Mechanisms of natural and forced variability in the southern ocean

• Main Author: Roncoroni, Sebastiano
• Format: Thesis
• Language: English
• Published: 2023
• Subjects: Antarctic; Southern Ocean; The Antarctic; Antarc*
• Online Access: https://centaur.reading.ac.uk/111427/; https://centaur.reading.ac.uk/111427/1/26022597_Roncoroni_thesis.pdf; https://centaur.reading.ac.uk/111427/2/26022597_Roncoroni_form.pdf

The Southern Ocean is an important regulator of global climate, and accurately predicting its future evolution under climate change constitutes a critical scientific challenge. Mesoscale eddies are key to the dynamics of the Southern Ocean, but the mechanisms and time scales of their natural and forced variability are not completely understood. Motivated by the dynamical analogy between the Antarctic Circumpolar Current and the tropospheric jet stream, the natural variability of eddymean flow interaction is studied by adapting a two-dimensional model of storm track variability to the oceanic case. It is found that eddies and the mean flow interact according to a predator-prey oscillatory relationship in both an idealised, eddy-resolving, channel configuration and the SOSE state estimate product of the Southern Ocean. The oscillatory nature of the dynamics reflects in the structure of the phase space diagrams, where quasi-periodic cycles with typical timescales of a few weeks can be observed. The simplified mathematical model qualitatively captures the statistical properties of the interaction well. The time scales of forced adjustment are investigated by means of an ensemble of wind step-change experiments run with the idealised channel configuration. It is found that the temperature response is driven largely, but not exclusively, by changes in the ocean’s circulation, with enhanced mixing also playing an important role. Circulation changes have a rich spatial structure, and vertical/meridional displacements of the residual overturning circulation cells have a large impact on the temperature response even though the channel is strongly eddy-compensated. The time scales of the response vary across the domain, and are set by the spin-up of baroclinic eddies. The results presented in this Thesis bring the fundamental mechanisms of eddy variability into clearer focus, and inform the interpretation of more realistic numerical simulations of the Southern Ocean.