These precipitation episodes are coincident with and probably related to multi-decade variation of global sea-surface temperature and atmospheric pressure (Dettinger and Cayan, 1995; Minobe, 1997; Zhang and others, 1997). The present regime began about 1978 and is characterized by higher sea-surface temperature and lower atmospheric pressure in the eastern Pacific Ocean than in the preceding period, 1942-1977 (Zhang and others, 1997). Precipitation in the Mojave Desert is generally enhanced since 1978 by these conditions, as shown in figure 6. These shifts in sea-surface temperature and atmospheric pressure have occurred about every 23-35 years since the 18th century (Minobe, 1997). Assuming the present episode is similar to earlier ones, dryer conditions may prevail again very early in the next century.

In addition to variation of total precipitation, the number of days annually with precipitation (Fig. 7) and precipitation intensity (Fig. 8; precipitation in 24 hours) have apparently changed in this century. The number of rainy days has increased, which in turn has reduced annual rainfall intensity. Both of these trends are statistically significant at the 0.05 probability level. At this time, however, it is somewhat uncertain whether these changes are related to climate or to gage location and accuracy.