Summer monsoon rains are a critical source of water for the southwestern United States, accounting for over half the total precipitation in some areas, yet the source(s) of this moisture remains contentious. The conventional explanation is that southeasterly upper-air flow brings moisture from the Gulf of Mexico. However, some researchers have questioned this. For example, Hales (1972) suggested that the major source of moisture in northwestern Mexico is the tropical Pacific Ocean; Carleton (1986) found that episodes of extensive cloudiness during the monsoon season were associated with strong southwesterly flow from the Gulf of California and the eastern Pacific; while Stensrud et al. (1995) proposed that under certain conditions low-level moisture from the Gulf of California is the dominant source for the southwestern United States – during the so-called “gulf surge”. Finally, vertically-integrated, time-averaged fluxes of water vapor for July and August (Schmitz,1995) indicate that upper-level moisture arrives from the Gulf of Mexico, while low-level moisture originates from the northern Gulf of California, i.e., that moisture arrives simultaneously from two sources. Thus there are at least three potential source regions (Gulf of Mexico, Gulf of California, eastern Pacific), or a combination of them, that could be responsible for monsoon rains.

One potential means for identifying the source region(s) is through analysis of the stable isotopic composition (¹⁸O and ²H) of regional sea water, water vapor and precipitation. Since there is practically no information on the isotopic composition of the Gulf of California, or on the isotopic composition of southwestern water vapor (as opposed to precipitation), we conducted a pilot sampling campaign during a surge event in 1998 collecting samples at La Paz, near the southern tip of the peninsula; at Puerto Peñasco, near the northernmost tip of the Gulf of California; and downwind at Yuma and Tucson, AZ. Delta ¹⁸O and ²H data will be reported and the results of the pilot study will be discussed.