Internal waves within Whittard Canyon: searching with a seaglider

Tahmeena Aslam is a PhD student at the University of East Anglia studying internal waves within submarine canyons using numerical models

The main goal of our cruise to Whittard Canyon is to map habitats, test models that predict where they occur, and see what changes have occurred since previous cruises. As a physical oceanographer, however, I’ll also be interested in looking for something else that lives beneath the surface: internal waves.

What are internal waves?

Internal waves are just like the waves that you see at the seaside, except that rather than occurring between the air-sea interface, they occur at the interface between two fluids of different densities. The sea is made up of many layers of different sea water densities, so there are plenty of opportunities for internal waves to occur. All that the layering needs is something to disturb it, in the Earth’s case, the moon is a periodic source of disturbance. The tide drags the layered sea water over features on the seafloor, such as seamounts and submarine canyons, and this sets up an oscillation, i.e., an internal wave is born! Internal waves are big (100s of metres), you can see them from space and they occur all around the globe (see video below).

So what have internal waves got to do with Whittard Canyon?

When internal waves reach features on the seafloor, such as a submarine canyons, they can break and cause lots of turbulent mixing, or they can be reflected and be focused by the steep walls of the canyon. This leads to enhanced currents, which may cause nutrient-rich organic matter to be re-suspended and transported throughout the canyon.  This has potential implications for habitats within the canyon, for example, high current speeds may keep corals from being buried by sediment and transport nutrients towards them, but if too large, they may actually damage the coral. By understanding internal wave behaviour within the canyon, we can start to include such information within predictive habitat models, and hopefully improve their predictive ability.

How do you look for internal waves?

On this cruise, we will be using a seaglider from the University of East Anglia to look for disturbances in temperature and salinity within the canyon, which mark the passage of internal waves. Seagliders are underwater robots that can dive up and down and take measurements. One of their benefits is that they are small and manoeuvrable, handy when we will be trying to navigate one through steep canyon walls. But first, we need to prepare the glider and get it in the water!