Southwestern United States Summer Monsoon Source—Gulf of Mexico or Pacific Ocean?

Abstract

Moist air from the tropics moves into northwest Mexico and the southwestern portion of the United States during the early part of the summer season. This is analogous to the monsoon circulation in other sections of the globe. The annual change in circulation has been related to the northward and westward development of the large subtropical high pressure system over the southern United States. With this change the moist tropical air over the Gulf of Mexico is believed to be carried across Mexico and into the southwestern United States. The mountains of Mexico provide a formidable barrier for the movement of the Gulf of Mexico moisture westward. It has been shown that half of the precipitable water vapor in the atmosphere over the Arizona deserts in the summer months is below the 800-mb level. This is inconsistent with the idea of the Gulf of Mexico supplying the greatest proportion of the moisture. This paper presents support for the greatest percentage of tropical moisture in the southwest United States and northwest Mexico coming from the Pacific Ocean by way of the Gulf of California rather than from the Gulf of Mexico.