A mild center in the midst of U.S. warming?

According to output that blends global and regional climate models, the central U.S. Great Plains may warm far less than other parts of the continent by the 2040s. The results were published in Geophysical Research Letters on 10 September by Zaitao Pan (Saint Louis University) with coauthors from Iowa State University (Raymond Arritt, Eugene Takle, William Gutowski, Christopher Anderson, and Moti Segal).

Even though parts of the central United States cooled by as much as 0.8°C (1.4°F) from 1976 to 2000, most global simulations of future climate have not shown this “warming hole,” according to the authors. The culprit, they believe, is the global models’ coarse resolution. In this study, the authors began with output from version 2 of the Hadley Centre global model, then simulated present and future U.S. climate using version 2 of the regional climate model RegCM, originally developed at NCAR. By comparing the years 2040–49 to 1990–99 (see graphic, with legend in degrees Celsius), the authors found that U.S. daily highs in summertime warm an average of about 3°C (5°F) nationwide by the 2040s, but they climb less than 1.0°C (1.8°F) in a patch centered on eastern Kansas and western Missouri.

The reduced warming appears to be the result of heavier showers and thunderstorms across this area, enhanced by a southerly low-level jet stream that strengthens to the south (due in part to drier Texas soils) but weakens to the north. With Gulf of Mexico surface waters some 2°C (3°F) warmer in the 2040s than at present, the strengthening jet carries about 20% more water vapor to the central Great Plains. In turn, summer rainfall amounts rise by an average of about 3 centimeters (1.2 inches) per month. The moister soil and cloudier skies provide a cooling influence on daytime temperatures.

“We conclude that in order to produce accurate projections of changes in the climate of the central U.S., it is necessary to simulate the mesoscale processes that convert converged moisture into rainfall near the northern terminus of southerly [low-level jets],” write the authors. “Regional climate models simulate these processes reasonably well,” they add, while current global models “cannot resolve the mesoscale processes that play key roles in the central U.S. summer climate.”

Saint Louis University
Iowa State University