Ocean internal waves and turbulence

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. 

Numerical simulation

 

Direct numerical simulations of waves and turbulence

Tidal flows push water back and forth over the ocean bottom. The vertical deflection of water parcels over sloping topography generates internal waves as shown in the image below. The frequency spectrum of the waves is concentrated around the tidal frequency (12.4 hours, for the dominant M2 tide). Wave-wave interactions lead to higher frequency waves, which then break to cause turbulence and mixing.

Numerical simulation

 

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.

 

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