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Alex Dickinson

Alex Dickinson

PhD student

Physical Oceanography and Reflection Seismology

Bullard Laboratories,
Madingley Rise,
Madingley Road,
Cambridge CB3 0EZ,
UK


Office Phone: +44 (0) 1223 33400

Biography:

2010 - 2014: MSci Natural Sciences (specialising in Physics), University of Cambridge

2014 - now: PhD, "Four Dimensional Imaging of Thermohaline Structure in the Water Column", University of Cambridge

Research Interests

I use marine seismic reflection profiling to image thermohaline structure within the oceans. Acoustic energy injected into the water column by air guns located at the sea surface is reflected from contrasts in temperature and, to a lesser extent, salinity at depth. This reflected energy is recorded at the sea surface by multiple hydrophones. Careful processing of recorded data yields images that approximate maps of the smoothed vertical gradient of temperature, with resolvable temperature contrasts as small as 0.03 degrees C. Critically, such images can achieve spatial resolutions on the order of 10 m in both the horizontal and the vertical, providing observations of oceanic fine structure that are unprecedented in their detail. Over the past decade, physical oceanographic phenomena such as internal waves, fronts and eddies have been spectacularly imaged in different parts of the world. These images have been exploited to create maps of temperature and salinity, to measure geostrophic velocities and to measure the speed of propagation of internal waves.

My research is divided into two main areas. Firstly, I have explored the potential of seismic images to study turbulent mixing. Within the oceans, internal waves transfer energy from large-scale flow to small length scales, at which turbulence irreversibly dissipates kinetic energy. Turbulence drives small-scale overturning, which mixes properties across density gradients. This mechanical mixing is a significant component of global thermohaline circulation and accurate measurements of its intensity are essential for a first-order understanding of global dynamics. However, observed levels of mixing are much lower than predicted levels, and it is thought that mixing must be concentrated above 'hotspots' of rough bathymetry such as mid-ocean ridges and continental margins. The spectral analysis of seismically imaged fine structure represents a novel method of measuring turbulent mixing in such environments. Previously, researchers have identified the spectral signature of turbulence in seismic images and have related this signature to the turbulent dissipation of kinetic energy. I have adapted this technique by exploiting the spectral subrange corresponding to the oceanic internal wave field, which occurs at longer wavelengths and is hence less affected by ambient noise. The energy of the observed internal wave field can be related to the turbulent dissipation rate using a parametrization based on numerical models and previous empirical studies. I have applied this technique to a seismic image that traverses the Sigsbee Escarpment in the northern Gulf of Mexico. Observed mixing rates are elevated above the shoaling bathymetry of the continental slope, and have a mean value that closely matches independent estimates. Crucially, the seismic image maps the spatial distribution of mixing in much greater detail than collocated conventional data. It is hoped that future analysis of seismic images will relate variations in mixing rate to the passage of mesoscale features such as fronts and eddies.

Secondly, I have used seismic images to map the evolution of mescoscale features in the Faroe-Shetland Channel over spatial scales of ~ 50 km and time scales of hours to months. The Faroe-Shetland Channel is a major conduit for northwards transport of heat and salt from the Atlantic Ocean to the Arctic Mediterranean. Northwards-flowing North Atlantic Water within the upper 400 - 750 m is separated from southwards-flowing Norwegian Sea Deep Water at greater depths by a prominent thermocline, which gives rise to bright seismic reflections. The depth and thickness of this thermocline is observed to vary in > 300 seismic images acquired in April - October of 1995 - 1999. I aim to understand this variation by integrating seismic images with high-resolution satellite images of sea surface temperature. Excitingly, each of these seismic images is three-dimensional, which raises the tantalising prospect of mapping the internal wave field in three dimensions and relating its evolution both to the development of mesoscale features such as eddies and to changes in rates of turbulent mixing.

Research Supervision

Supervisors: Professor Nicky White, Professor Colm-cille Caulfield.

Collaborators

Key Publications

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