We apply the multitaper frequency domain--singular value decomposition signal detection method to the investigation of coherent patterns of variation in seasonal Northern Hemisphere sea level pressure and conterminous U.S. summer drought during the period 1895--1995. The analysis identifies statistically significant patterns of spatiotemporal variability on interannual and bidecadal timescales indicative of both cold-season and warm-season atmospheric influences on North American drought patterns. The most robust signal found appears to be associated with the influences of the El Ni¿o--Southern Oscillation (ENSO) on North American summer drought. Evidence is also found to support the existence of a roughly bidecadal drought signal tied to warm-season atmospheric circulation changes. The Dust Bowl conditions of the 1930s appear to result from a combination of these bidecadal influences on drought conditions that coincide with a decrease in the amplitude of interannual ENSO-related variability during the 1930s.