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

Effects of prescribed CMIP6 ozone on simulating the Southern Hemisphere atmospheric circulation response to ozone depletion

The Antarctic ozone hole has led to substantial changes in the Southern Hemisphere atmospheric circulation, such as the strengthening and poleward shift of the midlatitude westerly jet. Ozone recovery during the twenty-first century is expected to continue to affect the jet's strength and position, leading to changes in the opposite direction compared to the twentieth century and competing with the effect of increasing greenhouse gases. Simulations of the Earth's past and future climate, such as those performed for the Coupled Model Intercomparison Project Phase 6 (CMIP6), require an accurate representation of these ozone effects. Climate models that use prescribed ozone fields lack the important feedbacks between ozone chemistry, radiative heating, dynamics, and transport. In addition, when the prescribed ozone field was not generated by the same model to which it is prescribed, the imposed ozone hole is inconsistent with the simulated dynamics. These limitations ultimately affect the climate response to ozone depletion. This study investigates the impact of prescribing the ozone field recommended for CMIP6 on the simulated effects of ozone depletion in the Southern Hemisphere. We employ a new state-of-the-art coupled climate model, Flexible Ocean Climate Infrastructure (FOCI), to compare simulations in which the CMIP6 ozone is prescribed with simulations in which the ozone chemistry is calculated interactively. At the same time, we compare the roles played by ozone depletion and by increasing concentrations of greenhouse gases in driving changes in the Southern Hemisphere atmospheric circulation using a series of historical sensitivity simulations. FOCI captures the known effects of ozone depletion, simulating an austral spring and summer intensification of the midlatitude westerly winds and of the Brewer–Dobson circulation in the Southern Hemisphere. Ozone depletion is the primary driver of these historical circulation changes in FOCI. The austral spring cooling of the polar cap in the lower stratosphere in response to ozone depletion is weaker in the simulations that prescribe the CMIP6 ozone field. We attribute this weaker response to a prescribed ozone hole that is different to the model dynamics and is not collocated with the simulated polar vortex, altering the strength and position of the planetary wavenumber one. As a result, the dynamical contribution to the ozone-induced austral spring lower-stratospheric cooling is suppressed, leading to a weaker cooling trend. Consequently, the intensification of the polar night jet is also weaker in the simulations with prescribed CMIP6 ozone. In contrast, the differences in the tropospheric westerly jet response to ozone depletion fall within the internal variability present in the model. The persistence of the Southern Annular Mode is shorter in the prescribed ozone chemistry simulations. The results obtained with the FOCI model suggest that climate models that prescribe the CMIP6 ozone field still simulate a weaker Southern Hemisphere stratospheric response to ozone depletion compared to models that calculate the ozone chemistry interactively

On the Ridging of the South Atlantic Anticyclone Over South Africa: The Impact of Rossby Wave Breaking and of Climate Change

Ridging South Atlantic Anticyclones contribute an important amount of precipitation over South Africa. Here, we use a global coupled climate model and the ERA5 reanalysis to separate for the first time ridging highs (RHs) based on whether they occur together with Rossby wave breaking (RWB) or not. We show that the former type of RHs are associated with more precipitation than the latter type. The mean sea level pressure anomalies caused by the two types of RHs are characterized by distinct patterns, leading to differences in the flow of moisture-laden air onto land. We additionally find that RWB mediates the effect of climate change on RHs during the twenty-first century. Consequently, RHs occurring without RWB exhibit little change, while those occurring with RWB contribute more precipitation over the southern and less precipitation over the northeastern South Africa in the future. Key Points: - Ridging South Atlantic Anticyclones are accompanied by Rossby wave breaking (RWB) aloft in 44% of the cases - Ridging highs that are accompanied by RWB lead to more precipitation over South Africa than those that are not - Ridging highs bring more precipitation over the southern and less precipitation over the northeastern part of South Africa in the futur

Ocean bottom pressure variability: Which part can be reliably modeled?

Ocean bottom pressure (OBP) variability serves as a proxy of ocean mass variability. A question how well it can modeled by the present general ocean circulation models on time scales of 1 day and more is addressed. It is shown that the models simulate consistent patterns of bottom pressure variability on monthly and longer scales except for areas with high mesoscale eddy activity, where high resolution is needed. The simulated variability is compared to a new data set from an array of PIES (Pressure-Inverted Echo Sounder) gauges deployed along a transect in the Southern Ocean. We show that while the STD of monthly averaged variability agrees well with observations except for the locations with high eddy activity, models lose a significant part of variability on shorter time scales. Furthermore, despite good agreement in the amplitude of variability, the OBP from the PIES and simulation show almost no correlation. Our findings point to limitations in geophysical background models required for space geodetic applications. We argue that major improvements in OBP modelling require data assimilation in order to increase the coherence between modelled and observed signals

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

Twenty-first century Southern Hemisphere impacts of ozone recovery and climate change from the stratosphere to the ocean

Changes in stratospheric ozone concentrations and increasing concentrations of greenhouse gases (GHGs) alter the temperature structure of the atmosphere and drive changes in the atmospheric and oceanic circulation. We systematically investigate the impacts of ozone recovery and increasing GHGs on the atmospheric and oceanic circulation in the Southern Hemisphere during the twenty-first century using a unique coupled ocean–atmosphere climate model with interactive ozone chemistry and enhanced oceanic resolution. We use the high-emission scenario SSP5-8.5 for GHGs under which the springtime Antarctic total column ozone returns to 1980s levels by 2048 in our model, warming the lower stratosphere and strengthening the stratospheric westerly winds. We perform a spatial analysis and show for the first time that the austral spring stratospheric response to GHGs exhibits a marked planetary wavenumber 1 (PW1) pattern, which reinforces the response to ozone recovery over the Western Hemisphere and weakens it over the Eastern Hemisphere. These changes, which imply an eastward phase shift in the PW1, largely cancel out in the zonal mean. The Southern Hemisphere residual circulation strengthens during most of the year due to the increase in GHGs and weakens in spring due to ozone recovery. However, we find that in November the GHGs also drive a weakening of the residual circulation, reinforcing the effect of ozone recovery, which represents another novel result. At the surface, the westerly winds weaken and shift equatorward due to ozone recovery, driving a weak decrease in the transport of the Antarctic Circumpolar Current and in the Agulhas leakage and a cooling of the upper ocean, which is most pronounced in the latitudinal band 35–45∘ S. The increasing GHGs drive changes in the opposite direction that overwhelm the ozone effect. The total changes at the surface and in the oceanic circulation are nevertheless weaker in the presence of ozone recovery than those induced by GHGs alone, highlighting the importance of the Montreal Protocol in mitigating some of the impacts of climate change. We additionally compare the combined effect of interactively calculated ozone recovery and increasing GHGs with their combined effect in an ensemble in which we prescribe the CMIP6 ozone field. This second ensemble simulates a weaker ozone effect in all the examined fields, consistent with its weaker increase in ozone. The magnitude of the difference between the simulated changes at the surface and in the oceanic circulation in the two ensembles is as large as the ozone effect itself. This shows the large uncertainty that is associated with the choice of the ozone field and how the ozone is treated