One of the more robust 21st century projections from the most recent Intergovernmental Panel on Climate Change report (IPCC AR4) is a northward shift in the mean location of the Northern Hemisphere storm track. In the western United States, cool-season precipitation provides most of the water for domestic and industrial consumption, irrigation and power generation. In addition, winter precipitation is particularly effective in recharging soil moisture; as a result, it provides a strong control on the productivity of vegetation and on wildfire. Consequently, there is great interest in understanding 1) how changes in the storm track influence regional climate; 2) spatial and temporal variability in its impact; 3) how well general circulation models simulate the regional climate dynamics that bring precipitation to the West; and 4) whether errors in climate simulation might impact assessment of ecological changes. In order to investigate climate change in the western United States associated with shifts in the storm track, I analyzed the relationship between climate and the Northern Annular Mode (NAM). When the storm track is displaced to the north, there is an earlier transition to warm-season circulation patterns and weather conditions. However, the relationship between the winter NAM and climate is not stable over time. Further analysis identified changes in the correlations between the NAM and tree-ring width, precipitation and temperatures associated with changes in the phase of the Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation. Examining 18 of the IPCC AR4 coupled climate models demonstrated cool-season precipitation errors approaching 300% over western North America. These errors are related to difficulties in representing orography, given the coarse resolution of models, and they may influence the quality of precipitation projections into the future. A simple test using the Köppen classification system found that these precipitation errors lead to underestimating the area of the United States in dryland ecosystem types by up to 89% and consequently allowed for much greater expansion of dryland in the future than is actually likely. These studies suggest that the West is likely to experience greater drought in the future. However, no single tool can yet quantify that change.
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