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Abstract
The main objective of this thesis is to evaluate the simulation of heatwaves in climate models and to better understand their relevant physical drivers. The thesis objective is motivated by the desire for climate science to provide more actionable information to decision-makers and planners on climate extremes. This thesis employs multiple state-of-the-art data products. CMIP5 coupled climate models were evaluated in terms of simulating large scale circulation patterns and their representation of heatwaves. Heatwaves were calculated from a gridded observationally-based Australian temperature dataset and compared against heatwaves in climate models. Reanalysis data was included to examine circulation and land surface conditions that drive heatwaves in different regions of Australia. Future projections of these driving conditions were then assessed in multiple climate models to characterize uncertainty.
Evaluating the representation of large scale circulation patterns over the Australian region, including the frequency, persistence and transition states of these patterns, revealed a relatively large diversity of model performance across these metrics. The spatial patterns of heatwave trends and climatology in CMIP5 also presented a diverse range of patterns and model skill. There was some association between model skill in representing large scale circulation and skill in representing the heatwave climatology. Models that represented circulation best were typically higher resolution models which also provide a reasonable representation of the heatwave climatology; models that represent circulation poorly were typically low resolution models with a poor representation of the heatwave climatology.
Examination of the linkages between large scale circulation patterns and heatwave occurrence in reanalysis revealed that the land surface state often modulated the circulation heatwave association. Analysis of future projections in CMIP5 revealed uncertainty concerning the magnitude of the frequency changes in relevant atmospheric circulation features and concerning the direction of change in relevant land surface conditions. Reducing uncertainty in how these driving conditions are projected to change is necessary to improve the robustness of regional heatwave projections at scales relevant for planning and decision-making.