Dynamics of the Southern Hemisphere extratropical atmospheric circulation

Abstract

The Southern Hemisphere extratropical westerly winds are the strongest time-averaged surface winds on Earth, having a profound impact on weather systems, ocean circulation, Antarctic sea-ice as well as oceanic uptake of heat and carbon. The westerly winds have shown poleward intensification in the last few decades driven primarily by stratospheric ozone depletion with a secondary role played by increasing greenhouse gases. In recent years we have seen early signs of ozone recovery as a result of the Montreal Protocol. Part 1 of this thesis demonstrates that by curbing CFC emissions, the Montreal Protocol also played a critical role in mitigating future surface climate change, equivalent to approximately 25% reduction in global surface warming by 2050. A major feature of the Southern Hemisphere extratropical atmospheric circulation is its strong zonal coherence. However, there are notable zonal asymmetries embedded in the flow, with two important examples being the zonal wave 3 (ZW3) and Amundsen Sea Low (ASL). Although these features have received significant attention from the scientific community, the mechanisms responsible for their presence are still not clear. In Part 2, model experiments suggest that the ZW3 pattern is generated remotely by tropical deep convection and not by the presence of three extratropical landmasses as had previously been assumed. Quantification of ZW3 impacts requires a way to consistently characterize this variability. In Part 3, I formulate a new index for ZW3 which accounts for variability in the structure, phase and amplitude of ZW3. In Part 4, I provide evidence that in contrast to ZW3, the ASL is generated primarily by the interaction between westerly winds and Antarctic orography. Zonally asymmetric features are not only present in the mean circulation but also in the past and projected westerly wind changes in the Southern Hemisphere. These are characterized in Part 5 in reanalysis and models. Following on from this, I demonstrate in ocean model simulations that future projected zonally asymmetric atmospheric changes can drive substantial changes in the ocean circulation in the Pacific and Indian Oceans, accounting for more than 30% of the projected surface ocean warming around parts of Australia (Part 6).