SOUTHWESTERN USA

Contemporary climate

The precipitation regime of the southwestern USA (Fig. 5.1: south of latitude 40° N) is a function of the interplay between the westerlies system over the Pacific and the monsoonal system over the Gulf of Mexico. It has two rainy periods. The summer rains result mainly from monsoonal air masses, which originate in the Gulf of Mexico, and air masses coming from the Pacific. In addition, the height of the Colorado plateau magnifies the thermo-synoptic contrast between continent and sea, forcing air masses to rise towards the low-pressure area over the plateau.

California has a moderate climate with an average annual precipitation in excess of 1000 mm. During the summer, temperatures vary between 27 and 28 °C; in winter, between 7 and 8 °C.

On the Pacific coastal plain (in Mexico), the climate is dry and very hot. Here, cold winds from the northwest blow for about 8 months of the year. During the summer, southwesterly winds bring torrential rains.

The factors that play a role in deciding the relative influence of the two systems (i.e., the westerlies and the monsoons) are the circumpolar vortex (the strength) of the sub-tropical westerlies, and large-scale anomalies in temperatures of the sea surface, primarily those associated with the ENSO and the NAO (Hughes and Graumlich, 1996). The climate of California is especially influenced by the California current. This current is a branch of the southward flowing system of currents of the northeastern Pacific.