Background
The authors write that "climate records derived from tree core measurements (e.g., Stahle et al., 2007; Cook et al., 2004; Woodhouse et al., 2006) or vegetation growth in lake beds (e.g., Stine, 1994) from the past millennia indicate that the southwestern United States is susceptible to severe impacts from multi-decade droughts ("megadroughts"; Woodhouse and Overpeck, 1998; Meehl and Hu, 2006)." And they say that "the potential economic and social cost of such intense and sustained events such as these motivates the need to understand the mechanisms for drought variability and persistence," which led them to conduct the research described herein.

What was done
Langford et al. examined the mechanisms responsible for decadal precipitation variability in 47 global climate model historical simulations that were performed for phase 5 of the Coupled Model Intercomparison Project (CMIP5).

What was learned
With respect to problems they identified, the three U.S. researchers report that (1) "the CMIP5 models have higher climatological precipitation in southwestern North America than reanalysis products," that (2) "shortcomings in summer precipitation in southwestern North America are more severe than for winter precipitation," and that (3) "climatological winds along the Gulf of California in July are misrepresented in CMIP5 models." On the other hand, they report that "robust coincident anomaly patterns in the tropical and North Pacific Ocean and low-frequency (5 yr) winter California precipitation exist in the CMIP5 historical simulations," but they say that these associations are "unable to explain more than 20% of the decadal variability."

What it means
In discussing the significance of their findings, Langford et al. conclude that "the small fraction of explained variance will limit the predictability of precipitation associated with the decadal variability and persistence offered by the ocean," which means that it's back to the drawing board - or in this case to the super computers - to try yet again to get the models to where they need to be in order to do any real good.

References
Cook, E., Woodhouse, C., Eakin, C., Meko, D. and Stahle, D. 2004. Long-term aridity changes in the western United States. Science 306: 1015-1018.

Meehl, G. and Hu, A. 2006. Megadroughts in the Indian monsoon region and southwest North America and a mechanism for associated multi-decadal Pacific sea surface temperature anomalies. Journal of Climate 19: 1605-1623.

Stahle, D., Fye, F., Cook, E. and Griffin, R. 2007. Tree-ring reconstructed megadroughts over North America since A.D. 1300. Climatic Change 83: 133-149.

Stine, S. 1994. Extreme and persistent drought in California and Patagonia during mediaeval time. Nature 369: 546-549.

Woodhouse, C. and Overpeck, J. 1998. 2000 years of drought variability in the central United States. Bulletin of the American Meteorological Society 79: 2693-2714.

Woodhouse, C., Gray, S. and Meko, D. 2006. Updated streamflow reconstructions for the Upper Colorado River basin. Water Resources Research 42: 10.1029/2005WR004455.