The Change of the North American Monsoon Seasonal Precipitation in the CCSMv4 under IPCC CO2 Emission Scenarios

The North American monsoon (NAM), characterized by distinct seasonal precipitation over western Mexico and the Southwestern United States, is a summertime phenomenon that depends on complex interactions between the Pacific Ocean, Gulf of Mexico, and the North American land mass. Thus, the NAM is strongly influenced by the El Niño Southern Oscillation, a dominant mode of interannual Pacific sea surface temperature (SST) and atmospheric variability, as well as the North Pacific Oscillation, a low-frequency (decadal) Pacific variation. This study assesses present day and projected changes in the NAM precipitation on a yearly and seasonal basis. Observations from the NCEP-NCAR Reanalysis project are compared to the Community Climate System Model version 4 (CCSM) from 1980 to 2000. Spatial patterns agree well, but still show an overestimation in precipitation within the NAM region. Fifteen CCSM ensemble runs, for various IPCC AR4 emission scenarios (A1, B1, and constant CO2), are assessed within each specific scenario and averaged, for comparisons between 1980-2000 and 2080-2100. In the NAM region we find yearly and seasonal decreases in precipitation and increases in temperature for all IPCC emission scenarios. Our analysis further finds statistical significance to the differences in mean precipitation and temperature over the NAM region, due in part to different levels of CO2 in the atmosphere.