Mechanisms of Changes in Precipitation and Atmospheric Circulation from Anthropogenic Forcing

of a dissertation at the University of Miami. Dissertation supervised by Professor Brian Soden. No. of pages in text. (118) Simulating and understanding the anthropogenic changes in atmospheric circulation and precipitation is a great challenge in climate change studies. Current climate projections rely primarily on coupled atmosphere-ocean models (CGCMs). Due to limited computational resources, these CGCMs have to be run at resolutions that are too low to adequately resolve the climate system. In addition, the climatological biases in CGCMs could also undermine their skillfulness. An alternative model framework – the atmosphere-only models (AGCMs) has been long proposed owing to its superior computational efficiency and better sea surface temperature (SST) climatology. However, AGCMs are often criticized for its lack of coupling with an underlying ocean, and the fidelity of AGCMs for the simulation of anthropogenic climate change is yet to be demonstrated. The first part of this dissertation seeks the optimal modeling framework for the projection of regional climate change by assessing the advantages and disadvantages of CGCMs and AGCMs. Results show that AGCMs are able to perfectly reproduce the anthropogenic climate change from the CGCMs despite the lack of two-way air-sea coupling. In addition, obtaining the pattern of SST change from the CGCMs is in general unnecessary for the simulation of anthropogenic climate change over land. Furthermore, results show that the inability of the CGCMs to simulate climate change largely results from the inability to simulate the present-day climatology. The biases in climatology can be largely reduced in AGCMs with observed SST climatology. These results point to a greater utility of AGCMs for the projection of regional climate change. The second part of this dissertation investigates the relative importance of direct radiative forcing and changes in SST for the anthropogenic changes in atmospheric circulation and precipitation. The global mean SST warming dominates many aspects of the atmospheric responses, including changes in moisture, the mean hydrological cycle and the extratropical precipitation. On the other hand, certain aspects of the atmospheric responses are driven by mechanisms other than the global mean SST warming. In the tropics, both the direct radiative forcing and the pattern of SST change contribute to weakening of the atmospheric circulation. In the subtropics, the precipitation declines are independent of the global mean SST warming but are dominated by fast responses to the direct radiative forcing and changes in SST pattern and land surface temperature.