Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States

Abstract

More than half (52%) of the spatial and temporal variance in multidecadal drought frequency over the conterminous United States is attributable to the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). An additional 22% of the variance in drought frequency is related to a complex spatial pattern of positive and negative trends in drought occurrence possibly related to increasing Northern Hemisphere temperatures or some other unidirectional climate trend. Recent droughts with broad impacts over the conterminous U.S. (1996, 1999-2002) were associated with North Atlantic warming (positive AMO) and northeastern and tropical Pacific cooling (negative PDO). Much of the long-term predictability of drought frequency may reside in the multidecadal behavior of the North Atlantic Ocean. Should the current positive AMO (warm North Atlantic) conditions persist into the upcoming decade, we suggest two possible drought scenarios that resemble the continental-scale patterns of the 1930s (positive PDO) and 1950s (negative PDO) drought.

Fig. 1.

Scores from the first three components (PC1, PC2, and PC3) of a rotated principal components analysis of 20-year moving drought frequency in the conterminous United States, compared to standardized departures of 20-year moving averages of the annual PDO, AMO, and NH temperature (NH Temp). The NH Temp values are multiplied by –1 for easy comparison with PC3 scores. All values are plotted at the centers of the window periods.

Fig. 2.

(A–C) Loadings for the first three components of a rotated principal components analysis of 20-year moving drought frequency in the conterminous United States. (D) Correlations between 20-year moving drought frequencies and 20-year moving annual PDO. (E) Correlations between 20-year drought frequency and 20-year moving AMO. (F) Inverse of trends in 20-year drought frequency. The correlations in F were multiplied by –1 for easy comparison with other figures. Positive values are shaded red, and negative values are shaded blue. Darker shades indicate values >0.4 or <–0.4.

Fig. 3.

Observed (A and B) and simulated (C and D) 20-year drought frequency (in years) for 1924–1943 (the 1930s drought) and 1947–1966 (the 1950s and 1960s droughts). Areas with drought frequencies of 6 years or more are indicated in red.

Fig. 4.

Time series of the annual PDO and AMO. Shaded areas indicate combinations of positive (+) and negative (–) PDO and AMO periods.

Fig. 5.

Drought frequency (in percent of years) for positive and negative regimes of the PDO and AMO. (A) Positive PDO, negative AMO. (B) Negative PDO, negative AMO. (C) Positive PDO, positive AMO. (D) Negative PDO, positive AMO.