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Thesis (Ph. D.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2007. / Committee Chair: Dr. Robert E. Dickinson; Committee Member: Dr. Irina Sokolik; Committee Member: Dr. Judth Curry; Committee Member: Dr. Peter Webster; Committee Member: Dr. Rong Fu.

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Dissertation submitted to the Faculty of Science, University of the Witwatersraild, Johannesburg for the Degree of Master of Science. / Six general circulation model simulations of present-day southern African climate are assessed, Each of these models are early-generation equilibrium climate models linked to simple mixed-slab oceans. Simulations of surface air temperature over the subcontinent are sensitive to the grid-scale parameterisation of convection in summer. At high latitudes, large simulation errors are caused by errors in the specification of sea-ice albedo feedbacks. Increased spatial resolution and the inclusion of a gravity wave drag term in the momentum equations results in a markedly-improved simulated mean sea level pressure distribution. Tho models successfully simulate the pattern of rainfall seasonality over the Subcontinent, although grid-point simulation of precipitation is unreliable. Treatment of convection, cloud radiative feedbacks and the oceans by this generation of models is simplistic, and consequently there is a large degree of uncertainty associated with predictions of future climate under doubled-carbon dioxide conditions. For this reason, more reliable estimates of future conditions will be achieved using only those models which reproduce present climate most accurately. Early-generation general circulation models suggest a warming of 4°C to 5°C for the southern African region as a whole throughout the year. Over the subcontinent, warming is expected to be least in the tropics, and greatest in the dry subtropical regions in winter. Estimated changes in mean sea level pressure indicate a southward shlft of all pressure systems, with a weakening of the subtropical high pressure belt and mid-latitude westerlies. Little agreement exists between the models concerning predictions of regional precipitation change. However, broad scale changes in precipitation patterns are in accordance with predicted circulation changes over the subcontinent. Generally wetter conditions may be expected in the tropics throughout the year and over the summer rainfall region during summer. Decreased winter rainfall may be expected over the winter rainfall region of the south-western Cape. However, estimated precipitation changes are grid-point specific and therefore must riot be over-interpreted. The present climate validation has resulted in more reliable estimates of future conditions for the southern African region. This approach should be extended to recent slrnulations which include more comprehensive treatment of important physical processes. / Andrew Chakane 2018

The Mediterranean region has experienced persistent drying since the middle of the 20th Century and global climate models project further drying in the future as a consequence of increasing greenhouse gases. The Mediterranean region is also known to oscillate between decades of relatively wet and dry conditions due to the strong influence of multidecadal North Atlantic Oscillation (NAO). It is therefore of great importance to understand the relationship between forced long-term drying resulting from human influences and those due to natural variability. To this end, we used observations, reanalyses and comprehensive global climate models in this thesis research. The roles of anthropogenic climate change and internal climate variability in causing the Mediterranean region's late 20th Century extended winter drying trend were examined using 20th Century observations as well as 19 coupled climate models from the CMIP3. The drying was strongly influenced by the robust positive trend in the NAO from the 1960s to the 1990s. Model simulations and observations were used to assess the probable relative roles of radiative forcing and internal variability in explaining the circulation trend that drove much of the precipitation change. It was concluded that the radiatively forced trends were a small fraction of the total observed trends. Instead it was argued that the robust trends in the observed NAO and Mediterranean rainfall during this period were largely due to multidecadal internal variability with a small contribution from the external forcing. Differences between the observed and NAO associated precipitation trends are consistent with those expected as a response to radiative forcing. The radiatively forced trends in circulation and precipitation are expected to strengthen in the current century and these results highlight the importance of their contribution to future precipitation changes in the region. The Mediterranean precipitation climatology and trend were further examined by comparing the newest generation of global climate models (CMIP5) used in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report, to the previous generation (CMIP3) and to observations over the latter half of the 20th Century for both the summer and winter half years. The observed drying trend since 1950 was predominantly due to winter drying, with very little contribution from the summer. However, in the CMIP5 multimodel mean, the precipitation trend since 1950 is evenly divided throughout the seasonal cycle. This may indicate that in observation, multidecadal internal variability, particularly that associated with the NAO, dominates the wintertime trend. An estimate of the observed externally forced trend showed that winter drying dominated in observations but the spatial patterns were grossly similar to the multimodel mean trend. The similarity was particularly robust in the eastern Mediterranean region, indicating a radiatively forced component being stronger there. These results also revealed modest improvement for the CMIP5 multimodel ensemble in representation of the observed six-month winter and summer climatology. We further explored the detailed mechanisms leading to the NAO-associated precipitation change, such as the role of the change in mean circulation versus that of the storm tracks in the regional moisture budget, which had not been investigated previously. We employed a moisture budget analysis using 15 CMIP5 models and the ERA-Interim Reanalysis to investigate the relationship between the NAO and the various moisture budget terms for the six-month winter and summer. Compared with the ERA-Interim, the models performed well in their simulation of the relationship between the naturally varying NAO and the large-scale moisture budget. Our results indicated that the shift in the midlatitude transient eddies induced modest moisture convergence, rather than divergence, over the Mediterranean under a positive NAO. The reduction in precipitation in this region during a positive NAO was dominated by the mean moisture divergence, which opposed the transient contribution. There were significant differences between the patterns of NAO-induced moisture budget anomaly and changes due to external radiative forcing. Under radiative forcing there was enhanced evaporation over the Mediterranean Sea, Italy and eastern Europe and drying by the shift in the wintertime storms over nearly all of Europe and the Mediterranean. Under a positive phase of the NAO, on the other hand, there was modest reduction in evaporation and wetting by the storms over the Mediterranean, and drying over northern Europe. The dependence of the Mediterranean moisture budget on the NAO was similarly explored in the summer half of the year and in this season the models exhibited more disagreement with observations, but otherwise showed the similar results as winter. The stronger anthropogenic induced drying signal over the eastern Mediterranean provided a basis to examine the possible cause and impact of the recent severe and persistent drought in Syria that occurred directly prior to the uprising of 2011. The drought devastated Syrian agriculture, resulting in food shortages, widespread unemployment, the collapse of rural social structure and a mass migration of agricultural refugees to Syria's urban areas. Anger at the government's failure to ameliorate conditions was one spark for the uprising that evolved into civil war. We found that though droughts occur periodically in Syria due to natural causes it is likely that the recent drought was more extreme due to the century long drying trend caused by increased radiative forcing. It was estimated that the anthropogenic trend made a drought of such severity several times more likely. Droughts as persistent as the recent one are projected to be commonplace in a future warmer world.

Due to the wide range of physical scales involved, clouds cannot be fully resolved in models of the global climate, and so are parameterized. The resultant model deficiencies in simulating important cloud processes within the current climate are strongly implicated in the large uncertainty in model predictions of future climate changes. Previous work has highlighted the uncertainties in predictions of future climate related to thermodynamic cloud changes, understanding of which requires detailed observations of small-scale cloud microphysics. In this thesis, we argue that understanding the linkages between mid-latitude clouds and the general circulation of the atmosphere can advance efforts to constrain their response to climate forcing. We make this argument with three main methods of analysis: 1) observations, 2) state-of-the-art general circulation models, and 3) experiments with an idealized model of the global climate. First, we perform a comprehensive investigation of the observed inter-annual relationships between clouds, their radiative effects, and key indices of the large-scale atmospheric circulation. Using reanalysis data and satellite retrievals, we find a relationship between the edge of the Hadley circulation (HC) and the high cloud field that is largely robust against season and ocean basin. In contrast, shifts of the mid-latitude eddy-driven jet latitude, which had been the focus of previous work on the coupling between mid-latitude clouds and circulation, only correlate with the high cloud field in the wintertime North Atlantic. In that season and basin, poleward shifts of the circulation are associated with anomalous shortwave cloud radiative warming. During all seasons in the Southern Hemisphere, however, poleward shifts of the circulation are associated with anomalous shortwave cloud radiative cooling. Second, we examine Coupled Model Intercomparison Project phase 5 (CMIP5) model output to evaluate the models' simulation of the inter-annual co-variability between the Southern Hemisphere HC extent and the shortwave cloud radiative effect. In the control climate runs, during years when the HC edge is anomalously poleward, most models reduce their cloud cover in the lower mid-latitudes (approximately 30$^\circ$S - 45$^\circ$S) and allow more sunlight to warm the region, although we find no such shortwave radiative warming in observations. We correlate these biases in the co-variability between the HC extent and shortwave cloud radiative anomalies with model biases in the climatological HC extent. Models whose climatological HCs are unrealistically equatorward compared to the observations exhibit weaker climatological subsidence in the lower mid-latitudes and exhibit larger increases in subsidence there with poleward HC extent shifts than models with more realistic climatological HCs. This behavior, based on control climate variability, has important implications for the model response to forcing. In 4$\times$CO$_2$-forced runs, models with unrealistically equatorward HCs in the control climatology exhibit a stronger shortwave cloud radiative warming response in the lower mid-latitudes and tend to have larger values of equilibrium climate sensitivity than models with more realistic HCs in the control climatology. The above correlative analyses suggest that uncertainty in the linkages between mid-latitude clouds and the general circulation of the atmosphere contributes to uncertainty in the model response to forcing. Finally, we use simulations of the global climate in an idealized aquaplanet model to show that the biases in the climatological Southern Hemisphere circulation do indeed contribute to much of the model spread in the cloud-circulation coupling. We find that for the same 1$^\circ$ latitude poleward shift, simulations with narrower climatological HCs exhibit stronger mid-latitude shortwave cloud radiative warming anomalies than simulations with wider climatological HCs. The shortwave cloud radiative warming anomalies result predominantly from a subsidence warming of the planetary boundary layer, which decreases low-level cloud fraction and is stronger for narrower HCs because of a tighter mean meridional circulation. A comparison of the spread across aquaplanet simulations with that across CMIP5 models suggests that about half of the model uncertainty in the mid-latitude cloud-circulation coupling stems from this impact of the circulation on the large-scale temperature structure of the boundary layer, and thus can be removed by improving the representation of the climatological circulation in models. Therefore, a more realistic representation of the Hadley circulation in models can improve their representation of the linkage between mid-latitude clouds and the atmospheric circulation in the current climate and increase overall confidence in predictions of future climate.

Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Meteorology. / Microfiche copy available in Archives and Science. / Vita. / Bibliography : leaves 206-218. / Ph.D.