Observed and CMIP5 Modeled Influence of Large-Scale Circulation on Summer Precipitation and Drought in the South-Central United States

The U.S. Third National Climate Assessment (NCA3) highlighted drought and more frequent extreme heat days as the major factors affecting the long-term viability of ecosystems in the South-Central U.S. region in the coming decades. Observed trends and future projections suggest that, although annual precipitation may not change significantly, climate change is likely to affect the frequency and severity of future heat waves and growing season drought risk in the region. To build confidence in future projections, we need to improve our understanding of the processes driving the droughts and how climate models simulate those processes. While modeled precipitation depends on a complex mix of the physical parameterizations used to represent convective processes, boundary-layer processes, and land-surface interactions, the large-scale circulation features that drive variability and trends in regional precipitation and drought risk occur at spatial scales that should be reasonably simulated by climate models. Therefore, examining how climate models reproduce these patterns and their relationship to precipitation variability and drought, and the extent to which future changes in precipitation and drought risk are linked to shifts in these large-scale patterns, may provide insight into interpreting projected future change.

Annual precipitation in the largely agricultural South-Central U. S. is characterized by a primary wet season in May and June, a mid-summer dry period in July and August, and a second precipitation peak in September and October. Of the 22 CMIP5 global climate models that archived the variables needed for this analysis, sixteen models are able to reproduce this bimodal distribution (so called “BM” models), while six models have trouble simulating the mid-summer dry period, instead producing an extended wet season (“EW” models). In BM models, the timing and amplitude of the mid-summer westward extension of the North Atlantic Subtropical High (NASH) are realistic, while the magnitude of the Great Plains Lower Level Jet (GPLLJ) tends to be overestimated, particularly in July. In the EW models, although temporal variations and geophysical locations of the NASH and GPLLJ appear reasonable compared to reanalysis, their magnitudes are too weak to suppress mid-summer precipitation. During warm-season droughts, however, both groups of models reproduce the observed tendency towards a stronger NASH that remains over the region through September and an intensification and northward extension of the GPLLJ. Similarly, future simulations from both BM and EW models under an increase in global mean temperature ranging from +1 to +3^oC show decreases in summer precipitation that seems to be related to the combined influences of an enhanced NASH and an intensified GPLLJ. These results suggest that projected decreases in summer precipitation over the South-Central region appear to be closely related to the projected anomalous patterns of the large-scale circulation similarly observed and modeled during historical dry years, which is consistently reproduced by the models regardless of the model ability to simulate the large-scale drivers of the annual cycle of precipitation.