Simulating Multi-Season Past Droughts
Abstract

Abstract Author: Tsing-Chang Chen, Joseph J. Tribbia, Shih-Yu Wang, and Adam J. Clark

Abstract Title: Droughts in the Central Plains of the United States

Abstract: The potential causes for the geographic preference and remote forcing of warm-season droughts in the U.S. Central Plains were identified:
1) The large-scale flow pattern of Central Plains droughts is characterized by a ridge in the western U.S. and a trough in the eastern U.S. This flow pattern resembles the spring-summer flow regime change across the U.S. continent which is accompanied by a drastic June-July decrease in the Central Plains rainfall maximum. This favorable continental-scale environment for droughts is established by the global spring-fall annual and the March-June semiannual modes which are much weaker than the winter-summer annual mode.
2) Without any special interannual forcing, the atmospheric circulation should only undergo a regular seasonal variation in response to the seasonal cycle of solar heating. Thus, to identify possible interannual forcing and the atmospheric response to this forcing, the regular seasonal cycle should be removed. Applying this approach, a cross-Pacific short-wave train emanating from an interannual rainfall anomaly center over the western subtropical Pacific appears to couple with the Central Plains drought flow pattern across the U.S. continent.
Using these findings, two research tasks were pursued: 1) to evaluate whether the seasonal-cycle modes forming favorable environments of Central Plains droughts are well simulated by global (SMIP2, DEMETER and US) climate models, and 2) to test whether simulated Central Plains droughts are stimulated by the subtropical forcing in the North Pacific. Based on these findings, new mechanisms leading to Central Plains droughts are introduced.