Research links long droughts in U.S. to ocean temperature variations

Large-scale, long-lasting droughts in the United States – such as the present one in the West — tend to be linked to warmer than normal sea surface temperatures in the North Atlantic Ocean, and not just cooling in the tropical Pacific, according to a USGS study published today in the Proceedings of the National Academy of Sciences.

The study statistically associates the patterns of U.S. droughts during the last century to multi-decade variations in North Pacific and North Atlantic sea surface temperatures, said USGS lead author Gregory McCabe and his co-authors, USGS scientist Julio Betancourt and Mike Palecki of the Midwestern Regional Climate Center at the Illinois State Water Survey.

Although droughts remain largely unpredictable, McCabe suggests that “this research, as well as that of others, “increases concern that the current drought in the West could persist due to continuing above normal North Atlantic sea surface temperatures.” The focal region of the drought may shift with the more variable North Pacific sea surface temperatures, he said.

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 1930’s, 1950s-60s, and 1996-2004). Although researchers believe that such large and sustained shifts in U.S. precipitation are linked with the natural variability of sea surface temperatures, the mechanisms are not well understood and cannot yet be used to help predict the likelihood of droughts.

These sea surface temperature variations are characterized by climatic indices dubbed the Pacific Decadal Oscillation, or PDO, and the Atlantic Multidecadal Oscillation, or AMO. The Pacific Decadal Oscillation, or PDO index, reflects the geographic pattern of temperatures in the North Pacific Ocean and the AMO reflects basinwide temperatures in the North Atlantic Ocean. When the PDO is positive, temperatures of the central North Pacific are cool and the eastern tropical Pacific Ocean is warm. When PDO is negative, the central North Pacific is warm and the eastern tropical Pacific is cool. A positive AMO indicates warm temperatures across the entire North Atlantic Ocean and negative AMO indicates cool North Atlantic temperatures.

Both negative and positive PDO “events” in the North Pacific Ocean tend to last 20-30 years, with recent research increasingly associating these events with regional temperature and precipitation variability across the country. For example, most scientists think the PDO variability is linked to changes in the frequency and duration of El Niño or La Niña events over the course of decades.

The AMO association with U.S. climate is less well known, but recent studies suggest that variation in water temperatures in the North Atlantic affects summertime precipitation and could also regulate the strength of El Niño/La Niña effects on weather year-round, particularly in the Midwest.

The statistical study was aimed at delineating temporal and geographical variations in drought frequency and then correlating these variations with indices of Pacific and Atlantic Ocean climate . The researchers were able to correlate two of the three leading modes of drought frequency with PDO and AMO variations. The other pattern of variability, the researchers suggest, may represent a complex pattern of trends in drought frequency related to increasing Northern Hemisphere temperatures, or some other as-yet unidentified climate trend.

In general, McCabe and his coauthors suggest that large-scale droughts in the United States are likely to be associated with positive AMO — the kind of warming of sea surface temperatures that occurred over the North Atlantic in the 1930s, 50s, and since 1995.

In contrast, wet conditions prevail over most of the country during North Atlantic cooling (negative AMO). The researchers found that cool waters in the central North Pacific are associated with drought in the Northern Rockies and Pacific Northwest, whereas warm waters in the central North Pacific are generally associated with drought in the Southwest and central Plains.

The researchers note that the best hope for predicting long-term droughts seems to lie with identifying precursor states in oceanic climate that could lead to drought.

The authors noted that persistent and widespread droughts can potentially compromise crop and livestock production, revenues from outdoor recreation and tourism, international and interstate water agreements, sustained urban growth, management of wildland fires, and even conservation efforts nationwide.

“What we hope to do eventually,” said Betancourt, “is use the information on the relationship between sea surface temperatures and North American climate to help resource managers and policy makers guide the country in more effective and long-term water management strategies and to anticipate climatic effects on ecosystems.”

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