Impacts of Increasing Greenhouse Gas Concentrations and of Ozone Changes on the Atmospheric and Oceanic Circulation in the Southern Hemisphere

This thesis investigates the changes in the atmospheric and oceanic circulation in the Southern Hemisphere driven by the increase in anthropogenic greenhouse gases and by changes in stratospheric ozone during the second half of the twentieth century and the twenty-first century. The effect of the method used to account for ozone changes in coupled climate models on the results is additionally investigated.

It is found that in the past, the formation of the Antarctic ozone hole is the main driver of dynamical changes in the Southern Hemisphere, such as the acceleration of the Brewer-Dobson Circulation and the strengthening and poleward shift of the tropospheric westerlies. In the future, ozone recovery and the increase in greenhouse gases following the high emission scenario SSP585 drive changes of opposite sign, with the effect of greenhouse gases dominating. However, ozone recovery mitigates the impact of the increase in greenhouse gases on the westerly winds, the Agulhas leakage and the Southern Ocean sea surface temperature. In the stratosphere, changes of similar magnitude were found due to ozone recovery and increasing greenhouse gases in the future. The increase in greenhouse gases leads to a planetary wavenumber 1 response in austral spring and, surprisingly, to a weakening of the Brewer-Dobson Circulation. This contrast the strengthening of the Brewer-Dobson Circulation that occurs during the rest of the year due to greenhouse gases.

Greenhouse gases dominate the changes in precipitation over South Africa associated with ridging highs in the future. Ridging highs were categorized based on whether they are accompanied by Rossby wave breaking aloft or not and it was found that Rossby wave breaking mediates the impact of the increase in greenhouse gases on ridging highs and the amount of precipitation they contribute over South Africa.


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