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Title: Thermocline mixing in a seasonally stratified shelf sea
Author: Lincoln, Ben
Awarding Body: Prifysgol Bangor University
Current Institution: Bangor University
Date of Award: 2012
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Vertical mixing at the thermocline determines transport pathways for heat,nutrients and carbon dioxide, which are crucial to ecosystems and to the Earths climate. Current models are unable to predict the correct levels of mixing in the thermocline region, falling short by orders of magnitude. This thesis examines two candidates for this anomalous internal mixing; inertial shear spikes driven by wind stress and internal waves generated by tidal currents interacting with underwater topography. 50 days of vertical current and temperature measurements were made in the Western Irish Sea in spring and early summer are complemented by a 48 hour time series of turbulent dissipation. Inertial currents generated by strong winds were observed in the surface mixed layer with magnitudes of over 0.3ms−1, while internal tidal currents of 0.1ms−1 were observed at spring tides. Analysis of the bulk shear vector revealed that the frequency of the shear rotation switched many times during the observations depending on wind and tidal forcing. When wind stress was high, inertial shear spikes were observed, with maximum shear production occurring when the bulk shear vector and wind vector aligned. Maximum shear resulted 3.75 hours later when the surface motion was at 90◦ to the right of the wind direction. Inertial shear spikes were the most energetic baroclinic process but were found not responsible for the anomalous thermocline mixing because their dynamics were well reproduced by a 1D turbulence closure model and because of to the way in which they modified the vertical temperature structure. High vertical resolution observations showed that the spikes driven by the wind generated sufficient shear to reduce the gradient Richardsonnumberbelowonequarterandsustainmixingatthebaseofthesurfacemixedlayer. Each spike generated instabilities which eroded the top of the thermocline, deepening the mixed layer in a series of steps until the buoyancy frequency increased sufficiently to suppress mixing.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available