Your weekly dose of ocean science + comics!
What lives underwater, is hundreds of meters tall, and can swipe a submarine out of its way like it’s a fly? An internal wave, that’s what! They don’t all get to be so big, but once a wave is released from the shackles of the air-sea interface, who knows what crazy things it might get up to?
Oh wait – Andy Pickering knows! He and his lab group study internal waves. Internal waves are kind of like surface waves – they need that density gradient to even exist. So in order to figure out how they propagate under water, Andy needs to know how the density varies underwater. There is often a surface mixed layer where the density is constant for tens or hundreds of meters, and after that it tends to increase with depth.
Although density is a continuous function of depth, the density gradient can be modeled as a series of very thin slices of constant density. And once you do that, internal waves are no longer confined to the interface; they can propagate vertically as well. Internal waves can travel very far, and just like waves crashing on the beach, internal waves can break as they approach the seafloor.
One of the very important implications of internal waves breaking is that they can cause turbulence, which has a surprisingly significant effect on ocean circulation models. Because this kind of turbulence happens on such a small scale relative to a global ocean circulation model, it’s really tough to incorporate it properly. Especially since it is not fully understood yet.
Andy and his lab group are participating in a large project called IWISE (Internal Waves in Straits Experiment). The goal of the project is to better understand the behavior of internal waves in a strait environment by taking tons of measurements in one particular strait: The Luzon Strait.
If you were ever looking for an awesome spot to find out how tides interact with shallow ridges to produce gnarly internal waves, look no further, my friend. As the tides slosh back and forth over the two ridges that run between Taiwan and the Philippine island of Luzon, enormous internal waves are generated that roll west into the South China Sea and east into the Pacific Ocean. These waves are so massive that the perturbations that they cause at the sea surface can be seen from space!
In addition to all the craziness from tides barreling over the shallow ridge, there’s also the Kuroshio current to deal with. The Kuroshio is sort of like the Gulf Stream, except in the Pacific Ocean instead. It pushes warm, tropical water Northward along the western edge of the Pacific Ocean basin. Like other ocean currents, it moves around a bit over time, and sometimes it dips further into the Luzon Strait than others.
Data for the IWISE experiment were collected in 2010 and 2011, and both moorings and ship-based instruments were used. The two most important instruments for Andy’s research are the CTD (conductivity-temperature-depth sensor) and the ADCP (acoustic Doppler current profiler). The CTD measures the density of the seawater at different depths, and the ADCP measures current speed and direction at different depths. With these key bits of data, Andy can look at how the ridges affect the generation of internal waves, and even at how this Kuroshio current adds to the mix.
Andy has been looking at the data and in particular at the energy flux generated by the tides colliding with the shallow ridges. He’s seeing that the ridges are actually creating interference effects because they’re so close together. Some of that energy is dissipated away locally, but Andy is also interested in looking at how much might be escaping, and whether it could make a significant contribution to deep ocean mixing.
The amount of deep ocean mixing is critically important for constraining global ocean circulation models and also global climate models so we can better understand things like climate change and global warming. So in a way, when Andy is not wasting time doing interviews with me, he’s busy saving the planet! Awesome.
If you want to know more about Andy’s lab group, the Wave Chasers, check out this pilot put together by Wide Eye Productions: http://vimeo.com/wideeye/wavechasers. It’s really cool, and well-worth a couple of minutes of your time.