The Sensitivity of the North American Monsoon to Gulf of California Sea Surface Temperatures

Abstract The sensitivity of North American Monsoon rainfall to sea surface temperatures (SST) in the Gulf of California (GoC) is assessed using a set of convection-permitting regional climate models. Simulations are performed from May through September 1994 and consist of one control run and two experimental runs that positively and negatively perturb SSTs exclusively within the GoC. Averaged across the season, rainfall increases by 23% in response to spatial mean warming of 1.4 K for the GoC. The direct conversion of GoC moisture into precipitation accounts for 19% of this total increase in rainfall. Increases in rainfall early in the season are more strongly supported by changes in mean thermodynamics that favor stronger moisture flux convergence. Additionally, enhanced inland moisture transport from the GoC increases the amount of convective available potential energy across the region and allows for stronger vertical convective fluxes that intensifies precipitation. A secondary mean circulation response also favors greater precipitation and is related to the advection of cooler air off the mountains of the Sierre Madre Occidental over the GoC. This cooler air mass lowers geopotential heights and drives a cyclonic wind anomaly that favors enhanced flow from the GoC towards the Sierre Madre Occidental mountain range. Later in the season, comparatively smaller increases in rainfall are attributed to increases in local terrestrial evaporation that serve as a positive feedback to initial increases in rainfall driven by perturbed SSTs.