The Role of the Gulf of California in the North American Monsoon

Abstract The role of the Gulf of California (GoC) in the North American monsoon (NAM) is investigated using a global climate model with 50-km horizontal atmospheric resolution and prescribed SSTs. Specifically, two 135-yr simulations are compared to quantify the influence of the GoC on the NAM: in the first simulation a realistic representation of the GoC is incorporated, while in the second simulation the GoC is replaced with land surface. The results suggest that the GoC has a significant impact on circulation, with cooler surface air temperatures and lower surface friction allowing for south-southeasterly surface flow along the entire length of the GoC, in turn increasing low-level moisture fluxes into the NAM region. Cooler air over the GoC also leads to lower heights at 700–500 hPa, with a corresponding cyclonic moisture flux anomaly, further increasing moisture fluxes into the NAM region. Correspondingly, precipitation is substantially higher over the NAM region and even east of the Continental Divide in areas such as New Mexico and the U.S. Great Plains. July/August precipitation with a realistic GoC is generally 25%–50% greater in northwestern Mexico than the land-filled case, with precipitation 50% greater in much of the southwestern United States. Due to enhanced surface evaporation, areas with increased precipitation also tend to have lower surface temperatures, higher sea level pressure, and lower mid- to upper-tropospheric heights, thus altering the large-scale circulation. These results highlight the importance of the GoC in the NAM and demonstrate the necessity of resolving the GoC in climate simulations. Significance Statement This paper seeks to improve our understanding of the North American monsoon, an important moisture source in the relatively arid region of northwest Mexico and the Southwest United States. Specifically, we investigate the role of the Gulf of California: a long, narrow sea surrounded by elevated topography within the North American monsoon region, oriented roughly southeast–northwest. The results suggest the Gulf of California is an important component of the North American monsoon system. We find that the Gulf of California acts to enhance moisture transports into the North American monsoon, leading to substantially higher precipitation over a large region. Therefore, the Gulf of California should be carefully considered in climate simulations and future projections, especially given it is often not well resolved.