Jets, mixing, and topography in the Southern Ocean

Abstract

The Southern Ocean holds a unique place in our planet. It is home to the world’s longest and strongest ocean current, the Antarctic Circumpolar Current (or ACC), which is formed of jets (alternating velocity structures), thought to be significant surface transport barriers. The dynamical processes (particularly mixing processes) in the Southern Ocean are crucial to driving the global overturning circulation, which is in turn responsible for the global transport of heat, CO2, and nutrients. Despite the evident importance of the Southern Ocean to current and future climate, the important dynamical processes that occur there are poorly understood. This thesis attempts to contribute towards the understanding of some of the open questions in Southern Ocean dynamics. In particular, we investigate the effect that topography might have on the jets that form the ACC, with regards to their formation and in particular, their transport properties. Through a quasi-geostrophic model we investigate the properties of jets that form over a zonal slope in bottom topography, and find that the jets become tilted, aligning perpendicular to the large-scale barotropic potential vorticity gradient. As the jets tilt more, they become significantly more energetic, corresponding with an increase in across-jet transport. We compare various theories regarding the formation of such jets, involving linear analysis of the system. It is found that the analytical form of the Rossby wave frequencies correctly predicts the anisotropy of the energy spectra of simulations, and so the jet direction. Additionally, there is a need to characterise accurately the isopycnal mixing occurring throughout the Southern Ocean. We utilise satellite measurements to estimate isopycnal diffusivities in the Southern Ocean in two different studies. Using an effective diffusivity diagnostic to extend a previous study, we find reduced surface horizontal mixing at the latitudes of the ACC core. By comparing a tracer advection simulation with measurements from an experiment in the Southern Ocean, we find that simulations with a vertically averaged horizontal diffusivity of 20m2s−1 best match observations in the Pacific sector of the ACC.