Springtime Intensification of the Great Plains Low-Level Jet and Midwest Precipitation in GCM Simulations of the Twenty-First Century

Simulations from 18 coupled atmosphere-ocean GCMs are analyzed to predict changes in the climatological Great Plains low-level jet (GPLLJ) and Midwest U.S. hydrology resulting from greenhouse gas increases during the twenty-first century. To build confidence in the prediction, models are selected for analysis based on their twentieth-century simulations, and their simulations of the future are diagnosed to ensure that the response is reasonable. Confidence in the model projections is also bolstered by agreement among models, in a so-called multimodel ensemble, and by analogy with present-day interannual variability. The GCMs agree that the GPLLJ will be more intense in April, May, and June in the future. The selected models even agree on the reason for this intensification, namely, a westward extension and strengthening of the North Atlantic subtropical high (the Bermuda high) that occurs when greenhouse gas-induced warming over the continental United States exceeds that of the subtropical Atlantic in the spring. Accompanying the changes in the GPLLJ are springtime precipitation increases of 20%-40% in the upper Mississippi Valley, which are closely associated with intensified meridional moisture convergence by the jet, with decreases to the south, which results in reduced moist static stability in the region. The simulated differences in the Midwest circulation and hydrology in the spring for the twenty-first century are similar to the observed moisture balance and circulation anomalies for May and, especially, June of 1993, a year of devastating floods throughout the Mississippi Valley.