Wind and boundary driven planetary geostrophic circulation in a polar basin

PhD ThesisThe 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

Topographical control of the source‐sink and wind stress‐driven planetary geostrophic circulation in a polar basin

The effects of topography on the barotropic circulation in a polar basin are examined analytically and numerically. New approximate linear analytical solutions are presented for steady‐state wind and boundary forced barotropic planetary geostrophic circulation in a circular polar basin with a step shelf. The solutions are obtained by retaining the full spherical geometry in the derivation of the forced potential vorticity equation; thereafter the colatitude is fixed in the coefficients of this governing equation. The accuracy of the analytical solutions is evaluated by comparing them with the equivalent numerical solutions obtained using the NEMO modeling system. Subsequently, the impact of a nonuniform width shelf on source‐sink‐driven circulation is investigated numerically. The equipartition of fluid entering the source strait into cyclonic and anticyclonic shelf currents, exiting the basin at the sink strait, in a basin with a uniform width shelf is shown to be modified when the shelf width varies. In general, the wider shelf supports a current with larger transport, irrespective of the azimuthal extent of the wider shelf. The study concludes with a numerical investigation of wind‐driven circulation in a basin with a step shelf, three straits, and a transpolar ridge, a prototype Arctic Ocean simulation. Topographic steering by the ridge supports a transpolar drift current, the magnitude of which depends on the ridge height. Without the ridge, the transpolar drift current is absent and the circulation is confined to gyres on the shelf and in the deep basin

Interconnectivity between volume transports through Arctic straits

Arctic heat and freshwater budgets are highly sensitive to volume transports through the Arctic‐Subarctic straits. Here we study the interconnectivity of volume transports through Arctic straits in three models; two coupled global climate models, one with a third‐degree horizontal ocean resolution (HiGEM1.1) and one with a twelfth‐degree horizontal ocean resolution (HadGEM3), and one ocean‐only model with an idealized polar basin (tenth‐degree horizontal resolution). The two global climate models indicate that there is a strong anti‐correlation between the Bering Strait throughflow and the transport through the Nordic Seas, a second strong anti‐correlation between the transport through the Canadian Artic Archipelago (CAA) and the Nordic Seas transport, and a third strong anti‐correlation is found between the Fram Strait and the Barents Sea throughflows. We find that part of the strait correlations is due to the strait transports being coincidentally driven by large‐scale atmospheric forcing patterns. However, there is also a role for fast wave adjustments of some straits flows to perturbations in other straits since atmospheric forcing of individual strait flows alone cannot lead to near mass balance fortuitously every year. Idealized experiments with an ocean model (NEMO3.6) that investigate such causal strait relations suggest that perturbations in the Bering Strait are compensated preferentially in the Fram Strait due to the narrowness of the western Arctic shelf and the deeper depth of the Fram Strait