Physical oceanography

Abstract:

Mesoscale ocean eddies have a profound influence on the large-scale oceanic flow, yet the effect of these eddies in driving rectified topography-following flows is often ignored in climate models. In this thesis, a new parameterization of barotropic eddy potential vorticity (PV) fluxes which is both energetically and enstrophetically constrained is developed for use in non-eddying models. The parameterization employs down-gradient PV mixing and incorporates a subgrid eddy kinetic energy and eddy potential enstrophy budget to constrain the parameterized PV mixing. The parameterization includes a mechanism through which kinetic energy can be exchanged bidirectionally between the resolved flow and the parameterized eddies, and a mechanism through which potential enstrophy can be converted from the resolved flow to the parameterized eddies.

The parameterization is tested in highly idealised simulations of barotropic freely decaying turbulence in a doubly periodic domain on an f-plane over variable topography. An emergent topography-following flow is produced by the parameterization. The constraints imposed in the parameterization are successful in constraining the resolved kinetic energy and potential enstrophy such that they are realistic when compared with an eddy-resolving model. The kinetic energy and potential enstrophy pathways associated with the parameterized and eddy-resolving models are quantified and compared. The parameterized transfer rates initially resemble those of the eddy-resolving simulation, but diverge from the eddy-resolving simulation when biharmonic diffusion dominates the flow.

The kinetic energy supercharger mechanism is introduced as a method of compensating for the erroneous kinetic energy and potential enstrophy transfers when biharmonic diffusion dominates the flow. The parameterization with the additional kinetic energy supercharger mechanism is tested in the setup described above, and produces kinetic energy and potential enstrophy transfers which more closely resemble those of the eddy-resolving simulation.

A method of implementing the parameterization of barotropic eddy PV fluxes in a baroclinic setup is outlined and referred to as GM+PV. The Gent and McWilliams (1990) parameterization is employed alongside the barotropic parameterization, and the two are integrated by the APE-to-EKE conversion mechanism which mimics the process of barotropization. GM+PV is implemented in the Massachusetts Institute of Technology general circulation model.

GM+PV is tested in semi-realistic simulations of the Arctic Ocean. An emergent cyclonic flow around the Arctic Basin at depth, broadly resembling the Arctic Circumpolar Boundary Current and not present in simulations without GM+PV, is produced. The kinetic energy pathways associated with GM+PV simulations are quantified and utilised to explain the maintenance of the quasi-steady state as well as the functioning of GM+PV in a baroclinic setup.