Multidecadal Megadrought

Megadrought events are thought to have had devastating impacts on past civilizations in the American Southwest and elsewhere; they are more extreme than any droughts seen during the 20th century, and last for multiple decades. I began studying megadrought dynamics during my postdoc in Hawaii, but am now part of a collaborative project between NCAR and the University of Arizona dedicated to the topic. You can find our project website at http://www.megadrought.org/

 
 

Megadrought and Climate Variability over the Last Millennium

There is still a great deal of debate about how external forcing (i.e. influences from changes to solar irradiance, or volcanic eruptions) and coupled atmosphere/ocean variability have contributed to the megadrought variations seen in paleoclimate reconstructions over the last millennium. I have been using the CESM Last Millennium Ensemble to study these questions: because there are so many simulations included in this ensemble, it is possible to build up statistical relationships between forced and internal variations and megadrought properties. 

This work is still in progress, but will be submitted to the Journal of Climate in April 2017.


Climate Variability and Mitigation of Forced Climate Trends

The presence of strong warming trends over the 21st century is expected to complicate the relationship between climate variability and megadrought. Previous work (Ault et al. 2016) showed that the mean trend tends to dominate over changes to variability for the American Southwest, but it is not clear whether this result generalizes to all arid regions worldwide, or how nonlinearities in the relationship between mean trends and variability may complicate things. 

I am using the CESM Large Ensemble to understand the potential for changes to climate variability to mitigate drying/warming trends in the future: this work is also in progress, but will be complete sometime in summer 2017.


Figure 8 from Stevenson et al. (2015), Journal of Climate Comparison of simulated and observed drought properties with an AR-1 null hypothesis. Filled circles indicate the mean value of the drought characteristic in CTL (fully coupled simulation; red), CLIM (simulation with prescribed climatology of sea surface temperature; blue), and the NADA (North American Drought Atlas reconstruction; black). Error bars indicate the 90% confidence interval on the mean generated from 1000 AR-1 samples.

Figure 8 from Stevenson et al. (2015), Journal of Climate
Comparison of simulated and observed drought properties with an AR-1 null hypothesis. Filled circles indicate the mean value of the drought characteristic in CTL (fully coupled simulation; red), CLIM (simulation with prescribed climatology of sea surface temperature; blue), and the NADA (North American Drought Atlas reconstruction; black). Error bars indicate the 90% confidence interval on the mean generated from 1000 AR-1 samples.

Internal Atmospheric Variations and Megadrought

I designed simulations with the CESM in which the ocean temperatures were fixed at a repeating 12-month cycle, to determine how the statistics of megadrought events would be affected by the absence of modes of variability like ENSO. When this was run for 1000 years, we found that megadrought events can indeed occur due to internal atmospheric variability interacting with the land surface, with statistics extremely similar to the coupled control simulation. This means that one must be quite careful in making statements about megadroughts being forced by atmosphere/ocean interactions!
(Stevenson et al. 2015)