From our partners at Earth Gauge:

 United States 2012 Drought

Drought is a cycle largely driven by changes in long term averages of sea surface temperatures in remote locations around the world. Dust can amplify this cycle.

As of August 14, 2012, 61.77 percent of the contiguous United States was under drought conditions, a slight decline from the record 63.9 percent set on July 24. During July, the percent of the country in extreme to exceptional drought conditions more than doubled. The epicenter of the drought is in the southern Midwest and southern Great Plains regions, where months of record breaking heat beginning in March and below normal spring precipitation created these record drought conditions. What are the fundamental drivers of drought?

1. Drought is a “vicious,” self-reinforcing cycle. Much of the summertime rainfall in the Midwest and Great Plains is convective “recycled” rainfall that originates from evaporated soil moisture. Drier soils at the beginning of summer make precipitation less likely. The lack of water in the soil also means that the soils absorb heat and warm more quickly, helping to wring yet more moisture out of the soils. For more examples of “vicious” or amplifying feedback loops, visit Climate Concepts: Analogies and Useful Descriptions: http://www.earthgauge.net/2011/system-dynamics#feedbackloop.

1. North America is sensitive to drought induced by “top-down,” large-scale sea surface temperature (SST) Patterns. A warm tropical North Atlantic weakens the North Atlantic Subtropical High (NASH) over the Gulf of Mexico and the Southeast United States, allowing drier northerly and southwesterly winds to blow into the Great Plains region, limiting warm season precipitation. North American precipitation is strongly influenced by the eastern tropical Pacific. Cool (La Niña) conditions there during the spring of 1988 supported an upper-air high pressure system that sat for weeks over the Midwest, causing lack of rainfall. Midwest summertime rainfall in 1988, which is in large part recycled spring rainfall, was almost non-existent, leading to a major drought. For a map of the optimal tropical sea surface temperature anomalies for drought in corresponding regions in North America, visit http://www.earthgauge.net/2012/climate-fact-united-states-2012-drought.
3. 3.     “Bottom-up” local conditions can reinforce drought. An initial drying of a region caused by sea surface temperature shifts can dry soil to the point where large amounts of dust are lifted aloft. This dust can block sunlight and reduce surface heating and associated convection, reducing rainfall and exacerbating the dry conditions. This is likely the mechanism that drove the “Dust Bowl” of the 1930s.

The optimal tropical sea surface temperature anomalies for drought in corresponding regions in North America (red areas are warmer than average, blue are cooler). Image: Shin et al, 2010.

(Sources: Svoboda, M et al. “Drought Reinforcing Drought in the U.S. Southern Plains.” ClimateWatch 16 August 2012. Accessed Online 19 August 2012 <http://www.climatewatch.noaa.gov/image/2012/drought-reinforcing-drought-in-the-u-s-southern-plains> and NOAA: State of the Climate. Accessed Online 19 August 2012 <http://www.ncdc.noaa.gov/sotc/>  and Cook, BI et al. “Forced and unforced variability of twentieth century North American droughts and pluvials.” Climate Dynamics 37 (2011): doi:10.1007/s00382-010-0897-9 and Feng, S et al. ”Influence of Atlantic sea surface temperature on persistent drought in North America.” Climate Dynamics 37 (2011): 569-586 and Shin, S et al. “Optimal Tropical Sea Surface Temperature Forcing of North American Drought.” Journal of Climate 23 (2010): 3907-3917)