Evidence of a 22-year rhythm of drought in the western United States related to the Hale solar cycle since the 17th century.

Families of Drought Area Indices (DAI) have been derived from tree-ring data for the entire U.S. west of the Mississippi River, for each year back to either 1700 or 1600 A.D., depending on the data base used. Three families of DAI are considered in the analysis reported in this paper. The solar control is best described as a modulation of terrestrial drought-inducing mechanisms, such that it alternately encourages and discourages the development of major continental droughts which are set up by evolutionary climatic processes unrelated to solar activity. - Authors