A 15-year numerical simulation of the geographical sources of water for precipitation in the North American Monsoon is examined. The simulation is performed with the NASA Data Assimilation Office (DAO) Finite Volume General Circulation Model (FVGCM). The spatial resolution of the model is 1° x 1.25° horizontally and 32 levels in the vertical. The dynamical core is the Lin-Rood finite volume semi-lagrangian model, and the atmospheric physics are from the NCAR Community Climate Model (version 3). Twenty-two passive tracers are implemented to simulation the water vapor integrated pathway, from the evaporative source to the precipitation sink.

Results indicate that the model is able to reproduce the observed onset signature, namely an increase in precipitation over western Mexico and decrease in precipitation over the southern Great Plains. Evaluation of the geographic sources of water for precipitation indicates that the fraction of water from the tropical Atlantic Ocean (including the Gulf of Mexico and Caribbean Sea) increases across Mexico and the southern United States. The local continental sources of water from the southern Great Plains decrease substantially during onset. However, the local continental source in Western Mexico maintains its contribution to precipitation. This indicates that the local source of water (namely, continental evapotranspiration) is critical to the intensity of the precipitation during the onset of the monsoon, as indicated in the model simulation. Further evaluation of the 15-year simulation, such as interannual variability and atmospheric circulation, will also be presented.