing sometimes. And that's going to more common in more places.

HOST: Is this mostly an East Coast thing, or is this all over the place?

GREG DUSEK: High tide flooding happens everywhere. It's less common in places, for instance, like Alaska where water levels are actually dropping relatively, because the land is moving up there. So even with sea level rise, you're not getting increases in water levels at many places in Alaska. So combine that with the steep topography there, and buildings are further away from the water typically. So you typically might not see it in a place like that. Same thing with like around maybe Maine and up in the Northeast where you have steep topography, it's not as much of an issue. But pretty much everywhere else in the U.S., even the Pacific islands, it's a problem. And if it's not that much of a problem, it's going to be as we get continuing sea level rise, it's going to become more and more of one over the next decade.

HOST: You mentioned regional winds as one of the factors that can change flooding levels, high tide flooding. Could you explain a little bit more about how the winds play in to this?

GREG DUSEK: I think it was two years ago, it was like record high water temperature in New Jersey. It was like 83 degrees, or like mid-80s, in August. And everyone is like, "Wow, this is great! The water's awesome!" and then all of a sudden, two days later, it's 60 degrees. And everyone's like, "What happened?!" It went from being like bath water to no one wants to go into the water. We had some questions about it and basically what happened is the winds shifted. And all of a sudden you were getting winds that were pushing the water offshore, and when that happens you push water on the top layer which is really warm. You have this warm top layer which is warmed up over the summer. That get's pushed offshore. And then you upwell, or bring up, this cold water from underneath, and all of a sudden your water temperature drops, you know, 20 degrees almost overnight. And that's all caused by Ekman Transport, which is basically the idea of pushing water to the right of the winds. So you get winds out of the south, which most people might think, "Oh, well it's coming from the south so it's going to move warm water to me." But actually what happens is the water wants to go to the right of the winds, and so that top layer of water gets pushed offshore and you actually get a decrease in water level on the coast when you have that, so you get lower water level and then you get this upwelling of cold water.

HOST: Usually when I think of upwelling, I think of California and the West Coast. So is it more prevalent on the West Coast?

GREG DUSEK: Yeah, because of the directions of the winds there. And the fact that you have no continental shelf really, so you drop off water depth really quick. So those really deep waters are really close to shore and so you bring up this nutrient rich, cold, deeper water very quickly close to shore when you're pushing that surface layer away from shore. On the East Coast it's not as significant because you have such a broad continental shelf, so your water column isn't as stratified typically and so you don't really have that deep bottom water close to shore. But you still get those same kinds of effects. Particularly, you see it more in the summer months where you start really warming up that surface layer, and the water underneath is a little bit cold.

The context, of course, is that this is caused by the Earth's rotation. Because of the Earth's rotation, because of the Coriolis Effect which a lot of people might remember from back in school hearing about how if you shoot a missile across the Earth and it wants to veer to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. And that's all caused by the Earth's rotation. So that's really important for oceanography because the water, when you're moving water over long distances, it wants to go to the right. And so you tend to move it, depending on where you are along the coast, you tend to move it 45 and 90 degrees from the direction of the wind stress when you have sustained winds over large distances. And that affects water levels too, because you're pushing either water away from shore so you have a little bit of a decrease in water level, or you're pushing water towards shore and you have an increase in water level. If I'm teaching a class on ocean currents, especially large-scale ocean currents, that's the first thing you've got to talk about, though, is Coriolis and why it's important. Because the Earth's rotation is really what drives all of our large-scale ocean currents. So that's why those occur the way they do, because of the Earth's rotation. So Coriolis and that idea of water turning to the right is principal in all of physical oceanography. It's really important.

HOST: That's a good segway to talking a little bit about the Gulf Stream and how that affects sea level along the coast.

GREG DUSEK: So first, just some background on the Gulf Stream probably, right?

HOST: Yeah. What is the Gulf Stream?

GREG DUSEK: Yeah, right, because a lot of people probably don't know. They hear about it all the time probably, right? But they don't really know what it is. So the Gulf Stream is a large fast-moving ocean current and it is really important for moving lots of warm water from near the equator up to the poles.

HOST: And it's always there, or is it just sometimes there?

GREG DUSEK: It's always there. It moves around a little bit and it can speed up and slow down depending on conditions. But it's always present. And it transports a massive amount of water. So what I've asked people to do is, in their mind, think about all the rivers in the world. The Amazon, the Mississippi ... all the big rivers you can think about. And think about all the water that is being put in to the ocean by those big rivers. Add them all together, then multiply it by 50, and that's how much water the Gulf Stream transports along the East coast. So it is a massive amount of water that's it moving. So it has a huge impact on the oceanography of the entire Atlantic ocean.

HOST: That's a lot of water. (laughs)

GREG DUSEK: Yeah. I think one of the things that people don't know about the Gulf Stream is that the height of the water actually changes across the Gulf Stream. So if you think about mean sea level — some flat, level surface — across the Gulf Stream, you actually get an increase in height of sea level by about three or four feet. So it's a substantial increase in water level there, and that's in part driven by the Earth's rotation and also because that water's moving so quickly. So what can happen is if the Gulf Stream slows down or moves in certain ways, the water that's kind of bulged as part of this stream can relax and actually push it's way up against the coast. When the Gulf Stream slows down, you can see increases of water level along the Southeast coast by several inches to maybe a foot. In some cases, it happens pretty regularly. So in North and South Carolina, you get a regular increase in water level in the fall because the Gulf Stream tends to be a little bit slower during those time periods. So it's really important to coastal sea levels, so understanding when the Gulf Stream is going to slow down and how it moves is really an important area to study when we think about how coastal water levels are changing in time and potentially changing in the future, if there's any ever longterm change in the way the stream moves.

HOST: Is that a possibility? That there's going to be a longterm change?

GREG DUSEK: That's an open question. There's been some research that suggests that the Gulf Stream ... you know, there's kind of some longer period cycles in the Gulf Stream which are known over tens of years and things like that. But there's some speculation that you could see a longterm trend in decrease in Gulf Stream transport, which of course would result in an increase in sea level along the coast, more than what we see just from global sea level rise alone. So the jury's still out a little bit on exactly how that's happening, if it's happening. We need more data, I think. But it's a possibility that's been studied in the research.

HOST: And there are major streams like this all over the world?

GREG DUSEK: That's right. So the Gulf Stream is what we call a western boundary current. We call it that because it's along a western boundary of an ocean basin. In this case, the Atlantic Ocean. In the Pacific, we have a similar current along the coast of Japan called the Kurushio Current. There are also other western boundary currents. But basically along every western edge of large ocean basins, we have these fast-moving, large ocean currents.

HOST: And that's mainly due to the rotation of the Earth.

GREG DUSEK: That's right. The Earth's rotation effectively pushes the water over to that side of the basin, and so you get this compressed, fast-moving water. In the case of the Gulf Stream, it's about 50-60 miles wide in most places. And compare that to the other side of the basin, where you have these broad, diffuse currents which transport water south. They move much slower and they might be thousands of miles wide. So you have this kind of opposite effect on both sides of the Atlantic ocean.

HOST: OK, top thing. If there's one thing you want people to know when they walk away from this interview, what's that going to be?

GREG DUSEK: I think probably one of the most important things is to know that we can predict how the water is moving in the coastal ocean pretty well in relying on what we know about the tides, because that causes a lot of that motion. Just by understanding that, being able to predict that, it's really important for things like coastal hazards, like high tide flooding. It's important for things like commerce and navigation. It really helps drive a lot of what happens along our nation's coasts: just being able to predict at it's most basic level what the tide is doing, what the tidal currents are doing. We're really good at it, and it's really important for a lot of different reasons.

HOST: What's the most fascinating things about tides and currents that you want people to know? Something that's super interesting that nobody knows.

GREG DUSEK: I think one of the interesting things about currents, and even tidal currents, that people don't realize is, one, they change with depth. So you can have really strong currents up at the surface, and then down at the bottom, you know, tens of feet or more down, the currents can be quite a bit different. And the reason that they're different is a combination of, one, the friction from the bottom. So when water rubs up against the bottom it slows down, so you typically have slower currents toward the bottom of the water. But then you also get changes in water density over the water column with depth. So you might have warm, fresher water towards the surface where your boat is, and down by the bottom you might have colder, saltier water. And those changes in density actually cause changes in the currents. And so the density of the water, or the warmth and the saltiness of the water, actually cause differences in how the water moves and differences in the currents, which I think is pretty interesting.

HOST: It is. And it makes me appreciate how complex of a system it is. You don't really think about it, but there's a lot going on.

GREG DUSEK: It's incredibly complex. You know, you can simplify a lot of it and get decent results. But if you really want to understand what's going on, there's a lot of layers. There's a lot that goes into it.

That was Greg Dusek, a physical oceanographer in NOAA’s tides and currents office. We hope you enjoyed the talk and we look forward to talking to Greg again in a future episode so we can dive into more topics about the physics of our ocean. This is the NOAA Ocean podcast. Head to oceanservice.noaa.gov for show notes and to listen to all of our other episodes. Subscribe to our podcast in your pod catcher of choice — and leave us a review on iTunes. It really helps us out to get more listeners.