Could U.S. extreme droughts have been anticipated?—A NASA NEWS initiative on extremes

The 2012-2013 drought may not have been predictable as based on current schemes employed for such purposes, but it may have been anticipatable due to knowledge of key precursors such as favorable (remote) SST patterns, and reduced regional soil moisture and winter snow packs. A working group was assembled under the NASA Energy and Water cycle Study (NEWS) to examine the extent to which the 2012 drought could be anticipated and to put recent severe droughts in perspective.

A recent NOAA report analyzing the drought of 2012 in the central US has concluded that the drought was not inherently predictable, representing a very anomalous atmospheric circulation pattern. It is important to recognize that this ‘predictability' is based on what happened in the atmosphere, and further, depends on the capabilities of the predictive schemes currently employed. The current prediction schemes emphasize the role of the large-scale atmospheric circulation, but the extent to which the long wave patterns and subsequent short wave effects can be predicted in advance remains unclear. These schemes generally lack full consideration of the local surface state, especially the effect of precursor anomalies in key elements such as soil moisture and snow pack. It is also not clear how well they account for the effects of either interannual or lower-frequency oceanic anomaly patterns.

The role of the aforesaid precursors, combined with knowledge of their state, allow some assessment of the ‘likelihood' of drought that is not currently being considered. For example, by late winter of 2012 much of the central US was already experiencing dry conditions, including reduced soil moisture, and the snowpack in the Rockies was well below normal. SST patterns appear to have been largely neutral. While the manifestation of the resultant drought also critically dependent on the large-scale atmospheric circulation that subsequently developed, it is clear that the region was preconditioned towards being dry. The other factor about precursors of the 2012 drought is the previous ‘Texas drought' in 2011, which was stronger than the 2012 one in the ‘exceptional' category. This feature reflects the different scales in the atmospheric teleconnection pattern and the comparison of the two events might help determine the soil moisture impact of 2011's intensive drought on 2012's widespread drought.

Our hypothesis is that even if one cannot predict the future atmospheric circulation patterns with much certainty for a given year, we may still be able to make some assessment of whether or not a drought may be likely to occur. We refer to this as anticipating drought. As precursors such as soil moisture and snowpack become important in potentially enhancing and prolonging the drought as it occurs, the actual drought that does subsequently occur will depend closely in magnitude and duration on the atmospheric circulation that unfolds. These factors will be discussed.