The swirling oceans around us

How do the oceans move?
10 November 2020

Interview with 

Erik van Sebille, Utrecht University


An underwater view of the ocean surface.


We may call the planet “Earth”, but its surface is mostly water. And fuelling giant ocean currents, that water is constantly on the move and has huge impacts on the climate and environment. Erik van Sebille is an oceanographer from Utrecht University, and he spoke to Adam Murphy about our swirling oceans.

Erik - Yeah. So you may have heard of the Gulf Stream or the Kuroshio of Japan, or maybe the East Australian current off of Sydney. And those are what we call Western boundary currents. And they are the Western monikers of the giant gyres. Gyres are gigantic, basin-spanning swirls, essentially, there's one in each ocean basin so there are five in total. And the highest speed in each of these gyres is not much more than walking pace. And most of it is much slower. So it takes maybe five years or so to do a total loop around one of these gyres. And what are they forced by? Well, the gyres are forced by the winds that are blowing over the ocean. And in particular there are the easterly flowing trade winds near the equator, and the westerly flowing winds at the solar latitudes, and all of this creates those gyres. But really the biggest current of all is the Australian circumpolar current, as we call it. It runs from West to East around Antarctica. It is more than two kilometers deep. It is hundreds of kilometers wide. And in total it transports 150 million cubic metres per second. And that is roughly a hundred times more than the combined outflow of all the rivers in the world.

Adam - Wow. That's huge! And you mentioned that the Gulf stream, so what is that, how is it formed and what does it do for us?

Erik - Yeah, exactly. So the Gulf stream is the Western side of the North Atlantic gyre. It's on the Eastern side of the US but as oceanographers, we like to have an ocean centric world view, right? Where we look at the ocean itself! Now each basin, like the North Atlantic, has a Western boundary current as they call it, a current on the West, has a beautiful mathematical proof. And it's called a sverdrup solution. And in short it has to do with the rotation direction of the earth, and the fact that the Earth is a sphere and not an infinite cylinder. So that actually the rotational speed varies with latitudes. Now, because of that, really the only possible mathematical solution is a gyre with a very strong current on the Western sides. But note that this solution can't actually explain why the ocean is the way that it is, because yes, it explains the Western boundary currents, but on top of those large scale gyre flows, the ocean is also chock full of what we call eddies. Now eddies are much smaller vortexes, maybe a few hundred kilometers in diameter or so, and they are kind of like the bubbling turbulence nature of the entire ocean as it moves around.

Adam - And another thing people might have heard of that they might not be too sure of - I know I'm one of those people - you sometimes hear about something called El Niño in the news. What is that? And what does that do?

Erik - Yeah, so El Niño is not per se about currents. El Niño is a massive redistribution of heat in a tropical Pacific ocean. So sometimes, and especially during what we call La Nina periods, the heat goes from the atmosphere into the ocean. And it means that worldwide, the atmosphere cools a little bit for a few years. And then during an El Niño, the heat comes back and the atmosphere warms up extra fast globally. So this is really a measurable effect. While winds are certainly important in El Niño, it's not really connected with the current. So I guess that even if the ocean would not move there would still be things like the El Niños I think

Adam - Those ocean currents, you mentioned earlier, they're not exactly clean, are they?

Erik - No, I know what you're speaking about. They're not clean at all. The ocean currents carry enormous amounts of plastic around. And what happens actually is that within the gyres, so these five gigantic swirls around the ocean basins, the flow's actually slowly inwards. So as the flow spirals slowly inwards, the plastic goes with it until then, in the middle of the gyres, so in the centers of the basin, the water sinks down a little bit. But the floating plastic is of course too buoyant. So it stays behind. And as new plastic spirals in, there's a constant more and more plastic piling up. It's a bit, I guess like the turd that won't flush!

Adam - And then how are things going to change and change as we see the climate changing more and more?

Erik - Yeah. So that's the big question. So the ocean by itself is absolutely going to change. And that is, of course, the sea level rise that we know is the ocean warming up, the ocean becoming more acidic because CO2 is actually captured within the ocean. So 25% of all the carbon dioxide that we've burned as society has gone into the ocean. It's a bit as if you get a 25% discount on our fossil fuel emissions. So that is all absolutely happening. But what the currents are doing is a little bit less clear. Now we know that winds are probably going to be intensifying. So you would expect the gyres to spin up a little bit and to go a little bit faster. And that probably means that they also become more unstable. So it would probably more eddies, but exactly what that then means we don't know. So it's a bit unclear. We don't even know about the large scale, kind of like what we call the thermohaline circulation, where all the ocean basins are connected and where heat is redistributed between the gyres, rather than the flow within the charters. We don't even really know what that is going to do in a changing climate. So as oceanographers, we have a really difficult time to predict what will happen. We don't have much evidence from the, about the strength of the currents per se. So what exactly it is, yeah, we just don't know.


Add a comment