Background
The authors write that "the North American monsoon (NAM) is the regional-scale atmospheric circulation system (Stensrud et al., 1997) responsible for the dramatic increase in precipitation during the summer in northwestern Mexico and the southwest United States (Grantz et al., 2007)," and they say that "understanding the mechanisms that govern the timing and intensity, as well as the impacts of climate change on the NAM, is a priority for the scientific community, watershed managers and farmers in the NAM area," since "the impacts of droughts/floods are devastating."

What was done
In light of the great significance of the NAM system, Cerezo-Mota et al. investigated the degree of realism in its simulation by a major regional climate model (RCM) -- the Hadley Centre Regional Model version 3P (HadRM3P), analyzing the moisture sources of the NAM by employing two different boundary-condition datasets used to drive the model, which allowed them to assess the ability of the RCM to reproduce rainfall under climate-change conditions in the NAM region as predicted by global climate models (GCMs).

What was learned
As a result of their various tests, the three UK researchers determined that "two of the most commonly used GCMs that simulate well the NAM precipitation (HadCM3 and MIROC) do not reproduce correctly the Great Plains low-level jet nor the moisture in the Gulf of Mexico," both of which factors play major roles in the northern portion of the NAM.

What it means
The implication of their results, in the words of Cerezo-Mota et al., is that "precipitation in Arizona-New Mexico would not be correctly represented by a regional model driven by these GCMs." Thus, they write that RCMs driven by the "most commonly used" GCMs "would not give realistic simulations of the current climate of the region and therefore would not offer a realistic projection of climate change of the NAM."

References
Grantz, K., Rajagoopalan, B., Clark, M. and Zagona, E. 2007. Seasonal shifts in the North American monsoon. Journal of Climate 20: 1923-1935.

Stensrud, D.J., Gall, R.L. and Nordquist, M.K. 1997. Surges over the Gulf of California during the Mexican monsoon. Monthly Weather Review 125: 417-437.