You might have seen a video like this before: a ship out in the open ocean approaching a distinct line of water. On one side of the line the water is dark blue and clear, and on the other it's greenish and silty looking. Many of these videos explain that this is the separation line between the Atlantic and Pacific oceans — a place where they claim water defies all its own laws and refuses to mix.
But as you can probably guess, water doesn't mix with itself in every situation, all over the world, and then start acting like two positive ends of a magnet in the place where the Atlantic and Pacific oceans come together between South America and Antarctica. So, yes, the waters of the Atlantic and the Pacific definitely do mix. But what's going on in these videos?
The Atlantic and Pacific oceans meet in the Drake Passage, which is a 528-mile-wide (850-kilometer-wide) bottleneck of ocean between South America and Antarctica. It's a turbulent little spot, feared by mariners since it was first discovered in the 1500s.
It makes a lot of sense that a cartographer long ago looked at a map of the gigantic Atlantic and Pacific and decided the Drake Passage would be the gateway from one to the other. Drawing a little line on a map between Cape Horn at the southernmost tip of Chile, and the Antarctic Peninsula was just the simplest way to do it.
But the boundaries of oceans are pretty arbitrary, just like neighborhoods in your town. There's nothing really different about the waters to the east and west of the longitudinal line drawn by that guy looking at a map somewhere in Europe hundreds of years ago. But there are lines out there in the ocean — they're just not the kinds cartographers find very useful.
"We have to think about two things when we're considering this question: what's happening at the surface where people are seeing these lines of different color or turbidity, and what's happening under the surface?" says Sally Warner, a professor and physical oceanographer at Brandeis University.
Although these videos probably aren't doctored, it's unclear where they were filmed. Of course it's possible they were taken in the Drake Passage, but they could also be showing something happening in a completely different part of the world.
Ocean fronts are masses of surface water that have different temperatures or salinity. Fronts out in the open ocean can be extremely sharp, and they can sometimes come together in a way that looks like two flavors of ice cream sitting next to each other in the carton.
For us landlubbers, the easiest place to see this is where two rivers flow together, or better yet, where a river flows into the ocean. River water is often very silty by the time it makes it to the ocean, giving it a chocolate milk look, which contrasts sharply with the dark water of the ocean it feeds into. Not only that, river water is fresh and ocean water is salty, giving them different densities. If you're crossing over a bridge or out in a boat, it might seem as if the river water remains separate from the ambient water of the ocean. They are definitely going to mix eventually — it might just take a day or two to blend completely.
There are places all over the world where fronts of waters come together creating visible lines in the surface water. As with the fresh waters from a river meeting and visibly tangling with the salty ocean waters, fronts of different temperatures can create clear delineations in the open ocean. For instance, at the equator, you find tropical instability waves, where colder waters from the north and south meet the bathwater of the equator and create visible delineations in the water.
The waters of the Atlantic and Pacific definitely do mix, and according to Warner, they may mix more than waters in most places in the world's oceans.
The Antarctic Circumpolar Current is a band of water that travels all the way around the globe, hugging pretty close to Antarctica. It's got a pretty clear shot on its entire journey, with the exception of the tight spot it has to squeeze through at Drake Passage. This makes the waters of this particular spot in the ocean very turbulent.
But if there's a visible line in the water anywhere in the area, it most likely has to do with waters of two different temperatures coming together:
"The water around Antarctica is colder than the water to the north. Probaby what people think is delineating the Atlantic water from the Pacific water is more likely a front that's delineating the colder water from Antarctica from the warmer water in the north."