Changes in North American Monsoon Precipitation in a Warmer Climate

A monsoon is a system characterized by seasonal changes of wind. Summertime monsoon precipitation is responsible for a significant percentage of annual rainfall for many places, such as sub-Saharan Africa, Northern Australia, Southern Asia, Northwestern Mexico, and the Southwestern U.S. The North American Monsoon (NAM) directly affects life for those living in Arizona, New Mexico, and surrounding states. The NAM provides almost half of the annual rainfall for parts of those states and well over half for northwest Mexico, making it very important for farmers and others who rely on water resources. This gives researchers impetus to understand how the NAM may change with rising temperatures in a future climate and to find ways that they can better predict the extreme events that could occur. This study examined whether the superparamaterized Community Earth System Model (sp-CESM) shows changes in precipitation when levels of carbon dioxide were quadrupled from preindustrial levels. CMORPH precipitation data was used as a reference observational dataset. The monsoon region was defined as 18-33 N and 102-112 W and the ‘monsoon season’ is defined to be the months of July, August, and September. In the 4xCO2 environment, boreal summer NAM rainfall substantially decreases, and the monsoon season becomes ill-defined in precipitation. Modest increases in precipitation may occur to the north of 26° N late in the monsoon season (e.g. October) in the 4xCO2 climate. Such decreases in precipitation would be of significant concern for regions that depend on monsoonal precipitation to provide a large fraction of annual precipitation, and may exacerbate droughts and wildfires. Model biases in the annual cycle of precipitation in the NAM region provide a caveat to the results.