The University of Arizona Desert Laboratory Past Research Highlights

Pacific and Atlantic Ocean Influences on Multidecadal Drought Frequency in the U.S.

Greg McCabe, USGS, WRD, Denver CO
Mike Palecki, Midwestern Regional Climate Center, Champaign IL
Julio Betancourt, USGS, Desert Laboratory, Tucson AZ

The U.S. climate of the last century was marked by three prolonged continental-scale wet spells (1905-1930, the 1940s, and 1976-1995) and three dry spells (the 1930s, 1950s-60s, and 1996-2004). Such large and sustained shifts in U.S. precipitation are thought to originate in low-frequency variability in ocean temperatures. Two important modes of decadal to multidecadal ocean variability are the Pacific Decadal Oscillation, or PDO, and the Atlantic Multidecadal Oscillation, or AMO.

The Pacific Decadal Oscillation, or PDO index, reflects the spatial pattern of temperatures in North Pacific Ocean. When the PDO is positive, there is colder water in the central and western Pacific and warmer waters in the eastern Pacific; under negative PDO, the reverse is true. Such negative and positive PDO "events" tend to last 20-30 years and have been associated with regional temperature and precipitation trends across the U.S. PDO variability is thought to be linked to the frequency and magnitude of El Niño or La Niña events over the course of decades.

The Atlantic Multidecadal Oscillation, or AMO index, represents annual ocean temperature anomalies averaged across the North Atlantic (0-70° N). Its association with U.S. climate is less well known, but recent studies suggest that the AMO affects summertime precipitation and could also regulate the strength of El Niño/La Niña effects on weather year around, particularly in the Midwest (Enfield et al. 2001).

McCabe et al. (2004) recently decomposed U.S. drought frequency into its principal modes of temporal and geographic variability without a priori consideration of PDO or AMO effects. Drought was defined as annual precipitation that fell into the lowest 25 percent of all years, and drought frequencies were calculated for 20-year moving windows for all 344 climate divisions in the conterminous U.S. Principal components analysis of the 344 time series of drought frequency identified three independent modes of drought frequency. The time series of the two leading modes closely track PDO and AMO variations, as do maps of the statistical correlations between drought frequency and PDO and AMO.

This statistical study shows that large-scale droughts in the U.S. tend to be associated with positive AMO, or the kind of warming over the North Atlantic that occurred in the 1930s, 1950s and since 1995. Wet conditions prevail over most of the country during North Atlantic cooling. Positive PDO conditions tend to shift drought areas to the Northern Rockies and Pacific Northwest, whereas negative PDO conditions generally shift drought to the Southwest and central Plains. Although droughts remain largely unpredictable, there is concern that the current drought in the West could persist due to continued North Atlantic warming, although the focal region of the drought may shift with the more variable North Pacific sea surface temperatures.

Time series of the PDO and AMO.
Time series and complimentary modes of the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). Note -PDO, +AMO modes in the 1950s and 1999-2003, when severe drought gripped much of the western U.S.

Map of drought frequencies in the USA.
Areas of high (red > 25%) and low (blue < 25%) drought frequencies associated with complimentary modes of the PDO and AMO. Note the greater extent of U.S. drought associated with warming in the North Atlantic Ocean.