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Title: Forcing of the Southern Ocean meridional overturning circulation by internal wave breaking
Author: Broadbridge, Maria Barbara
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2011
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The Southern Ocean is governed by strong wind forcing, energetic eddies and probably intense internal wave fields, which are considered to be generated in part by interaction of the eddy field with bottom topography. While wind and eddy forcing have been recognized in the dynamical balance of the Southern Ocean Meridional Overturning Circulation (MOC), the role of internal waves remains uncertain. The present study aims to investigate the extent to which the interaction of an energetic eddy field with realistic bottom topography, leading to diabatic forcing of the interior stratification through internal wave breaking, can sustain a deep overturning circulation and provide an additional leading order term in the Southern Ocean MOC that is unaccounted for by current theories and numerical models. A hierarchy of experiments to test this hypothesis is conducted in this thesis. The MIT General Circulation Model (MITgcm) is configured to simulate a circumpolar current in two individual versions of a zonal channel, namely a coarse-resolution setup with Gent-McWilliams eddy parameterisation and a mesoscale eddy-resolving setup. The energy input that arises from internal wave breaking and forces the buoyancy budget of the MOC is parameterised in the model via a vertical diffusivity profile. Initially forced to follow a simple constant shape, this diffusivity profile is subsequently estimated online via the internal lee wave energy generation rate, which is calculated from the model stratification, velocities and from prescribed sub-grid-scale topography, following nonlinear internal lee wave theory. Results from the individual experiments are discussed in view of the extent and mechanisms by which internal waves generated by eddy-topography interaction force the cross-channel MOC. The main results of this thesis show that the internal lee wave energy generation rate enhances the diapycnal diffusivity near the bottom, which results in a substantial increase in the strength of the residual MOC of the lower limb and a warming of the deep interior of the channel.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available