The Role of Eddy Effects in the Sensitivity of the Southern Ocean Circumpolar Current to Changes in Wind Stress

Abstract

The Antarctic Circumpolar Current (ACC) transport is connected to the global ocean stratification and the global meridional overturning circulation, and accurately predicting its responses to changes in surface forcings such as wind stress is important in predicting global effects of anthropogenic climate change. The ACC region is very turbulent, and mesoscale eddy effects have significant effects on ACC dynamics. However, coarse-resolution models are known to poorly represent eddy effects, and the mechanisms behind major eddy-related phenomena are yet to be fully understood.

This thesis investigates the equilibrium and transient responses of the ACC to changes in wind stress over models of differing complexities with parameterised eddies. The GEOMETRIC scheme is known to effectively capture eddy phenomena that more widely used parameterisation schemes do not, including that of eddy saturation, the relative insensitivity of the ACC transport to changes in wind stress. GEOMETRIC is further shown here to exhibit 'negative sensitivity' of the ACC transport when the residual meridional overturning (MOC) is the opposite of the conventional direction, in line with some eddy resolving calculations. The time-dependent responses of the ACC are also explored; when the wind forcing changes, the eddy energy adjusts faster than the stratification, and this lag suggests an innate oscillatory scale be-tween the eddies and stratification. This is supported by a dynamical system model, and further verified through wind perturbation experiments showing that the system exhibits oscillator-like behaviour when the eddy coefficient is allowed to vary sufficiently. This thesis concludes that mesoscale eddies play a critical role in both equilibrium and transient responses of the Southern Ocean, and a parameterisation scheme that allows the eddy transfer coefficient to adjust sufficiently fast in response to the mean stratification is necessary in order for coarse-resolution models to accurately represent such responses.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Julian MAK (Supervisor)