The North American Monsoon System (NAMS) is intimately linked to the large scale distribution of summer moisture in the western U.S. An inverse relationship exists between precipitation in the Great Plains and Southwest U.S. due to the formation of a monsoon ridge over the Colorado Plateau in late July and early August. Interannually, Castro et al. (2001) showed that the positioning of the monsoon ridge and NAMS onset are modulated by the Pacific Transition and East Pacific teleconnection patterns related to tropical and North Pacific SSTs, respectively. The present work extends this earlier observational study to understand the possible differences in evolution of NAMS hydrometeorological features resulting from different Pacific SST regimes.

The Regional Atmospheric Modeling System (RAMS) is employed over the course grid (100 km) domain of North America with a nested grid (33 km) domain over the NAMS region. The ability of RAMS to capture NAMS diurnal and seasonal variability is evaluated, with particular attention to the evolution of the monsoon ridge, Great Plains and Baja low-level jets, and convectively generated precipitation. As part of this model validation, model differences using the Kain-Fritch convection scheme are compared to using the Kuo convection scheme. Years of different Pacific SST distributions are simulated, such as 1988 and 1993, to investigate interannual differences in NAMS evolution, focusing on the time of monsoon onset. These include, for example, the frequency and intensity of Gulf surge episodes and the distribution of upper and low level moisture sources. Future sensitivity experiments incorporating RAMS nested within the NCAR CCM3 GCM will also be presented.