Unraveling the Weddell Gyre: exploring its dynamics and influence on Southern Ocean ventilation

Thesis

The Southern Ocean is an area of great importance when considering anthropogenic climate change but also an area of great uncertainty. Between climate projections, the Southern Ocean is where one finds the most disagreement in both heat and carbon uptake. The Weddell Gyre is the largest coherent circulation south of the Antarctic Circumpolar Current (ACC) and mediates the transport of water to and from the Antarctic shelf. Here we use a diverse range of methods to study the dynamics of the Weddell Gyre and its role in Southern Ocean ventilation.

An idealized model of the Weddell Gyre and the ACC is used to investigate the gyre’s sensitivity to mesoscale eddies. The gyre is particularly strong at eddy-permitting resolutions (45 Sv with a noisy bathymetry) and weakest at eddy-parameterized resolutions (12 Sv). Simulations with partially resolved eddies have the largest horizontal density gradients. The steep isopycnals intensify the gyre's thermal wind transport and fuel tall columns of eddy kinetic energy which excite the gyre circulation at the sea floor.

The vorticity budget of a realistic Weddell Gyre simulation is calculated to identify important forces for the circulation. The gyre’s vorticity budget is dominated by a spurious component of the Coriolis force which emerges from the C-grid geometry. We identify two topographic and two non-topographic components of the spurious force. The topographic components can be mitigated by using a terrain-following coordinate and all components vanish when the B-grid is adopted.

Backwards-in-time trajectories are used to identify spatial and temporal patterns of Southern Ocean ventilation on a thirty-year timescale. Practically all ventilation occurs between August and November. Sites of deep convection are responsible for 60% of open ocean ventilation. Almost no ventilation occurs within the subpolar gyres but their western boundary currents are the primary export pathway for shelf-ventilated water.

Authors: Styles, A

• DOI: 10.5287/ora-0zd97vrzg
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: climate change; Weddell Gyre; oceanography; subpolary gyre; model discretization; ocean circulation; southern ocean; gyre; vorticity; ocean modelling; eddies; ocean ventilation; shelf processes; Weddell Sea
• Subjects: Physical oceanography; Physics