Feb 20, 2017

Influence of the Ocean and Greenhouse Gases on Severe Drought Likelihood in the Central United States in 2012

David E. Rupp, Sihan Li, and Philip W. Mote
Journal of Climate
  • Investigates the impacts of sea surface temperature (SST) anomalies and anthropogenic greenhouse gases on the likelihood of extreme drought occurring in the central United States in the year 2012 using large-ensemble simulations from a global atmospheric climate model
  • Conducts two sets of experiments
  • The first compares the simulated hydroclimate of 2012 to a baseline period (1986–2014) to investigate the impact of SSTs
  • The second compares the hydroclimate in a world with 2012-level anthropogenic forcing to five “counterfactual” versions of a 2012 world under preindustrial forcing
  • Finds that SST anomalies in 2012 increased the simulated likelihood of an extreme summer precipitation deficit (e.g., the deficit with a 2% exceedance probability) by a factor of 5
    • In other words, in 2012, the likelihood of an extreme summer precipitation deficit as extreme as that observed in the central US increased due to sea surface temperature anomalies from being a 1 in 50 year event to a 1 in 10 year event
  • Finds that the likelihood of an extreme summer soil moisture deficit increased by a similar amount, due in great part to a large spring soil moisture deficit carrying over into summer
  • Finds that an anthropogenic impact on precipitation was detectable in the simulations, doubling the likelihood of what would have been a rainfall deficit with a 2% exceedance probability under preindustrial-level forcings
    • In other words, in 2012, human-caused greenhouse gas emissions doubled the likelihood of a 1 in 50 year rainfall deficit to 1 in 25 years in the central US
  • Finds that despite this reduction in rainfall, summer soil moisture during extreme drought was essentially unaffected by anthropogenic forcing because of:
    1. evapotranspiration declining roughly one-to-one with a decrease in precipitation due to severe water supply constraint and despite higher evaporative demand and
    2. a decrease in stomatal conductance, and therefore a decrease in potential transpiration, with higher atmospheric CO2 concentrations