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Title: Lagrangian study of the Southern Ocean circulation
Author: McAufield, Ewa Katarzyna
ISNI:       0000 0004 7661 0462
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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The Southern Ocean is an important region for the sequestration of heat, carbon dioxide and other tracers. The Southern Ocean circulation is typically described in a circumpolarly averaged sense as a Meridional Overturning Circulation (MOC), but the detailed 3-D pathways that make up this circulation remain poorly understood. We use Lagrangian particle trajectories, obtained from eddy permitting numerical models, to map out and quantify different aspects of the 3-D circulation. We first introduce various definitions used to quantify efficient export from the Antarctic Circumpolar Current (ACC) to the subtropical gyres. Using these definitions, we show that the permanent northward export varies by water mass and occurs in localised regions; with 11 key pathways identified. We then examine the dynamics setting the location and efficiency of the identified pathways, which includes the investigation of the role of diapycnal mixing and the impact of short and long time variability in the flow. Although we show that the flow of particles in the 3-D model is predominantly isopycnal, we find that particles that are forced to remain on isopycnals lead to approx. 60% lower export (mainly via three pathways) than identical releases where the diapycnal component of advection is included. Enhanced upward mixing near rough topography, and downward mixing in the southeast Pacific, were shown to be mostly responsible for the export. In addition, we show that most of the export pathways are mainly influenced by timescales from 90 days to 20 years, which suggests that mesoscale eddies are not the leading-order importance in the northward export from the ACC to the subtropical gyres. However, we also find that mesoscale eddies and the mean-ACC flow play a significant role in setting the export from the ACC in some pathways. These results highlight the role of temporal variability and vertical transport in enhancing the northward flow from the ACC by allowing transport across barotropic streamlines and onto more efficiently exporting isopycnals. In addition, the asymmetrical response of the studied quantities emphasises the importance of the three dimensions in understanding the dynamics driving the overturning circulation. We also demonstrated that the annually repeating velocity fields, which are commonly used for trajectory calculations, increase the diapycnal transport of particles and as a consequence, increase the overall 20-year northward export from the ACC by approx. 10%. In the study of the meridional overturning circulation, we diagnose the geographical distribution of the streamwise averaged diffusivity calculated from meridional displacements of the Lagrangian particles. We examine streamwise averaging using both latitude and equivalent latitude and argue that the latter gives a more useful measure. Reconciling tracer and particle horizontal diffusivities, we show that in the ACC, the average diffusivity peaks between 1500m and 2500m with an average value of 1500 m$^{2}$/s and that it is highest near the topographic features. We compare the exact diffusivity and its approximation to show that an assumption of time homogeneity does not hold and therefore that standard expressions for diffusivity that assume time homogeneity are of limited usefulness. Finally, we use the calculated trajectories to provide a streamwise averaged 2-D advection-diffusion model of the Southern Ocean MOC and then examine the extent to which this 2-D model can capture the overall effect of the actual 3-D transport.
Supervisor: Haynes, Peter Howard ; Meijers, Andrew Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Lagrangian particles ; three dimensional pathways ; Southern Ocean circulation ; Lagrangian diffusivity