Brüggemann, Nils (2013) Ageostrophic effects on large scale circulation, eddy mixing and dissipation in the ocean. (PhD/ Doctoral thesis), Universität Hamburg, Hamburg, Germany, 134 pp.

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Abstract

This thesis aims to provide a better understanding of the role of ageostrophic processes in ocean dynamics by analyzing three different case studies - the large-scale circulation, the mixing of eddies in the upper ocean and the ability of ageostrophic dynamics to feature a direct route to dissipation. Furthermore, it examines to which extent parameterizations can yield adequate simplifications of the more complex ageostrophic phenomena.

The first case study concerns zonally averaged models of the large-scale meridional overturning circulation. Ageostrophic processes need to be considered here to correctly describe the dynamics in western boundary currents, while the interior ocean can be described by a geostrophic balance. Both, interior geostrophic and ageostrophic dynamics in the western boundary current need to be considered for the zonally averaged flow. It is illustrated that many zonally averaged models which do not consider both regimes show dynamical inconsistencies in comparison with zonally resolved models. A new parameterization for the zonally averaged flow is developed, in which both dynamical regimes are directly represented and which does not suffer from those inconsistencies. Zonally resolved models show good agreement with the new zonally averaged model, demonstrating that the new parameterization is dynamically consistent.

The second case study deals with the mixing of eddies in the upper ocean. Since the stratification is often weak within the mixed layer, ageostrophic processes are likely to occur here. Two parameterizations for the eddy mixing are compared, which especially take ageostrophic dynamics into account. The first is based on linear stability analysis while the second is based on a scaling of the potential energy release. Numerical simulations for a wide range of dynamical conditions are used to diagnose the ability of these parameterizations to predict the mixing effect of the eddies. It turns out that the mean difference between both parameterizations and the diagnosed eddy fluxes is less than a factor of two. While the parameterization based on linear stability analysis performs slightly better in an equilibrated forced-dissipative flow scenario, the parameterization based on the scaling of the potential energy release performs better in a scenario of a re-stratifying density front. In addition it is found that the vertical structure of the eddy fluxes is better described by the former in both scenarios.

The third case study investigates the role of ageostrophic dynamics for kinetic energy dissipation. Numerical simulations for a wide range of different dynamical conditions characterized by their Richardson number are used to diagnose the energy flux in wavenumber space. It is found that quasi-geostrophic dynamics feature an upscale kinetic energy flux while kinetic energy is transferred towards smaller scales for ageostrophic dynamics. Horizontal divergent velocities evolving under ageostrophic conditions can be identified to be responsible for the downscale flux. An important consequence is that the small-scale dissipation is larger in the presence of ageostrophic dynamics. To quantify the effect of ageostrophic dynamics on the small-scale dissipation, its dependency on the Richardson number is investigated and a power law relating the energy dissipation with the Richardson number is estimated.

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