Sea surface temperature can nudge the atmosphere into conditions, subsequently exacerbated or moderated by atmospheric variability, that can lead to drought.
In this paper, authors Richard Seager (Lamont-Doherty Earth Observatory, Columbia University) and Martin Hoerling (NOAA ESRL) examine the causes of North American drought, investigate the predictability of drought, and explore the contribution of climate disturbances to drought.
Atmospheric models were forced using idealized and observed SST values to determine the contribution of tropical SST anomalies to precipitation and soil moisture as opposed to the contribution of atmospheric variability and other factors.
The authors find that up to 40 percent of the variability of annual mean precipitation in the North American south, southwest, and southern Great Plains can be explained by ocean forcing, and conclude that sea surface temperature can nudge the atmosphere into conditions, subsequently exacerbated or moderated by atmospheric variability, that can lead to drought.
The authors also find that the general warming trend over the United States, particularly in the southwest, may lead to a more rapid onset, and delayed resolution, of drought conditions in the future.
This paper is published in the June 2014 issue, and is part of a special collection on Global Drought Information System Worldwide.
Atmosphere and Ocean Origins of North American Droughts
The atmospheric and oceanic causes of North American droughts are examined using observations and ensemble climate simulations. The 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. In these regions, individual wet and dry years, droughts, and decadal variations are well reproduced in atmosphere models forced by observed SSTs. 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. 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. 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. 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.