Climate Variability and Change in the Southwest

Part II: Symposium

Chapter 6

Climate Patterns and Trends in the Southwest

Long-term Historical Patterns

Climate records for the Southwest have been kept since the turn of the century. However, it is possible to reconstruct the region's climatic past back to the late 1500s using dendrochronology studies.

Tree-ring growth is related to climate, with small ring growth indicating stress conditions (e.g., hotter and drier) and larger rings indicating cooler, wetter periods. While tree-ring growth cannot provide an exact reconstruction of rainfall totals, there is a significant correlation between growth and precipitation (r = 0.80).

These studies have revealed the complex and cyclical nature of past climate in the Southwest, including the pattern of the El Niño-Southern Oscillation (ENSO) (Swetnam and Betancourt, 1992). Figure 6.1 illustrates the reconstructed average tree-ring growth in the Southwest dating back to the year 1000.

The historical pattern of the pattern of the El Niño-Southern Oscillation can be seen in tree-ring growth. The periods from 1740 to 1780 and from 1830 to 1860 were abnormally wet years with large tree-ring growth. The interstitial period (1780 to 1830) was a dry period in the Southwest. Table 6.1 summarizes the extreme historical drought events based on dendrochronology research.

The first drought period is called the Great Drought by anthropologists and is linked to the disappearance of several indigenous tribes in the Southwest. The third drought is mentioned in the archives of the Spanish explorers in the area. Table 6.2 provides an overview of extreme historical wet events.

Current Climatology

Records of the more recent past also show that the Southwest has experienced large seasonal, year-to-year, and decade-to-decade climate fluctuations. For Tucson, the July maximum temperatures and rainfall for the 1961-1990 period show a large year-to-year variation (Figure 6.2, Plate 3).

Although many parts of the Southwest receive the majority of their precipitation from the summer monsoons, wintertime precipitation provides most of the annual runoff for the region.

Winter precipitation is considerably more variable than summertime precipitation, most of the latter being lost to evaporation (Figure 6.3).

The impacts of climate variability are also illustrated by Figure 6.4 (Plate 3), which shows that significant areas of the Southwest are affected by moderate to severe drought or wet conditions every year.

The droughts of the 1930s and 1950s are evident. These graphs show no distinct changes in the frequency or extent of severe events.

Are there any systematic patterns or trends in southwestern climate? Figure 6.5 shows annual runoff in the Salt, Tonto, and Verde rivers with high climate variability but no distinct trend.

Longer-term reconstruction of Colorado River flows, based on tree ring records, show decade-long fluctuations associated with sustained wet and dry periods in the Southwest (Figure 6.6, Plate 3). In Arizona, the period since 1960 shows a lower daily temperature range (the difference between the daily maximum and minimum temperatures) for Arizona than for the period prior to 1960. The difference is about 2.5° (F) in the autumn and 1.4° (F) over the year. This annual change is due to a 1.0° (F) increase in daily minimum temperatures and a 0.4° (F) drop in daily maximum temperature, and may be explained mainly by an increase in cloud cover over the same period (W. Sellers, pers. comm.).

Analysis of climate records for the last century for a broader region to include Arizona, New Mexico, Nevada and Utah (Figure 6.7) suggests that there has been a slight increase in both maximum and minimum temperature, but no detectable change in precipitation since the turn of the century.

Fluctuations in Pacific sea surface temperatures (SST) and atmospheric conditions known as the El Niño-Southern Oscillation (ENSO) influence climate and its variability in the Southwest. When SSTs are warm (El Niño), the Southwest often experiences relatively wet winters, with higher snow pack and water year stream flows. Cooler events are sometimes associated with droughts (Figure 6.8).

Values in Figure 6.8 above zero are warm sea surface temperature events, below are cold events (NOAA Web site).

Improved understanding now allows predictions of the climatic effects of El Niño and its influences up to one year in advance in many regions of the world. For example, climate-model simulations of monthly precipitation in the Southwest indicate that El Niño years have about 66 percent more precipitation than other (or control) years (Figure 6.9). Forecasts indicate both the evolution of sea surface temperatures and the probability of seasonal climate conditions. The following page shows some forecasts for the current El Niño.

The current ENSO event (1997-98) is one of the more intense of recent decades (Figure 6.10, Plate 4). It intensified through the autumn and winter of 1997 and faded out during summer 1998(Figure 6.11, Plate 4).

Using knowledge of El Niño, scientists forecast a wetter 1997-98 winter for the Southwest. Recent ENSO information is available at the NOAA Web site ( www.ogp.noaa.gov/enso/ ).

Analysis of Recent Trends

While precipitation in the Southwest continues to fluctuate over a several-year cycle, average daily temperature has increased. In addition, the average daily minimum temperature has increased more than the maximum temperature.

As a result, the diurnal temperature range in decreasing, which could have implications for such sectors as agriculture and rangelands.

The 1980s and 1990s have been climatic anomalies. Concerns about greenhouse gas emissions and global warming have prompted scientists to investigate the link between these concerns and the recent anomalies.

While we cannot claim that global warming has caused any single climate event, we do note that the frequency of extreme events is increasing.

Sources

Diaz, H.F., and Anderson, C.A. 1995. Precipitation trends and water consumption related to population in the southwestern United States: A reassessment. Water Resources Research.31: 713-20.

Dettinger, M. 1997. Coping with severe and sustained drought in the Southwest. Online paper for the USGS Web Workshop (geochange.er.usgs.gov).

Giorgi, F., C.S. Brodeur and G.T. Bates. 1994. Regional climate change scenarios over the United States produced with a nested regional climate model. Journal of Climate 7(3): 375-99.

Keane, J. 1991. Managing water supply variability: The Salt River Project. In Managing Water Resources in the West under Conditions of Climate Uncertainty. National Academy Press, pp. 303-23.

Meko, D, C. W. Stockton, and W. R. Boggess. 1995. The Tree-ring record of severe sustained drought. Water Resources Bulletin 31(5).

Swetnam, T., and J. Betancourt. 1992. Temporal patterns of the El Niño-Southern Oscillation--wildfire patterns in the southwestern United States. In El Niño: Historical and Paleoclimatic Aspects of the Southern Oscillation, H. F. Diaz and V. M. Margraf, eds., Cambridge University Press, pp. 259-70.

Quayle, R. 1997. NOAA National Climate Data Center, Asheville, NC.