Analysis of decadal moisture anomalies over the United States: 1500 - 1978

Instrumental Palmer Drought Severity Indices (PDSI) averaged over the western-central USA document three major decadal moisture regimes during the twentieth-century: an early twentieth-century pluvial, the Dust Bowl drought, and a drought in the 1950's. Tree-ring reconstructed PDSI for the United States replicate these regimes and have been used to search for possible analogs over the past 500 years. We also analyzed the sea surface temperature anomalies in the Pacific Ocean associated with these twentieth-century moisture anomalies. Spectral analyses were performed on the PDSI data to determine the spatial and temporal variation of instrumental and tree-ring reconstructed PDSI across five selected spectral intervals. Cross-spectral analyses were used to test the hypothesized influence of external and internal forcing mechanisms on observed and tree-reconstructed PDSI over the United States.;A wet regime from 1905--1917 appears to have been the wettest episode across the western U.S. since A.D. 1500, but similar pluvials occurred in the nineteenth-, seventeenth-, and sixteenth-centuries. The SST regime associated with this pluvial was dominated by cold waters in the eastern North Pacific and warm waters in the eastern equatorial Pacific. The Dust Bowl drought (1929--1940) was most severe over the northern Plains-northern Rockies. We did not find any close analogs for the Dust Bowl drought, but the sixteenth-century megadrought centered over the Southwest appears to have surpassed the Dust Bowl drought in coverage, duration, and severity. The Dust Bowl drought was associated with anomalously warm water in the eastern North Pacific and eastern equatorial Pacific. The 1950's drought (1946--1956) was concentrated over the Southwest and was replicated by 12 droughts of similar spatial coverage and duration. This drought was linked with cold waters in the eastern equatorial Pacific (La Nina-like conditions).;The strongest spectral power in reconstructed summer PDSI was the bidecadal period, and is coherent with the Hale solar magnetic cycle in the north central U.S. Spectral analyses of the 3.22--6.22 frequency band resemble the geography of the ENSO teleconnection, i.e., El Nino wet conditions in the Southwest and South and La Nina drought in the Southwest. The 7.0--8.0 year spectral interval was strong over the east-central and intermountain west and is coherent with the North Atlantic Oscillation.