1. Introduction

During the period July through September, northwestern Mexico receives about 60% of its total annual rainfall, while Arizona and New Mexico receive about 35% and 45% of their annual rainfall, respectively (Higgins et al. 1999). This precipitation pattern is often referred to as the Mexican or North American (NA) monsoon. The cause of the monsoon was believed by some to result from moist air advected from the Gulf of Mexico and the Caribbean, due to the change in mid-tropospheric winds at the onset of the monsoon, from westerly to easterly (Bryson and Lowrey 1955; Sellers and Hill 1974). Others (Reitan 1957; Hales 1974; Brenner 1974; Carleton 1986; Douglas et al. 1993; Douglas 1995; Stensrud et al. 1995) have argued and supplied persuasive evidence that the monsoonal moisture source is the Gulf of California and/or the tropical east Pacific. The recent review article by Adams and Comrie (1997) states that "there is general agreement that the bulk of monsoon moisture is advected at low levels from the eastern tropical Pacific Ocean and the Gulf of California". In recent work (Mitchell and Brown 1996; Brown and Mitchell 1997; Mitchell et al. 1999), it was suggested that the northward evolution of the 29°C SST isotherm into the Gulf of California (henceforth referred to as "the gulf") may govern the northward propagation of convection and rainfall. Moreover, the modeling study of Stensrud et al. (1995) revealed that for the monsoon to be successfully simulated, exceptionally warm water (29.5°C) in the gulf must be present. As discussed below, the warming of gulf SSTs may be partially due to a coastal warm current during Spring/Summer (Collins et al. 1997; Castro et al. 1994).

Recent work (Higgins et al. 1997; Higgins et al. 1998) suggests the Mexican monsoon exerts a strong influence on summer precipitation patterns over the contiguous United States, with strong monsoons well correlated with summer droughts in the midwest, and somewhat correlated with relatively wet summers in the southeastern United States. If the NA monsoon depends on SSTs in the Gulf of California, a relatively small body of water, then it is not surprising that current-generation global and regional climate models have difficulty in predicting the monsoon rainfall amounts, as well as NA summer precipitation in general.

The objective of this study was to determine whether the onset and subsequent evolution of monsoon rainfall was consistent with a postulated dependence on evolving gulf SSTs. This entails (1) determining whether monsoon rainfall is predicated by a threshold SST, where rainfall occurs only after local SSTs exceed this value, and (2) determining relationships between gulf SST increases and subsequent rainfall amounts. The next section describes the seasonal evolution of the monsoon region, including SSTs, convection, the gulf circulation, and wind fields, and offers some mechanistic understanding. Section 3 describes the methodology and the results of this study. Oceanic processes related to gulf SSTs are discussed in Section 4. Conclusions are given in Section 5.