A strong statistically significant spectral peak with a frequency of 7 - 8 years is computed for tree-ring reconstructed summer PDSI averaged at continental and regional scales over North America for the period 1645 - 1990. A similar, though non-significant spectral peak is computed for instrumental summer PDSI averaged over North America for the shorter period from 1900 - 1990. The winter NAO index ( 1781 - 2002) has a very strong spectral signature at this same 7 - 8 year sub-decadal frequency and is coherent with summer PDSI across a broad sector of the central U. S. Composite analyses confirm a PDSI response to extremes of the NAO over the central U. S., with drought prevalent during negative extremes, and wetness during positive extremes. The winter NAO index leads summer PDSI and a contingency table analysis indicates that extrema in the winter NAO index may have modest forecast value for the following spring-summer moisture regimes over the central U. S.
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