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Precipitation History of the Mojave Desert Region, 1893—2001
Richard Hereford, U.S. Geological Survey, Flagstaff, Arizona (firstname.lastname@example.org)
The results of this study indicate that precipitation has varied substantially in the 20th century. Episodes of drought or drought-like conditions (1893-1904 and ca. 1942-75) alternated with relatively wet periods (1905-ca. 41 and 1976-98). Moreover, these long-term changes were in phase with global-scale climate fluctuations that have a moderate degree of predictability. Based on recent patterns of global climate, one may tentatively infer that the moisture deficiency since 1998 signifies the return of conditions similar to the mid-century drought (ca. 1942-77) in the Southwest and Mojave Desert.
The Mojave Desert is characterized by complex geology, diverse topography, and distinctive plant communities. The desert covers 152,500 km2 of the Basin and Range physiographic province in eastern California, southern Nevada, the southwest corner of Utah, and northwest Arizona. On the west and southwest, the Mojave is bounded by the Sierra Nevada, San Gabriel, and San Bernardino Mountains. These imposing mountains alter the prevailing westerly winds, intercepting moisture derived from the Pacific Ocean. This produces a rain shadow effect and arid conditions on the lee side of the mountains. On the east, the desert is bounded by the mostly semiarid Colorado Plateau, a broad elevated-region rising far above the general elevation of the desert. The northern and southern boundaries of the desert are transitional. To the southwest, the desert grades into the Colorado Desert, to the southeast it grades into the Sonoran Desert, and to the north it merges with the Great Basin Desert. The climate and topography of the Mojave Desert are primary factors affecting the distribution and abundance of desert plant species (Rowlands, 1995).
Data and Methods
Daily precipitation amounts were the raw data used in the analyses. Records of daily precipitation were assembled from 52 weather stations in the Mojave Desert region. Digital data are typically available from only 1948 to the present. To develop long-term time series of daily precipitation, archival records (available on microfiche from NOAA, Asheville, North Carolina) were used to backfill records of the longest running stations. Ten of these (labeled in the map) have records beginning between 1893 and 1900. During settlement of the desert in the 20th century, the number of weather stations increased such that 16 stations were reporting by 1914, 25 by 1945, and 47 by 1959. Aside from this gradual increase in the number of stations, the number entering the calculations varies from year-to-year due to missing data. For a particular station and accounting period (calendar year or season), the total precipitation was assigned a missing value if more than 10 percent of the daily entries of the period were missing. The median record length of the data set is 58 years.
SEASONAL VARIABILITY-THE ANNUAL PRECIPITATION CYCLE
The annual precipitation cycle on average shows two distinctive patterns that divide the region roughly along the 117th meridian. A bi-seasonal pattern prevails east of the 117th meridian, whereas, a winter dominant pattern is typical west of 1170 (fig. X). In both cases, May to July is consistently dry accounting for less than 5 percent of annual rainfall. On average, in the western part of the region, October through April precipitation accounts for 82 percent of the annual total, while in the eastern portion it accounts for 66 percent. During the warm months of July to September, 13 and 29 percent of the annual total falls in the western and eastern Mojave, respectively.
The cool season precipitation regime is the most important and extensive source of rain in the desert region. It results largely from extratropical cyclones of the North Pacific Ocean that occur in conjunction with large synoptic and planetary scale tropospheric depressions and with the polar and subtropical jet streams. This pattern develops in fall and winter during the southward shift of the mean Pacific cyclone track and expansion of the semi-permanent Aleutian-low pressure center (Pike, 1972). Rainfall in the desert is widespread and of relatively long duration during the cool season.
The warm season precipitation regime is, broadly speaking, the northwesterly extension of the Mexican monsoon into eastern California. This monsoon is a seasonal reversal of atmospheric circulation that transports maritime tropical moisture into the desert region from the Gulf of Mexico and (or) the Gulf of California (Douglas and othersw, 1993). These storms result almost entirely from convective precipitation in the form of isolated or organized thunderstorms. Because of intervening mountainous terrain and the prevailing westerly winds, this low-level moisture does not regularly penetrate into the western Mojave Desert region, resulting in the weakly developed warm season precipitation regime west of 1170. Although rather infrequent, the most dramatic sources of precipitation are 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 (Huning, 1978).
The distribution of desert vegetation is strongly affected by the extent and magnitude of warm season rainfall. The presence of cacti, many yuccas, agaves, and agave-like plants is much greater in areas of relatively abundant warm season rainfall (Rowlands, 1995).SPATIAL COHERENCE OF PRECIPITATIONS
patial coherence is the consistency of precipitation among the weather stations. Coherence was evaluated using the distance and statistical correlation (i.e., the Pearson correlation coefficient, r) between all pairs of weather stations. The distances and correlations among 50 stations were calculated for the period 1950-2000, yielding 1,250 interstation distances and correlation coefficients. The median distance between stations is 250 km with interquartile range of 150-350 km. Regarding annual (calendar year), cool season (October 15-April 15), and warm season (July 4-October 14) precipitation, respectively, 91, 92, and 80 percent of the correlations are significant (P < 0.05). In the case of annual precipitation, the median interstation correlation is r = 0.65 with interquartile range of 0.4-0.8. As expected, the interstation correlation decreases with distance. The distance decay function indicates that on average the interstation correlations (or coherency) decline from near 0.8 at 50 km to 0.4 at 600 km. Generally, these results suggest that precipitation varies consistently across the Mojave Desert region on an annual and seasonal basis.
Annual Precipitation-Calendar Year
Average annual precipitation in the Mojave Desert region, based on analysis of daily records of the 52 weather stations, ranges from 47 to 587 mm/yr, with an overall average of 149 mm/yr. The driest year was 1953 while 1941 and 1983 were the two wettest. Long-term annual precipitation varied substantially during the 20th century. These variations are largely contemporaneous with well-known drought or drought-like episodes elsewhere in the Southwest; specifically an 11-year drought from 1893-1904 and a mid-century drought from 1942-77. The early part of the mid-century dry regime (1942-56) is recognized as a drought throughout the Southwest (Gatewood, 1962; Gatewood and others, 1963), perhaps the severest drought in the past 400 years in New Mexico (Swetnam and Betancourt, 1998). In the Mojave Desert region, this drought lasted until 1977, although precipitation gradually increased after 1956.
In the desert region, four multi-decadal precipitation regimes are apparent: 1893-1904; 1905-41, 1942 (or perhaps 1946)-75, and 1976-98. The choice of limiting dates for these regimes is somewhat subjective; the mid-century drought in the desert region may have begun in 1946 and ended in 1975. Regardless of the exact dates, the mid-century was clearly dry and was sandwiched between two wetter episodes. The period 1976-98 was the wettest of the 20th century, broken only by the relatively short drought of 1989. The droughts are also distinguished from the two wet episodes by the dearth of stations reporting unusually high precipitation (i.e., more than 1 standard deviation above the station average). Statistical analysis (multiple comparison analysis of variance) shows that average precipitation during the early and mid-century droughts was significantly less (P = 0.001) than the two wet episodes.
Seasonal Precipitation-Cool and Warm Season
Precipitation of the cool season, defined as October 15 to April 15, ranges on average from 34 to 475 mm/yr with an overall average of 104 mm/yr. The five driest years were 1904, 1934, 1951, 1972, and 1990; the five wettest were 1941, 1978, 1983, 1992, and 1993. The mid-century drought is well defined in the cool season precipitation time series between 1945 and 1977. The period 1978-98 is clearly the wettest of the 20th century. As with annual precipitation, statistical analysis shows that the early and mid-century droughts had significantly (P = 0.003) less precipitation than the wet episodes when the mid-century drought is placed at 1945-77.
Warm season precipitation, defined as rain falling between July 4 and October 14, averages 35 mm/yr and ranges from 10 to 93 mm/yr at the 52 stations. The two driest years were 1928 and 1944, although several others were nearly as dry. The wettest were 1939, 1978, 1983, and 1984. Although the early and mid-century droughts are visibly evident in the time series, the precipitation is not significantly (at the 0.05 probability level) less than the two wet episodes. Nevertheless, the post-1977 period is notable for having several unusually wet seasons with more than 50 percent of the stations reporting rainfall 1 standard deviation above normal.MOJAVE DESERT PRECIPITATION AND GLOBAL CLIMATE
Precipitation variability in the Mojave Desert region is linked spatially and temporally with events in the tropical Pacific and northern 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. 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. Multi-decadal climate variation follows a pattern best expressed by the Pacific Decadal Oscillation (PDO), a phenomenon of the North Pacific Ocean.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 portends 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 Niño-Southern Oscillation). El Niño conditions tend to bring wet winters to the Southwest and increased streamflow through southerly displacement of storm tracks, although drought may also occur, whereas La Niña conditions reliably bring dry winters.
Total seasonal precipitation (i.e., cool + warm season) is inversely correlated with the SOI averaged from the June preceding the season to May of the season; the SOI leads total seasonal precipitation by four months. This inverse correlation is shown in the with precipitation in standardized form (SAI or standardized anomaly index with mean = 0 and standard deviation = 1) and color coded by the type of ENSO activity (El Niño, La Niña, and non-ENSO). 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 percent of the cases whereas, El Niño conditions produced above normal precipitation in 55 percent of the cases and the precipitation amounts were significantly larger than the precipitation associated with La Niña. Regardless of the modest correlation, precipitation tracks the SOI (plotted as negative SOI) in time reasonably well.Long-Term Variation-The PDO
Recent and possible future climate variation related to the PDO and other ENSO-like indicators of multi-decadal climate variability is a recently developed tool for climatological research. The PDO is related to SST and atmospheric pressure of the North Pacific Ocean. Changes in these parameters evidently trigger sharp transitions from one climate regime to another, altering the climate of North America for periods of 2-3 decades. Phase shifts of the PDO are thought to affect the spatial connection between ENSO and precipitation in the western United States. During the warm PDO phase, the SST off the coast of western North America is relatively warm, whereas, it is relatively cool during the negative phase (illustrated upper right).
Precipitation in the Mojave Desert region is modestly but significantly correlated (P << 0.05) with the PDO in the year preceding (lag 1) and the year of the cool + warm (lag 0) season. The three regime shifts of the PDO are largely in-phase with the annual and seasonal precipitation time series, particularly since the mid-1940s. This in-phase relation is shown well by the mid-century drought, which corresponds to 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 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 regime shift in the PDO beginning in 1999 with concomitant decreased precipitation that has continued through the winter of 2002 and into winter 2003. The weather, SST, and surface-pressure patterns of the past four years suggest to climatologists that the transition to another regime is presently underway, ending the warm PDO phase and the wet climate of 1978-98.IMPLICATIONS
Precipitation of the Mojave Desert region has varied substantially during the past century. This multi-decadal variability has implications for ecosystem processes and land management. Precipitation along with other climate variables affects the spatial scale, frequency, and magnitude of natural disturbances to the ecosystem as well as 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 climate regime.
Recent trends in Mojave Desert precipitation and the PDO suggest that climate of the region may become drier for the next 2-3 decades in a pattern that could resemble the mid-century drought. Although there are many uncertainties and assumptions, including using a single index (PDO) to predict multi-decadal climate variability, 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 1942-77 drought; the population of the region has increased fourfold 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.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., and others, 2003, From anchovies to sardines and back: Multidecadal change in the Pacific Ocean: Science, v. 299 p. 217-221.
Douglas, M.V., Maddox, R.A., Howard, K, and Reyes, S., 1993, The Mexican monsoon: Journal of Climate, v., 6, p. 1665-1677.
Gatewood, J.S., 1962, The meteorologic phenomenon of drought in the Southwest: U.S. Geological Survey Professional Paper 372-A, p. A1-A43.
Gatewood, J.S., Wilson, A., Thomas, H.E., and Kister, L.R., 1964, General effects of drought on water resources of the Southwest, 1942-1956: U.S. Geological Survey Professional Paper 372-B, p. B1-B55.
Hoerling, M., and Kumar, A., 2003, The perfect ocean for drought: Science, v. 299, p. 691-694.
Huning, J.R., 1978, A characterization of the climate of the California desert: Contract No. CA-060-CT7-2812, Desert Planning Staff, Bureau of Land Management, Riverside California, 220 p.
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.
Pyke, C.B., 1972, Some meteorological aspects of the seasonal distribution of precipitation in the Western United States and Baja California: Los Angeles, University of California Water Resources Center Contribution No. 139, 205 p.
Ropelewski, C., 1999, The great El Niño of 1997-1998: Impacts on precipitation and temperature: Consequences, v. 5, p. 17-25.
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.
Swetnam, T.W., and Betancourt, J.L., 1998, Mesoscale disturbance and ecological response to decadal climate variability in the American Southwest: Journal of Climate, v. 11, p. 3128-3147.
Trenberth, K.E., 1997, The definition of El Niño: Bulletin of the American Meteorological Society, v. 78, p. 2271-2777.
Ze'ev, G., and Smith, D.J., 2001, Interdecadal climate variability and regime-scale shifts in Pacific North America: Geophysical Research Letters, v. 28, p. 1515-1518.
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 and Pamphlets
Climate Prediction Center, Previous ENSO events (1877-present): http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears_1877-present.html; accessed August 31, 2001.
Pacific Decadal Oscillation Data: N. Mantua http://tao.atmos.washington.edu/pdo; accessed January 14, 2003.
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.
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