An introduction to internal waves for scientists and future-scientist alike
What are internal waves?
Internal waves are similar to surface waves (the kind you see and surf at the beach). The biggest difference is that surface waves exist at the boundary between water and air while internal waves exist at a boundary between two layers of water that differ in density. Think of oil and vinegar in the same container: if you let the container sit for a while the oil settles on the bottom in a state of equilibrium (as oil is the denser of the two liquids). You can clearly see the boundary layer between them. Disturbances in this layer (e.g. a drop of oil pushed above the boundary) result in restoring forces trying to bring things back to equilibrium. The dissipation of these forces result in waves that propagate along the boundary between the two liquids. These disturbance-generated waves are just like the waves you see moving across a pond after you throw a rock in and disturb the pond surface layer. In the ocean heavy, cold, and dense water plays the role of the oil in our above thought experiment, and lighter, warmer, less dense surface waters play the role of the vinegar. The boundary between the two water masses in the ocean is called the pycnocline.
Most frequently internal waves are tidally generated, but they can also be created by underwater earthquakes, significant freshwater inputs flowing into the ocean, the mixing of the water-column by storms, or flow of water over certain bathymetric features.
Why are internal waves important?
Internal waves provide energy to ocean ecosystems. They induce mixing, and transport nutrients from depth to the surface where they can be utilized by photosynthetic plankton. Internal waves have also been shown to advect (micro)organisms through the water.
What are we studying them for?
The biota of the ocean are not evenly distributed. In fact there exists a patchiness that is documented throughout the literature which scientists have been trying to understand for quite some time. Areas of high biological activity are often called "hotspots". At a hotspot you might find 40 Eubalaelena glacialis (right whale) feeding on a dense patch of Calinus finmarchicus, or hundreds of seabirds following a pair of Megoptera novaeangliae (humpback whale) diving with a pod of Lagenorhynchus acutus (Atlantic white-sided dolphin) for krill or Ammodytes americanus (sandlance).
One of the goals of the EMLab is to understand this patchiness. To do this we're looking at internal waves and how they advect euphausiids (krill) as a potential driving mechanism in determining the patchy distribution of hotspots.