A time series showing the percent of the country meeting the above drought criteria is presented in Figure 1. As illustrated there, it is clear that the frequency and magnitude of droughts are not increasing. In fact, they have decreased over the past century.

Taking their findings a step further, Mo and Lettenmaier next filtered the data presented in Figure 1 to examine severe drought events that covered 50% of the continental USA for a period of six months or longer. Analyses of these data revealed that (1) severe drought events were chiefly located in the central USA, (2) "the 2012 drought event was not unique," considering that there were "16 great drought events in the 98 years of record [they] analyzed," (3) "great droughts occurred less often, and events were less severe as time progressed" and (4) all but two of the 16 great drought events occurred in the first half of the record.

To accomplish their objective Mo and Lettenmaier used "gridded observed precipitation and reconstructed total moisture percentiles and runoff from four land surface models" to develop an integrated drought index (IDI) for defining drought in the continental USA over the period 1916-2013. Using this resulting index, drought periods were then defined as having an IDI value less than 0.3.

With respect to the potential cause(s) of the 16 great drought events, Mo and Lettenmaier detected a relationship between the Atlantic Multidecadal Oscillation (AMO) and drought (there was a higher percentage of drought when the AMO was in a positive phase) and between ENSO and drought (12 of the 16 great drought events occurred during cold La Niña years). However, droughts did not always occur when the AMO was in a positive phase or when La Niña conditions existed.

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