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

Gulf of California moisture surges (GoC surges) are transient disturbances that propagate northward along the GoC, transporting lower-level moisture into the southwestern United States during the summertime North American monsoon. They are associated with periods of enhanced convection in northern Mexico and the southwest United States. The intensity of precipitation following a GoC surge can vary over a wide spectrum, going from drier-than-average to extremely intense and persisting events.

In this study we use a 50 km-horizontal resolution global coupled model (FLOR) developed at the NOAA Geophysical Fluid Dynamics Laboratory and featuring a realistic simulation of the GoC surges. We evaluate the model’s ability to reproduce the intensity of precipitation during GoC surge and non-surge periods in present and doubled CO₂ climatic conditions. We find that the mean number of GoC surge events per monsoon season (i.e., approximately 15) is not significantly affected by CO₂ forcing. Nevertheless, when SST biases are minimized through flux adjustment, FLOR predicts a reduction in monsoonal precipitation over the southwestern United States. Our simulations further suggest that surge-related rainfall adjusts towards lower and higher percentiles, while becoming less important at intermediate values. Convective precipitation not occurring during GoC surges is instead not coherently affected by doubled CO₂. Finally, the influence of CO₂ forcing on the large-scale drivers of monsoonal precipitation during GoC surge events, such as the position of the monsoonal ridge, is investigated and related to precipitation changes