Diabatic Eddies in Idealised Channel Models

Abstract

The Southern Ocean is a unique and climatically important region where mesoscale eddies are of first order importance. The circulation of the Southern Ocean controls the amount of carbon stored in the ocean and is an integral part of the worlds global overturning circulation. It is therefore of great importance to understand the dynamics of the Southern Ocean and the possible response of the overturning circulation to changing forcing i.e. climate change. Because of the role eddies play in setting that circulation, the dynamics are still not fully understood.
This thesis explores one poorly understood aspect of the Southern Ocean; dia- batic eddies. Using the Massachusetts Institute of Technology General Circulation Model (MITgcm) to set up an idealised channel we close the northern boundary which causes the Southern Ocean overturning to collapse. This is to be expected as the circulation is part of the global overturning and must be connected to the rest of the world’s ocean. We use Transformed Eulerian Mean (TEM) theory to suggest this collapse is achieved via diabatic eddies altering the effective surface buoyancy forcing. We then alter the northern boundary condition to show that diabatic eddies can be related to the northern boundary stratification. However the response of the diabatic eddies is also dependent on the surface forcing, partic- ularly the sense of the forcing: changing from fixed-fluxes to surface restoring can have dramatic effects. Surface restoring alters the dynamics of the mixed layer, eddy compensation and saturation. This suggests that when exploring Southern Ocean dynamics in regional models, the effect of the imposed boundary conditions must always be considered. Our results suggest if the background stratification is altered we may see a very different circulation in the Southern Ocean, but to what extent would also depend on the response of the atmosphere and hence the surface forcing.

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Identifiers

ORCID for Sybren Drijfhout: orcid.org/0000-0001-5325-7350

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