An Increased Incidence of Wet Years Over the Coterminous United States During 1973–1999

Interdecadal shifts in rainfall have been detected over the growing regions of the American Midwest, but the origins of this variability and its capacity to affect agriculture are not well documented. Using annual rainfall records derived from U.S. Climate Division Data, USDA/NASS agricultural yield records, and historical SST and SLP analyses, the roots of long-term climate variability and associated yield effects over key agricultural regions are being traced. Preliminary results show that ~12 year rainfall cycles over Kansas were matched by concurrent shifts in winter wheat yields between the late 1950's and mid-1980's. This climate-yield variation was subsequent to a more widely acknowledged interdecadal dry-wet-dry cycle observed over Kansas between the 1930's and 1950's, which is also apparent in wheat yields. Interestingly, past studies concentrating on interdecadal variability over the Atlantic have indicated the presence of 12-14 year cycles in sea-surface temperature. As a result, a preliminary hypotheses made here is that interdecadal variability in the tropical Atlantic, combined with the disposition of the Bermuda High, may be key influences on variation in rainfall over the central United States over similar time scales. This is currently being tested via principal component analysis of the Kaplan historical SLP and SST data sets.