The Largest Waves in the Sea Aren’t at the Beach

I am very excited to introduce Kim Martini as guest blogger here at DSN.  Kim is a physical oceanographer working at the University of Alaska, Fairbanks and as the post reveals below is interested in deep-ocean waves.  You can find her on Twitter at @rejectedbanana.  Make sure to comment below and welcome her to DSN.

When asked what lies beneath the surface of the ocean, most people think of the various numbers of terrifying sea beasties that lurk there. But as a physical oceanographer, when I look at the ocean I think about completely different phenomena. I think about waves. And not just that puny Hawaiian surf that is often tossing professional surfers. These are the biggest waves in the ocean and they can’t be seen from shore. They exist inside the ocean. These waves are called internal waves.

If you are like most people (or even most marine scientists), you probably haven’t even heard of an internal wave. I didn’t even know they existed until I went to graduate school. In many ways they are similar to waves you see at the beach, they undulate, have crests and troughs, and even break! But since internal waves occur deep in the ocean rather than at the sea surface, they have some unique characteristics.

First, what is the difference between a surface wave and an internal wave? Both waves occur at the interface between two fluids of different densities, but these interfaces are different for the two waves. At the sea surface the interface is where air and water meet, two fluids with two different densities. Sea water becomes denser the deeper it is, and we can think of the ocean as being made up of an infinite number of tiny layers each having a different density. Between these infinite number of density layers are an infinite number of interfaces where internal waves occur.

Now that we understand that internal waves occur because density changes, why are they so cool? Here’s a short list:

1. They so are VAST you can see them from space. Here is one of my favorite pictures of an internal wave. It’s a satellite photo of the Strait of Gibraltar and you can see the internal wave surface signature as crests. But the height of the crests, tiny. Maybe only a couple of inches. We can only see them because of the way that sunlight reflects off it’s crests and troughs. This particular wave is caused by the tides forcing water to flow back and forth over the Gibraltar sill, emitting internal waves.

   

2. Surface waves are suckers because they can only propagate horizontally between the interface of water and air. But an infinite number of density layer in the ocean interior means internal waves can propagate vertically! They have been observed bouncing between the seafloor and the sea surface.
3. Did I mention internal waves are big? As they travel, they can move water below the surface up and down over 200 meters. That’s twice the height of the Statue of Liberty. Internal waves and the large vertical displacements they cause have even been accused of sinking nuclear subs. 
4. They travel 1000s of kilometers across ocean basins. Internal waves generated in Hawaii have been observed using satellites propagating all the way to Alaska and vice-versa

   

5. Internal waves don’t just occur in the ocean. They occur in the atmosphere (mountain waves cause your plane to bounce around when you fly over mountains) and even on the sun (helioseismology). See diagram here.