Ocean internal waves and turbulence

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Turbulent mixing plays an important role in the oceanic circulation energy balance. Energy is input at large scales from tides and surface wind stresses, and provides the energy required to bring deep, dense water back towards the surface via mixing. Mixing can occur by a variety of mechanisms. Small-scale mixing driven by breaking internal waves is thought to make a first-order source, especially away from boundaries. Near-boundaries, processes such as shear instabilities associated with turbulent plumes or gravity currents cause significant mixing. This project seeks to improve knowledge of the physics of mixing and thereby enable better parameterisations for the mixing rates in computational models.

The turbulent energy cascade involves such a large range of scales it is impossible with current technology to measure or model numerically the full spectrum of processes in a single experiment. The present project thus uses a variety of methods to investigate different aspects of internal waves, turbulence and mixing. These include:

Ultra-high resolution wave-resolving numerical modelling

The energetics of internal waves are complex. Internal waves can gain energy from other ocean flows and be amplified, but they can also lose energy back to those same flows. Thus, energy going into the wave field is not necessarily contributing to mixing. The below image shows the wave field divergence (pink colours) and boundary flow speeds (rainbow colours) from a wave-resolving simulation of an idealised sector of the Southern Ocean. We use the simulations to develop a closed energy budget for the sources and sinks of wave energy. 

Numerical simulation

The second image below shows the hourly time evolution of the wave field in a transect across the model. In this case the only wave generation is occuring at the ocean surface. Internal waves are the banded structures emanating down from the upper boundary. 

Laboratory experiments

Gravity currents occur when there exists a horizontal gradient in fluid density. The denser fluid will tend to flow under the lighter one, generating turbulence along the interface. The below image from the GFD Lab shows a gravity current flowing off a shelf, and generating intense turbulence and mixing.

Gravity current

Lee waves are generated when fluid is forced to flow vertically into an ambient stratification, usually due to the presence of topography. The waves tend to stagnate in the flow just downstream of the topography --- hence the name "lee" waves. In the this video from the GFD Lab, waves are being generated by an obstacle (black "bump") being run over the surface of the tank. The waves break when their amplitude becomes large enough. This breaking produces a large amount of mixing.

Student Projects

Mean-to-wave energy exchanges in the ocean

Supervisor: Dr. Callum J. Shakespeare

The ocean is a sea of internal gravity waves. Similar to the gravity waves that propagate over the ocean surface and break along our coastlines, internal waves propagate great distances through the ocean interior. These waves are generated at the ocean surface and the seafloor by a variety of mechanisms. As the waves propagate, they interact with the ocean currents, jets and eddies. These interactions often involve an exchange of energy either to the internal waves (amplifying them) or away from the waves (weakening them). The location and direction of these energy exchanges tell us about the pathways by which energy moves through the ocean system from the very largest scales where it is injected by tides and winds, to the very smallest scales where it is dissipated as heat.

A number of projects involving the theory and/or modelling of internal waves are available for students at different levels (e.g. undergraduate projects, Hons, MSc, PhD) with an interest in applied mathematics/physics/climate science/oceanography.

Helpful skills: strong mathematics/physics background (essential), experience with scientific analysis software like Matlab/python (useful but not essential), experience with unix and coding (useful but not essential).