The North American Monsoon (NAM) is an integral part of the summertime climate over the United States. Thus, inter-annual variability of warm-season precipitation in the United States is determined to some extent by inter-annual variability of the NAM. Aside from its direct effect on precipitation in the southwestern US, the monsoon produces an out of phase relationship between southwestern and central US precpitation. That is, onset of monsoon precipitation over the southwestern US is associated with a seasonal decrease in rainfall over the central US.

We will present results from an intercomparison of regional climate models (RCMs), which nest a LAM within a global climate model, that examines whether the NAM is well simulated by RCMs. A model intercomparison provides an analysis framework for isolating systematic model errors and quantifying model uncertainty. The RCMs in this intercomparison have completed simulations with 52-km node spacing in domains that were centered on the United States during 1 July 1987 through 31 December 1994. Data from the NCEP/DOE Reanalysis-II were used for atmospheric and soil boundary, and SST boundary data were obtained from the Reynold's OI SST. Results to date indicate that RCMs are generally capable of capturing the main features associated with monsoon onset but that there are some inter-model differences in the details of the results (e.g., precipitation amount).