Regional models boost summer drying

U.S. trends could be more intense than global models suggest

September 20, 2013 | Parts of the central United States may become more prone to summertime drying than earlier thought, based on new simulations of climate change that involve both global and regional climate models. The new research, published in the journal Climatic Change, sheds light on a major question: how global climate change will affect specific regions.

While climate scientists have spent decades creating models that simulate global climate, only recently has computational power and scientific understanding grown enough to allow the use of regional models that can capture more fine-scale details of long-term climate. These regional models are typically embedded within global models, a process called dynamical downscaling.

NCAR scientist Linda Mearns and colleagues have analyzed the results of a number of combinations of global and regional climate models, focusing on future temperature and precipitation across North America. In line with previous work, the models generally agreed that for the period 2041–2070 compared to 1971-2000, the Midwest and Northeast are likely to be more moist during the winter, while western and southern states are likely to face increasingly dry summers.

But the models diverged on the extent of these changes. Simulations run on embedded regional climate models (RCMs) showed more dramatic precipitation changes than for global climate models (GCMs) alone. This is especially true for summertime drying in regions such as the Southern Plains and Southwest.

If the models are correct, this could have significant societal effects, the study warns.

“If further process-based analysis confirms that the RCM results are credible, this would imply more serious impacts on water and agricultural resources than inferred from GCM projections, and adaptation to these changes could be much more difficult,” the authors state.

The work was part of the North American Regional Climate Change Assessment Program (NARCCAP), which focuses on projecting future changes in regional climate patterns.

The intensified drying in summer in the RCMs may be tied to differences in moisture transport and convergence—as has been seen in other studies—as well as the general dominance of mesoscale precipitation processes in the summer. Regional models are better than global models at simulating those mesoscale processes.

The authors stress that uncertainty remains over the location of the future transition belt separating wetter conditions to the north and east and drying to the south and west. The regional models also show more agreement in the northeastern moistening than in the southwestern drying, although both phenomena are consistent with how U.S. climate is expected to unfold with global warming.

L. O. Mearns, S. Sain, L. R. Leung, M. S. Bukovsky, S. McGinnis, S. Biner, D. Caya, R. W. Arritt, W. Gutowski, E. Takle, M. Snyder, R. G. Jones, A. M. B. Nunes, S. Tucker, D. Herzmann, L. McDaniel, and L. Sloan (2013), Climate change projections of the North American Regional Climate Change Assessment Program (NARCCAP), Climatic Change, 120, DOI 10.1007/s10584-013-0831-

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