Now showing 1 - 2 of 2
ItemInvestigating stratospheric ozone change and associated impacts on circulation and climateStone, Kane ( 2015)With stratospheric ozone on the path to recovery, understanding its future role in modulating Southern Hemisphere circulation and climate is essential. This work contributes to answering this question through both observational and modelling studies. First, a new Dobson Umkehr retrieval technique optimised for resolution was developed, with the retrievals contributing in the evaluation of the Australian Community Climate and Earth-System Simulator-Chemistry Climate Model (ACCESS- CCM). This model was used to investigate future Southern Hemisphere stratospheric ozone changes and associated dynamical and climate responses, particularly in the Southern Annular Mode (SAM), the tropopause height and quasi-stationary waves. For this purpose, four simulations were completed comprising of historical, future projection and sensitivity simulations with fixed ozone depleting substances and greenhouse gases (GHGs) at 1960 levels. These simulations also act as Australia’s contribution to the international Chemistry Climate Model Initiative. The Dobson Umkehr retrieval technique developed here uses all available data, unlike other algorithms, which use designated solar zenith angles (SZAs) from a single wavelength pair (C), out of three (A, C and D). Investigating a test case from Melbourne, the degrees of freedom for signal increased from 3.1 to 3.4 when using all C-pair SZAs, and up to 6.5 when using all available SZAs and wavelength pairs; a significant improvement over current operational methods. ACCESS-CCM evaluation shows excessive ozone but an accurate distribution, and a temporally persistent ozone hole. Comparison with the Dobson Umkehr retrievals, total column ozone observations, ERA-Interim reanalysis and past modelling studies shows ACCESS-CCM produces excess ozone at altitudes above 25 km for Melbourne, but with substantial improvements in Antarctic total column ozone over it’s precursors (CCMVal-2 UMUKCA models). Comparisons with Davis and South Pole ozonesondes display a large disparity in the vertical location of perturbed ozone. Maximum depletion is seen between 100–50 hPa in ozonesondes, compared to above 50 hPa in ACCESS-CCM. This difference is likely caused by cold model biases enhancing polar stratospheric cloud formation and subsequent chlorine release at high altitudes. The lack of supercooled ternary solution may be a cause of less depletion between 100–50 hPa. Despite these inadequacies, ACCESS-CCM is simulating the amount of historical Antarctic October ozone depletion, the SAM and 50 hPa zonal wind anomalies well compared to ERA-Interim and past modelling studies. This gives confidence that the model simulates reasonable ozone-induced circulation responses. The model shows that October averaged Antarctic ozone is returning to 1980 levels just after 2060. Increasing GHG and ozone concentrations act to delay and advance the breakup of the polar vortex respectively. Regression analysis shows that in the future, increasing GHGs and ozone concentrations drive an increase and decrease in the SAM index respectively, effectively cancelling each other out. Contrary to the SAM, the high latitude tropopause changes, while influenced heavily by ozone changes in the past, is dominated by increasing GHGs in the future. The phase of spring and summer wave 1 in TCO, 50hPa temperature and 10hPa zonal wind undergo an eastward shift due to both ozone depletion and GHG increases. The wave 1 phase influence from GHGs is seen to originate from the troposphere, and therefore is influenced heavily by Andes orography. This is not the case for ozone concentration changes, indicating that the main influence is through modulation of the stratospheric polar vortex. A decrease in the amplitude is also seen due to GHG increases, primarily due to a decrease in the amplitude of the tropospheric wave 1.
ItemDrivers of Southern Hemisphere climate changeArblaster, Julie Michelle ( 2013)The climate of the Southern Hemisphere (SH) has undergone significant changes over recent decades, with additional warming expected under future emission scenarios. However, while temperature increases are robust across models there is more uncertainty around changes in rainfall, atmospheric circulation and extremes, all of which have a large impact on human society and ecosystems. The objective of this thesis was to increase our confidence in future projections by considering the relative importance of various drivers of past and future climate change, with a focus on the Southern Hemisphere. These drivers include sea surface temperatures (SSTs; which may or may not be anthropogenically forced), greenhouse gases and stratospheric ozone. To achieve this objective a hierarchy of model experiments were utilised, from idealised experiments to multimodel datasets. Insights were gained by exploring both the consistencies and the spread across the model results. The main results were: 1) The internal variability of the climate system, such as the El Niño-Southern Oscillation (ENSO), impacts the emerging signals of anthropogenic climate change and characterisations of this noise were explored. Opposite phases of ENSO were found to drive marked contrasts in maximum temperature extremes, with ENSO fidelity crucial in simulating the observed relationships. These patterns are unlikely to change substantially under future climate change. Over the Australian continent, future warming leads to increases in warm temperature extremes and a propensity for longer dry spells interspersed with heavy rainfall events. In general, the magnitude of changes in both temperature and precipitation extremes indices scaled with the strength of emissions. 2) Coupled model simulations were able to reproduce the large-scale features of SH climate trends since 1950, if observed changes in anthropogenic forcings were included. However, atmospheric models driven by observed SSTs and anthropogenic forcings were unable to capture wintertime trends, suggesting either deficiencies in the modelling framework, SSTs or models themselves or that internal variability has been largely responsible for these trends. The improvement of the simulated trends in experiments with partial coupling suggests the modelling framework plays some role in this deficiency. 3) Future changes in the SH atmospheric circulation will be driven by the competing effects of greenhouse gases and stratospheric ozone recovery. Climate sensitivity was found to largely explain the difference in Southern Annular Mode (SAM) projections between two coupled climate models under identical greenhouse gas and stratospheric ozone forcing. This result extended to multimodel simulations under transient carbon dioxide (CO2) conditions in all seasons, with the stronger the warming the larger the trend in the SAM. Tropical upper tropospheric warming was found to be more relevant than polar stratospheric cooling to the model spread in SAM responses to CO2. 4) Idealised SST experiments in two models showed a consistent poleward shift of the SH wintertime westerly jet under Southern Ocean cooling. However, the austral winter response to increased tropical SSTs was found to be model dependent, with opposite latitudinal shifts in the SH westerly jet in the two models. This finding was linked to different tropical rainfall and convective atmospheric heating responses in the models to identical SST increases. These results highlight the reliability of current climate model simulations as well as some of their limitations. Potential deficiencies in forcing datasets, modelling frameworks and the simulation of internal variability were identified. Understanding and improving these deficiencies is crucial for interpreting recent observed change and understanding future projections, particularly at the regional scale.