We’re all familiar with waves when we go to the coast and see them wash onto the beach. But since ocean waters are usually stratified by density, with warmer fresher waters on top of colder, saltier ones, waves can occur between water layers of different densities at depths up to hundreds of meters. These are called internal waves. They often have frequencies that are synched with the tides and can be pretty big–up to 200 meters in amplitude! Because of their immense size, these waves help transfer heat and nutrients from deep waters, meaning they have an impact on ocean current circulation and the growth of phytoplankton.
The line of foam on the surface of the ocean indicates the presence of an internal wave.
We still don’t understand a lot about how these waves work. Jenny Thomas is a PhD student working with Jim Lerczak in Physical Oceanography in CEOAS (OSU’s College of Earth, Ocean, and Atmospheric Sciences). Jenny studies the behavior of internal waves whose frequencies correspond with the tides (called internal tides) in ocean basins. This requires a bit of mathematical theory about how waves work, and some modeling of the dimensions of the basin and how it could affect the height of tides onshore.
Picture a bathtub with water in it. Say you push it back and forth at a certain rate until all the water sloshes up on one side while the water is low on the other side. In physics terms, you have pushed the water in the bathtub at one of its resonant frequencies to make all of it behave as a single wave. This is called being in a normal mode of motion. Jenny’s work on the normal modes of ocean basins suggests that the length-to-width ratio and the bathymetry of an ocean basin influence the structure of internal tides along the coast. Basically, if the tidal forcing and the shape of the basin coincide just right, they can excite a normal mode. The internal wave can then act like water in a bathtub sloshing up the side, pushing up on the lower-density water above it.
It turns out that water isn’t the only thing that can have normal modes. The air column in a wind instrument is another example. Jenny grew up a child of two musicians and earned a degree in trumpet performance from the University of Iowa, and she occasionally uses her trumpet to demonstrate the concept of normal modes. She can change pitches by buzzing her lips at different resonant frequencies of the trumpet–the pitch is not just controlled by the valves.
Jenny uses her trumpet to explain normal modes.
Near the end of her undergraduate degree at the University of Iowa, Jenny discovered that she had a condition called fibrous dysplasia that could potentially cause her mouth to become paralyzed. Deciding a career as a musician would be too risky, and realizing her aptitude for math and physics, she went back to school and earned a second undergraduate degree in physical oceanography at Old Dominion University. After a summer internship at Woods Hole Oceanographic Institution conducting fieldwork for the US Geological Survey, she decided to pursue a graduate degree at OSU to further examine the behavior of internal waves.
Tune in to 88.7 KBVR Corvallis to hear more about Jenny’s research and background (with a trumpet demo!) or stream the show live right here.
You can also download Jenny’s iTunes Podcast Episode!
Jenny helps prepare an instrument that will be lowered into the water to determine the density of ocean layers.
Jenny isn’t fishing. The instrument she is deploying is called a CTD for Conductivity, Temperature, and Depth–the three things it measures when in the water.