Decoding the dynamics of the Antarctic Circumpolar Current

The ACC is one of the most important components of the global climate system. It forms the link between the major ocean basins, and a barrier to heat transport to Antarctica, as well as being one of the most important routes for exchange of CO2 between atmosphere and ocean. The aim of this project is to produce a simplified model of how the ACC works, which will make it possible to predict how its global influence will change in response to climate change.

The ACC is presently very poorly understood, to a large extent because it doesn’t have a western boundary as other ocean basins do. The extra freedom produced by this lack of constraint means that there is no simple model for the first order flow field. The current is highly nonlinear and is strongly steered by seafloor topography such as ridges and other features. This complicates the flow so that, although some idealized theories can be solved, none are realistic. Realistic models at adequate resolution stretch the capabilities of the biggest of supercomputers, making long climate model runs impractical and slowing progress in the understanding of processes. We need a simpler, but realistic model to guide investigations into how the ACC works.

One simple linear theory is capable of producing quite realistic ACC flows, as long as some things are specified in advance. Observations and models show that the current is close to “equivalent barotropic”, meaning that the deep flow is simply the same as the surface flow but with amplitude reduced by an assumed function of depth (Killworth, 1992; Sun and Watts, 2001; Killworth and Hughes, 2002). If we assume a fixed equivalent-barotropic structure, then a solution can be obtained which looks similar to the real ACC (Krupitsky et al., 1996; Killworth and Hughes, 2002; LaCasce and Isachsen, 2010), though it misses some important features (Figure 1). This lack means that the modelled response to changes in forcing is unrealistic.

The most important missing features are meanders with wavelength 300-500 km, which can also be understood in an equivalent-barotropic framework (Hughes, 2005), but require nonlinear physics (vorticity advection) to produce (Figure 2). These meanders modify the important interaction with topography, and may be responsible for the surprisingly small response of the ACC to changing wind stress (Thompson and Naveira Garabato, 2014). By extending the equivalent-barotropic model to include nonlinear terms, we should be able to produce a much more realistic model of the ACC, but one which can be used to explore its response to changes on climatological or perhaps geological timescales.

Project Summary:

The first stage of this project is to test a nonlinear, equivalent barotropic ocean model recently produced by the project supervisors. You will explore different parameter ranges and representations of eddies (necessary to prevent instabilities) to assess how realistic an ACC can be produced in this way. For comparison, we will use the latest satellite measurements of the real ACC.

Following this stage, the project can continue in any of a number of directions. Ideas concerning momentum transport by eddies could be used to add more detail to the nonlinear terms (Hughes and Ash, 2001). Further ideas concerning eddy buoyancy transports could be introduced to provide a balance which determines the vertical structure, making the shape of the equivalent-barotropic mode part of the solution and permitting the model to make predictions of heat, freshwater and carbon fluxes. Alternatively, the vertical structure could remain specified and be treated as a forcing function. The ACC response to changing winds and temperatures can then be explored on climate change timescales, or the influence of tectonic changes on geological timescales could be explored to see how the opening of Drake Passage and other constrictions led to the initial development of the ACC.

The ideal candidate will have a strong mathematical or physical sciences background and programming skills. Experience with numerical modelling or ocean/atmosphere dynamics will be an advantage, but ability and interest count for more than subject-specific experience and we would be happy to consider candidates shifting from quite different study backgrounds.

There will be close involvement from the British Antarctic Survey supervisor and, should CASE funding be confirmed, a period spent with the supervisor in Cambridge particularly focused on the numerical modelling aspects of the project.