Jun 5, 2014

Atmosphere and Ocean Origins of North American Droughts

Richard Seager and Martin Hoerling
American Meteorological Society
  • Examines the atmospheric and oceanic causes of North American droughts using observations and ensemble climate simulations
  • Models indicate that oceanic forcing of annual mean precipitation variability accounts for up to 40% of total variance in northeastern Mexico, the southern Great Plains, and the Gulf Coast states but less than 10% in central and eastern Canada
  • Observations and models indicate robust tropical Pacific and tropical North Atlantic forcing of annual mean precipitation and soil moisture with the most heavily influenced areas being in southwestern North America and the southern Great Plains
  • Finds that in the southwestern North America and the southern Great Plains, individual wet and dry years, droughts, and decadal variations are well reproduced in atmosphere models forced by observed SSTs
  • Finds that oceanic forcing was important in causing multiyear droughts in the 1950s and at the turn of the twenty-first century, although a similar ocean configuration in the 1970s was not associated with drought owing to an overwhelming influence of internal atmospheric variability
  • Finds that up to half of the soil moisture deficits during severe droughts in the southeast United States in 2000, Texas in 2011, and the central Great Plains in 2012 were related to SST forcing, although SST forcing was an insignificant factor for northern Great Plains drought in 1988
  • Finds that during the early twenty-first century, natural decadal swings in tropical Pacific and North Atlantic SSTs have contributed to a dry regime for the United States
  • Concludes that long-term changes caused by increasing trace gas concentrations are now contributing to a modest signal of soil moisture depletion, mainly over the U.S. Southwest, thereby prolonging the duration and severity of naturally occurring droughts