Interactions of jets and eddies with topography in the Southern Ocean

Abstract

The Southern Ocean, which lies south of approximately 35 degrees S and completely encircles the Antarctic continent, is considered to be a unique and important component of the Earth's climate system. The Southern Ocean is home to the world's strongest current system, the Antarctic Circumpolar Current (ACC), which connects the Earth's major ocean basins, mediates the southward transfer of heat, and strongly influences climate on both short and millennial time scales. However, due primarily to lack of observations and insufficient computing power, the dynamics of the Antarctic Circumpolar Current are poorly understood. Modern ocean observing technology and advanced numerical modelling have revealed that the ACC is composed of numerous fine-scale features: strong, narrow currents called "jets" and ring shaped, turbulent features known as "eddies". Although it is widely acknowledged that the nature of the Southern Ocean's flow field has a dramatic influence on the dynamics of the ACC and its interaction with the global climate system, exactly how and why are still questions that perplex oceanographers. The work conducted in this thesis investigates how the fine scale nature of the Southern Ocean flow affects the system as a whole. A particular focus is the interaction of these small scale features with bathymetry. Using a combination of in-situ observations, data collected from satellites and the output of high-resolution numerical models, a newly discovered mode of low-frequency variability, dubbed {u0300}{u0300}jet--jumping", whereby two jets that pass close to each other near a particular topographic feature show strongly anti-correlated behaviour: one jet strengthening while the other weakens. A dynamical explanation of this behaviour is proposed: variability in the jets arises due to their interaction with a vortex generated through eddy-topography interaction. The predictions of this framework are tested in an idealised numerical model of the Southern Ocean as well as several case studies conducted using sea-surface height altimetry. By adapting methods from signal processing, improvements to an existing method for the detection of jets in the Southern Ocean are proposed. This new method is compared with several existing methods for jet detection, both quantitatively and qualitatively. The new method is found to have superior error performance when compared to with several existing methods. Qualitative comparisons are undertaken using output from an eddy-resolving numerical simulation. It is found that in a global sense, all methods compared are in broad agreement in the placement of jets. However, it is shown that the nature of detected jet field differs depending on the method used. In particular, the new method reveals a finer-scale, more complex jet field than popular contour methods. Finally, the dynamics of {u0300}{u0300}storm tracks", regions of anomalously high eddy kinetic energy, are explored using a primitive equation model with idealised bottom topography. Using the output of this model, a dynamical mechanism for the formation and downstream development of storm tracks is developed. In this mechanism, it is large scale, topographically forced stationary meanders that initiate the storm track, while eddy-eddy interactions cause the storm track to develop downstream.