Daily Archives: November 2, 2012

Severe droughts in North America are nothing new….

Cook, BI, Seaer, R., Miller, RL, and Mason, JA. Intensification of North American megadroughts through surface and dust aerosol forcing. Journal of Climate 2012 doi: http://dx.doi.org/10.1175/JCLI-D-12-00022.1

Abstract: During the late Medieval Climate Anomaly (MCA, 1100-1500 C.E.), much of western North America experienced recurrent periods of drought spanning decades or longer. These ‘megadroughts’ had exceptional persistence compared to more recent events and many climate models have difficulty reproducing droughts of similar duration. We conduct a suite of general circulation model experiments to test the impact of reconstructed sea surface temperature (SST) and land surface forcing during the megadroughts. The land surface forcing is conceptualized as a set of dune mobilization boundary conditions, derived from available geomorphological evidence and modeled as increased bare soil area and a dust aerosol source over the Central Plains (105oE–95oE, 32oE–44oE). Cold tropical Pacific SST forcing and other MCA radiative forcings (solar, volcanoes, etc) stimulate warming and drying over the Central Plains, but can not reproduce the persistence of the megadroughts. In our simulation with additional forcing from dust aerosols, the dust increases the shortwave planetary albedo, reducing energy inputs to the surface and the planetary boundary layer. This energy deficit increases atmospheric stability, inhibiting vertical movement and convection which reduces cloud cover and subsequent precipitation over the Central Plains. To compare against available drought reconstructions for the MCA, we calculated the Palmer Severity Drought Index (PDSI) from our model runs. Droughts simulated in our scenario with dust aerosols have significantly longer persistence time scales than the model experiments with only SST and bare soil forcing. Specifically, inclusion of dust aerosols extends significant autocorrelation out to a lag of 3 years, matching the persistence of tree ring reconstructed estimates of PDSI during the MCA. Results from this study provide the first model based evidence that dust aerosol forcing and land surface changes can explain the persistence of the MCA megadroughts, although uncertainties remain in the formulation of the boundary conditions and the future importance of these feedbacks as western North America moves into an anthropogenically warmer and drier world.

Click here for the full paper (fee).