The Influence of CO2 Forcing on North American Monsoon Moisture Surges and Precipitation Intensity in the Southwestern U.S. in a 50-km-Resolution General Circulation Model

It is well known that widespread convective bursts in the southwestern United States during the North American monsoon can be triggered by Gulf of California moisture surges (GoC surges). The surge-related precipitation is, however, directly influenced by large-scale mid-tropospheric conditions and can vary over a wide spectrum of intensity, ranging from drier- to heavier-than-average. How GoC moisture surges, and associated precipitation, will change in response to CO2-induced warming remains less understood, not least because state-of-the-art climate models do not currently resolve the relevant mesoscale dynamics. In this study, a 50-km resolution global coupled model (FLOR) is used to address this question. The FLOR model has a realistic representation of the mean synoptic patterns associated with GoC moisture surges characterized by different precipitation amounts and of the typical intensity of surge/non-surge precipitation. When CO2 concentration is doubled in this model, the mean number of GoC surge events per monsoon season (approximately 15) remains unchanged, but intermediate-to-high intensity surge-related precipitation tends to become less frequent, thus drying the monsoon. However, no evidence is found for a decrease in the number of surge-related very heavy precipitation events above 99th percentile. CO2 doubling also induces nonuniform changes in the mean mid-tropospheric geopotential height, which lead to a monsoonal ridge displaced to the west of its present-day climatological maximum and hence an environment less conducive to enhanced convective activity. Convective rainfall not occurring during surges days is less influenced by changes in large-scale mid-tropospheric conditions, suggesting terrain-locked convection may become increasingly dominant in the future over the Southwest elevated terrains.