The range of standardized anomalies is shown by the box plots in figure 5. The boxes show the 25th to 75th percentiles of the standardized precipitation of each station and the horizontal line in the box shows the median value. The thin vertical lines show the range of the 10th to 90th percentiles. Although there is substantial annual variation, this figure reveals unusual conditions when 90 percent of the stations had above or below normal precipitation. Particularly wet years were 1909, 1931, 1941, 1965, 1978, and 1983. Below normal precipitation occurs more frequently and was widespread in 1898-1900, 1903, 1917, 1924, 1928-1929, 1934, 1942, 1947, 1950, 1953, 1956, 1961, 1964, 1971, 1975, and 1989.

The standardized anomalies are shown as a time series in figure 6, which generalizes the information in figure 5. Two patterns of precipitation variability are evident. First, El Niño-Southern Oscillation (ENSO) conditions generally have a positive influence on precipitation in the Mojave Desert, although the effect is at times out of phase with annual precipitation and not every wet year is associated with ENSO conditions. Second, three episodes (dashed horizontal lines) of relatively wet or dry climate are apparent. Conditions from the early 1900s until about 1942 and from 1978 to at least 1996 were relatively wet overall, whereas, from about 1943 to 1977 the desert was comparatively dry.