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U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 117-03

2004

U N D E R S TA N D I N G E A R T H S U R FA C E P R O C E S S E S A N D G E O L O G Y — A K E Y T O E N S U R I N G F U T U R E P R O S P E R I T Y

Precipitation History of the Mojave Desert Region, 1893–2001

The Mojave Desert region covers 

152,000 km2 of the Southwestern 

United States. Recent studies by U.S. 

Geological Survey (USGS) and other 

scientists suggest that the region may 

become drier for the next 2 to 3 decades, in a pattern similar to mid-20th 

century dry conditions. Because the 

region’s population has increased rapidly since the mid-1950s, a repeat of 

such a dry episode could have severe 

consequences not only for residents 

but also for the desert’s diverse and 

fragile ecosystems.

Diverse topography, complex geology, and 

distinctive plant communities characterize 

the Mojave Desert region. The desert covers 

152,000 km2 of eastern California, southern 

Nevada, the southwest corner of Utah, and 

northwest Arizona (fig. 1). On the west and 

southwest, the Mojave Desert is bounded by 

the Sierra Nevada and the San Gabriel and 

San Bernardino Mountains. These imposing mountains alter the prevailing westerly 

winds and intercept moisture derived from the 

Pacific Ocean, producing a rain-shadow effect and arid conditions on the lee side of the 

mountains. The regional climate and topography of the Mojave (Rowlands, 1995) strongly 

influence the distribution and abundance of its 

diverse and fragile desert plant communities.

The Annual Precipitation Cycle

The annual precipitation cycle shows two 

distinctive patterns that approximately divide 

the region along the 117°W meridian, which is 

near Barstow, California. A biseasonal pattern 

prevails at 90% of the weather stations used to 

collect data in this study that lie east of 117°, 

whereas a winter-dominant pattern is typical 

of 70% of the stations lying west of 117° (fig. 

2). In both cases, May and June are consistently dry, accounting for less than 5% of annual 

rainfall. October through April precipitation of 

the winter dominant pattern accounts for 82% 

of the annual total, whereas the biseasonal 

pattern accounts for 66%. During the warm 

months of July through September, 13 and 

29% of the annual total falls in the winterdominant and biseasonal patterns, respectively.

Cool-season precipitation is the most important and extensive source of rain in the 

desert region. Rainfall is widespread and 

of relatively long duration during the cool 

season. Warm-season precipitation results 

largely from convective precipitation in the 

form of thunderstorms. Although rather infrequent, the most dramatic precipitation source 

is tropical cyclones and hurricanes (often 

referred to as chubascos) that drift across the 

region from off the coast of Baja California. These typically occur late in the warm 

season and are accompanied by widespread 

and severe flash flooding. The extent and 

magnitude of warm-season rainfall strongly 

influences the distribution of desert vegetation. The relative abundance of cacti, many 

Figure 1. Mojave Desert region and the 52 weather stations used in the precipitation history.

Figure 2. Precipitation cycle of the Mojave Desert 

region showing average monthly precipitation as a 

percentage of total annual precipitation. White lines 

are the precipitation cycles of individual stations. 

The two heavy lines are the dominant patterns. 

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yuccas, agaves, and agave-like plants is much 

greater where warm-season rainfall is abundant (Rowlands, 1995).

Precipitation History, 1893–2001

Annual Precipitation

Daily precipitation amounts from 52 

weather stations in the Mojave Desert region 

were the raw data used in the analyses. Precipitation time series were developed from 

digital and archival records. The former 

are available commercially and the latter 

are available on microfiche from National 

Oceanographic and Atmospheric Administration (NOAA). None of the stations have data 

for the entire 1893–2001 period. Average 

annual precipitation in the Mojave calculated 

with data from the 52 stations (fig. 1) ranges 

from 34 to 310 mm/yr, with a long-term 

average of 137 mm/yr. The driest year was 

1953, and 1941 and 1983 were the two wettest (fig. 3). Long-term annual precipitation 

varied substantially during the 20th century. 

Four multidecadal precipitation regimes 

are apparent: 1893–1904, 1905–1941, 

1942–1975, and 1976–1998. The choice of 

limiting dates for these regimes is somewhat subjective; the mid-century dry conditions in the desert region may have begun 

in 1946. Regardless of the exact dates, the 

mid-century was clearly dry and was sandwiched between two wetter episodes. The 

period 1976–1998 was the wettest of the 

20th century, broken only by the relatively 

short, intense drought of 1989. Droughts and 

dry conditions are distinguished from the 

two wet episodes by the sparse number of 

stations reporting precipitation substantially 

above normal (fig. 3).

These long-term variations are largely 

contemporaneous with well-known 

droughts elsewhere in the Southwest, specifically an 11-year drought from 1893–

1904 and a mid-century dry spell from 

1942–1977. The early part of the mid-century dry regime (1942–1956) is recognized 

as a drought throughout the Southwest 

(Gatewood, 1962). In the Mojave Desert region and adjacent Colorado Plateau (fig. 1), 

precipitation increased somewhat after 1953 

(1956 in the Colorado Plateau; Hereford 

and others, 2002), although it did not reach 

the levels of the early and late 20th century. 

Statistical analysis shows that average precipitation during the early and mid-century 

dry conditions was significantly less than 

the two wet episodes.

Seasonal Precipitation

Cool-season precipitation (calculated 

from October 15 to April 15) averages 

95 mm/season, with a range of 27 to 249 

were nearly as dry. The wettest were 1939, 

1976, 1983, and 1984. Although the early 

drought and mid-century dry conditions are 

visibly evident in the time series, the precipitation is not significantly (P>0.05) less than 

the two wet episodes. Nevertheless, the post1975 period is notable for having three unusually wet seasons (1976, 1983, and 1984) 

with more than half of the stations reporting 

rainfall well above normal (that is, greater 

than one standard deviation).

Mojave Desert Precipitation and 

Global Climate

Precipitation variability in the Mojave 

Desert region is linked spatially and temporally with events in the tropical and northern 

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Figure 3. Average deviation of annual precipitation (that is, the average deviation from the 

mean of the reporting stations) and bar graph showing percent of stations having precipitation more than 68% of normal.

Figure 4. Average deviation of cool- (upper) and warm-season (lower) precipitation. Bar 

graphs same as figure 3.

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mm/season (fig. 4 upper). The five driest 

seasons were 1904, 1934, 1951, 1972, and 

1990; the five wettest were 1941, 1978, 

1983, 1992, and 1993. The mid-century dry 

conditions are evident in the cool-season 

time series between 1945 and 1977. The 

period 1978–1998 is clearly the wettest of 

the 20th century. As with annual precipitation, statistical analysis shows that the early drought and mid-century dry conditions 

had significantly less precipitation than the 

wet episodes.

Warm-season precipitation (calculated 

from July 4 to October 14) ranges from 0.5 

to 125 mm/season and averages 35 mm/

season (fig. 4 lower). The two driest years 

were 1928 and 1944, although several others 
Pacific Oceans. Specifically, episodes of 

unusually wet or dry climate result from 

interrelated global-scale fluctuations of 

sea-surface temperature (SST), atmospheric 

pressure, and atmospheric circulation patterns (Cayan and others, 1999). These fluctuations operate on two time scales, providing an important means of understanding and 

predicting precipitation patterns. Short-term 

climate variation, with a period of 4 to 7 

years, is associated with El Niño and La 

Niña activity as expressed by several indicators, including the Southern Oscillation Index (SOI) and equatorial SST. Multidecadal 

climate variation follows a pattern best 

expressed by the Pacific Decadal Oscillation 

(PDO), a phenomenon of the North Pacific 

Ocean (Mantua and Hare, 2002).

Short-term Variation—El Niño and La Niña

The SOI is the standardized difference 

in sea-level atmospheric pressure between 

Darwin, Australia, and Tahiti. A sustained 

negative value of the SOI signals the 

large-scale, anomalous warming of SST 

in the tropical eastern Pacific Ocean. This 

phenomenon is generally referred to as 

El Niño, a term originally applied to the 

weak, seasonal (usually late December), 

warm, and south-flowing current off the 

coast of Peru. Warm SST in the eastern 

equatorial Pacific Ocean and sustained 

negative SOI indicate El Niño conditions, 

whereas cool SST and sustained positive 

SOI indicate La Niña conditions. The fully 

developed interaction between atmosphere 

and ocean is termed ENSO (El NiñoSouthern Oscillation). El Niño conditions 

tend to bring wet winters to the Southwest 

and increased streamflow through southerly displacement of storm tracks, although 

Figure 5. Standardized anomaly index (SAI) of 

cool+warm-season precipitation (classified by 

type of El Niño-Southern Oscillation or ENSO 

activity) as a function of the average Southern 

Oscillation Index (SOI).

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western North America is relatively warm, 

whereas it is relatively cool during the 

negative phase. These multidecadal swings 

in SST that modulate precipitation of the 

Mojave Desert region also affect important 

oceanic fisheries, causing decades-long 

changes in landings of sardines, anchovies, 

and salmon (Chavez and others, 2003). 

Precipitation in the desert region is 

modestly but significantly correlated with 

the average PDO (computed from October 

to September) in the year preceding (lag 

1) and the year of the cool+warm (lag 0) 

season (fig. 7). The three regime shifts of 

the PDO are largely in-phase with the annual and seasonal precipitation time series, 

particularly since the mid-1940s (fig. 8). 

The mid-century dry conditions show this 

in phase relation, which coincides with 

a period of low indices and a prolonged 

cool phase of the PDO. The early neutral 

to positive phase of the PDO is associated, 

although in a complicated manner, with 

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Figure 6. Time series (upper) of average negative Southern Oscillation Index (SOI) (El Niñolike conditions have positive values) and SAI of cool+warm-season precipitation (lower).

Figure 7. Standardized anomaly index (SAI) as a 

function of average Pacific Decadal Oscillation 

(PDO) during the year preceding (lag 1) and 

year of (lag 0) cool+warm-season precipitation, 

1900–2000.

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drought may also occur, whereas La Niña 

conditions reliably bring dry winters (Cayan and others, 1999).

Total seasonal precipitation (cool+warm 

season) is inversely correlated with the 

SOI averaged from the June preceding the 

season to May of the season (SOI leads total seasonal precipitation by four months). 

This inverse correlation (fig. 5) is shown 

with precipitation in standardized form (the 

SAI or standardized anomaly index with 

mean=0 and standard deviation=1) and coded by the type of ENSO activity (El Niño, 

La Niña, and non-ENSO or background 

climate). Although the strength of the correlation is relatively weak (r=–0.41), the 

probability that no relation exists between 

the SOI and precipitation is extremely 

small. In the Mojave Desert region, La 

Niña conditions produced above normal 

precipitation in only 27% of cases, whereas 

El Niño conditions produced above normal 

precipitation in 55% of cases, and the precipitation amounts were significantly larger 

than the precipitation associated with La 

Niña. Regardless of the modest correlation, 

precipitation tracks the SOI in time reasonably well with notable exceptions of 1909, 

1919, 1925–1926, and 1939 (fig. 6).

Long-Term Variation—The PDO

Recent and possible future climate variation related to the PDO and other ENSOlike indicators of multidecadal climate 

variability is a recently developed tool 

for climatological research. The PDO is 

related to SST and atmospheric pressure 

of the North Pacific Ocean (Mantua and 

Hare, 2002). Changes in these parameters 

evidently trigger sharp transitions from 

one climate regime to another, altering the 

climate of North America for periods of 

2 to 3 decades (Zhang and others, 1997; 

McCabe and Dettinger, 1999). During the 

warm PDO phase, the SST off the coast of 
For more information contact:

Scientist-in-Charge

U.S. Geological Survey

2255 North Gemini Drive

Flagstaff, AZ 86001

(928) 556-7024

or

http://climchange.cr.usgs.gov/

See also Precipitation History of the Colorado Plateau 

Region, 1900–2000 (USGS Fact Sheet 119-02).

This fact sheet and any updates to it are available online

at: http://geopubs.wr.usgs.gov/fact-sheet/fs117-03/

Richard Hereford, Robert H. Webb, and

 Claire I. Longpre

Edited by James W. Hendley II

Graphic design by Sara Boore and Susan Mayfield 

References

Cayan, D.R., Redmond, K.T., and Riddle, L.G., 

1999, ENSO and hydrologic extremes in the 

western United States: Journal of Climate, v. 

12, p. 2881-2893.

Chavez, F.P., Ryan, J., Salvador, E., and Miguel, 

N., 2003, From anchovies to sardines and 

back—Multidecadal change in the Pacific 

Ocean: Science, v. 299 p. 217-221.

Gatewood, J.S., 1962, The meteorologic phenomenon of drought in the Southwest: U.S. 

Geological Survey Professional Paper 372-A, 

p. A1-A43.

Gedalof, Z., and Smith, D.J., 2001, Interdecadal 

climate variability and regime-scale shifts 

in Pacific North America: Geophysical Research Letters, v. 28, p. 1515-1518.

Gedalof, Z., Mantua, N.J., and Peterson, D.L, 

2002, A multi-century perspective of variability in the Pacific Decadal Oscillation—

new insights from tree rings and coral: Geophysical Research Letters, v. 29, no. 2204, p. 

57-1 to 57-4.

Hereford, R. and Webb, R.H., and Graham, S., 

2002, Precipitation history of the Colorado 

Plateau region, 1900-2000: U.S. Geological 

Survey Fact Sheet 119-02, 4 p.

Hoerling, M., and Kumar, A., 2003, The perfect 

ocean for drought: Science, v. 299, p. 691-694.

Mantua, N.J, and Hare, S.R, 2002, The Pacific 

decadal oscillation: Journal of Oceanography, v. 58, p. 35-42.

McCabe, J.G., and Dettinger, M.D., 1999, 

Decadal variations in the strength of ENSO 

teleconnections with precipitation in the 

western United States: International Journal 

of Climatology, v. 19, p. 1399-1410.

Rowlands, P.G., 1995, Regional bioclimatology of 

the California Desert, in, Latting, J., and Rowlands, P.G., eds., The California Desert—An 

Introduction to Natural Resources and Man’s 

Impact: Riverside, University of California, 

Riverside Press, p. 95-134.

Schmidt, K.M., and Webb, R.H., 2001, Researchers consider U.S. Southwest’s response to 

warmer, drier conditions: Eos, Transactions, 

American Geophysical Union, v. 82, p. 475, 478.

Zhang, Y., Wallace, J.M., and Battisti, D.S., 1997, 

ENSO-like interdecadal variability: 1900-93: 

Journal of Climate, v. 10, p. 1004-1020.

Websites

Climate Prediction Center, Previous 

ENSO events (1877-present): site

accessed August 31, 2001.

Pacific Decadal Oscillation Data: N. Mantua; 

http://tao.atmos.washington.edu/pdo; accessed January 14, 2003.

Printed on recycled paper

the relatively wet conditions during the 

early half of the century. The strong warm 

phase of the PDO beginning around 1977 

is readily associated with the wet climate 

beginning in 1978. Of particular interest is 

the downward shift in the PDO beginning in 

1999 with concomitant decreased precipitation that has continued through the winter 

of 2002 with only slight relief in winter 

2003. The unusually dry climate in the Mojave Desert region since 1998 is likely associated with a nearly continuous belt of high 

pressure in the northern mid-latitudes that 

produced drought conditions elsewhere in 

the United States, the Mediterranean region, 

southern Europe, and central Asia. This 

global-scale drying was evidently related 

to unusually cool and persistent SST in the 

eastern Pacific Ocean (Hoerling and Kumar, 

2003). The weather, SST, and surface-pressure patterns of the past several years suggest that a transition to another PDO regime 

is presently underway (Gedalof and Smith, 

2001). This transition could affect the climate of the Mojave Desert region.

Implications

Precipitation in the Mojave Desert region 

varied substantially during the past century. 

This multidecadal variability has implications for ecosystem processes and land 

management, because precipitation strongly 

affects the recovery rates from natural and 

human disturbances (for example, the results of studies of floral and faunal population dynamics and the affects of grazing 

are dependent on the prevailing climate). 

Inferences and projections based on these 

studies may not be valid or may need adjustment or reappraisal if applied during a 

different, potentially drier climate regime.

Recent trends in Mojave Desert precipitation and the PDO suggest that climate of the 

region may become drier for the next 2 to 

3 decades in a pattern that could resemble 

the mid-century dry conditions. Although 

there are many uncertainties and assumptions, including using a single index (PDO) 

to predict multidecadal climate variability 

(Schmidt and Webb, 2001; Gedalof and others, 2002), it is important to consider the 

potential affects of climate variation on the 

human and natural resources of the region. 

Water resources were heavily affected during 

the early part of the 1942–1977 dry conditions, and the population of the region has 

increased greatly since the mid-1950s, substantially increasing the demand for water in 

an arid region and creating the possibility of 

severe or catastrophic consequences if such a 

drought were repeated.

The work of USGS and other scientists 

is leading to a better understanding of the 

past and probable future climate of the 

Mojave Desert region. This work is only 

part of USGS efforts to provide information 

critical to creating policies for the informed 

management of land, energy, and other 

resources. These efforts are also helping 

to protect lives and property from drought, 

landslides, and other natural hazards.

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Figure 8. Time series of average monthly Pacific Decadal Oscillation (PDO) (upper, smoothed) and 

standardized anomaly index (SAI) of cool+warm-season precipitation (lower). Arrows indicate 

regime shifts of the PDO.