Observed and CMIP5 modeled influence of large-scale circulation on summer precipitation and drought in the South-Central United States

Annual precipitation in the largely agricultural South-Central United States 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 with sufficient output available, 16 are able to reproduce this bimodal distribution (we refer to these as “BM” models), while 6 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 EW models, temporal variations and geophysical locations of the NASH and GPLLJ appear reasonable compared to reanalysis but 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 model groups under a +1 to +3 °C transient increase in global mean temperature show decreases in summer precipitation concurrent with an enhanced NASH and an intensified GPLLJ, though models differ regarding the months in which these decreases are projected to occur: early summer in the BM models, and late summer in the EW models. Overall, these results suggest that projected future decreases in summer precipitation over the South-Central region appear to be closely related to anomalous patterns of large-scale circulation already observed and modeled during historical dry years, patterns that are consistently reproduced by CMIP5 models.