Rapid changes in frontal position in a simplified model of the Southern Ocean

The General Circulation of the Southern Ocean is known to consist of a series of strong, narrow, and generally eastward jets (often known as “fronts”). These jets are transient, undergoing splitting, merging and large latitudinal shifts in position. They do, however show a particular, quasi-stable feature: jets are “pinned” to particular subsurface topographic features. For example, a particular jet may deviate from a purely zonal path to detour around a subsurface plateau. This meander will not show the time dependant behaviour evident in regions where the flow is not strongly affected by topography.

However, recent observations of the Southern Ocean obtained from satellite altimetry and fixed moorings have indicated that jets will suddenly change “preference” of topographic feature. A jet that previously skirted a plateau to the north will, almost instantaneously, shift to skirt the plateau to the south and obtain a new quasi-steady state. Shifts in the latitudinal positions of jets of more than 10 degrees have been observed. As jets are associated with sea-surface temperature fronts, shifts of this magnitude can result in large changes in local climate. The dynamics of this phenomenon are unclear.

We will present a theoretical investigation of this phenomenon, utilising a simplified numerical channel-ocean model with idealised topography. The model itself is a high resolution, quasi-geostrophic model forced with an idealised surface stress. While the model employs simplified dynamics, it does allow for the non-linear effects of meso-scale eddies to be included.

Topography consists of a smooth meridional ridge with a variable number of “canyons”; zonally oriented gaps in the ridge, which act to organise the jets. Under particular conditions, jets are shown to shift their position. We will show a detailed exploration of the parameter space, including the geometry of the topography, the shape and magnitude of applied surface stress.

We will also present a series of forcing perturbation experiments, in which the location of and magnitude of peak surface stress is perturbed. Such changes in surface stress are crude approximations of the dominant modes of mid-latitude southern-hemisphere atmospheric variability that together make up the Southern Annual Mode. The results from these experiments will be discussed in terms of the predicted changes in the Southern Annular mode and their likely impacts on the Southern Ocean's circulation.