Coherent structures in baroclinic turbulence: jets, eddies and their influence on ocean circulation

The discovery of basin-scale jets in observations and numerical models of oceanic flows, satellite images of remarkably steady zonal bands of east-west flow on Jupiter's atmosphere and high resolution numerical models depicting highly structured turbulent geophysical flows have all contributed to a resurgence in the study of jets in the atmopshere and ocean. Because of the Earth's curvature, the effects of the Earth's rotation on fluid motion changes with latitude. Thus large-scale environmental flows tend to align in east-west, or zonal, directions in structures referred to as jets. Recent studies have made progress in understanding the mechanisms behind jet formation and jet maintenance, however, less attention has been paid to how these jets influence larger-scale circulation patterns. The goal of the proposed research is to better understand how oceanic jets, with their associated vertical and latitudinal structure, interact with flow patterns at the ocean basin scale and within the Antarctic Circumpolar Current (ACC) of the Southern Ocean. Jets are typically described as barriers to transport that limit the exchange of heat, chemicals and plankton across the core of the jet. In reality, though, jets are complex features that exhibit both horizontal and vertical variability. The effectiveness of jets as barriers to transport is found to vary with depth, often with sharp transitions. The dynamics that determine these transition depths are not well understood. Ocean circulation patterns also tend to exhibit vertical structure. For example, in the ACC, which is a region with a number of jets that encircle Antarctica, meridional, or north/south, flow is driven equatorward by winds near the surface, while at depth, fluid is carried poleward by ocean eddies. The most recent descriptions of the ACC's meridional circulation do not include jet features and are therefore incomplete. With the help of a three-dimensional model that produces a series of jets that are similar to those found in the ACC, an important objective is to understand how and at what depth heat is transported across the jets in order to maintain the ACC's strong flow. Ocean eddies are closely linked with jets. Eddies tend to form at jet cores by extracting energy from the jet, and eddies can track along jets for long distances. Eddies have also been shown to enhance the transport of heat, chemicals and biology in the ocean, by trapping these properties within the cores and carrying them along as they move. Thus, the role of eddies is crucial for understanding how heat and other tracers move across jets. Unfortunately, traditional methods of analyzing turbulent jets in the atmosphere and ocean have relied on statistical techniques that are unable to capture the behaviour of individual eddies accurately. Analysis of a three-dimensional model that is able to resolve eddy features within the jets will provide new and important insight into how local fluxes of heat and tracers influence the larger-scale transport of these properties in ocean basins. Ultimately the goal of this research is to improve our understanding of realistic oceanic flows. One objective of the research is to determine how more complicated flows changes the vertical and horizontal jet structure as compared to jets in idealised flows. One modification is to include the effects of steep topography, which is of particular relevance to flows in the ACC, where topographical features have the ability to steer jets across lines of longitude. Finally, through contact with the oceanography community, information gained from this research on the specific role of jet structures on transport properties will be used to improve realistic ocean models and ocean observation programmes.