Anatomy of an Extreme Event: The 2011 Texas Heat Wave and Drought

The record-setting 2011 Texas heat wave/drought is examined to identify key physical processes, ascertain underlying causes, and determine predictability. Observations and climate models are used to assess the role of specific factors contributing to the extreme magnitude of the event and the probability of its occurrence in 2011. While the heat wave magnitude (2.9 K above the 1981-2010 normal) was nearly a factor of two larger than the previous summertime record, it was within the range of unforced model variability, suggesting that natural processes alone can produce such an extreme heat wave. The principal physical factor contributing to the 2011 heat wave magnitude was a severe rainfall deficit during antecedent and concurrent seasons related mostly to naturally-varying sea surface temperature (SST) patterns. Natural climate variability, including both slow SST variability and random atmospheric variability, explains about 80% (2.3 K) of the heat wave magnitude and virtually all of the precipitation deficits. About 20% (0.6 K) of the heat wave magnitude relative to the 1981-2010 climatology was attributable to human-induced climate change. This is above and beyond any human-induced contribution to 1981-2010 average temperatures, but in Texas there had been no century-scale temperature increase, making quantitative attribution of the overall human-induced contribution since pre-industrial times difficult.

Multiple factors increased the probability of eclipsing the prior record Texas summer heat wave. Human-induced climate change increased the probability of a new record from approximately 4% during the 1981-2010 reference period to 7% in 2011, while the observed 2011 SSTs increased the probability from approximately 4% to 24%. Two forecast systems initialized in May demonstrate predictive skill in anticipating much of this latter enhanced risk for a Texas heat wave event during summer 2011.