Wind and boundary driven planetary geostrophic circulation in a polar basin

Abstract

The Arctic Ocean circulation is controlled by the interaction of many factors such as bathymetry, wind stress and volume transport across the straits connecting the basin to its marginal seas. In addition, stratification plays an important role in the 3–dimensional circulation, shielding the deep warm, salty water of Atlantic origin from the surface cold, relatively fresh layer. However, it is not clear how these factors interact together and how their relative contribution to the circulation will change as the Arctic warms. This thesis focuses on a subset of the factors determining the circulation of the Arctic. We confine our attention to homogeneous wind and boundary forced flows in a polar basin with a range of idealised topographies. New analytical solutions using a beta–sphere approximation first proposed by Imawaki and Takano (1974) are obtained for boundary and wind forced planetary geostrophic circulation. These solutions are compared with equivalent numerical solutions using the NEMO modelling system to evaluate the fidelity of the beta–sphere approximation. Then, numerical solutions are determined for planetary geostrophic flow in basins more representative of the Arctic, containing a transpolar ridge and variable width continental shelves. We found the role of shelf break currents connecting the straits is ubiquitous. A new dispersion relation for planetary waves is derived on the beta–sphere and compared with the equivalent dispersion relation on the polar plane (LeBlond, 1964). The thesis also examines numerical time dependent solutions of the unsteady circulation driven by harmonically perturbation transport varying in time across one (typically the Bering) of three straits. Vorticity waves then determine the evolution of the resulting sea surface height anomaly field. It is demonstrated that a non–uniform width shelf fundamentally controls the partition of the circulation between the Davis and Nordic Strait when the Bering Strait transport is perturbed. The final chapter of the thesis briefly sums up the most important results obtained in this study.