Climate Variability and Western Water
Introduction
Variability is a fundamental characteristic of the water resources of the Western U.S. Water availability varies geographically, and streamflows fluctuate on daily, seasonal, annual, and decadal time scales. Runoff depends on climatic processes that, to some extent, follow regular seasonal patterns. For example, there are regular seasonal changes in the global distribution of solar radiation, leading to predictable patterns of seasonal change in global atmospheric circulation. However, within those seasonal patterns, there is a great deal of intraseasonal and interannual variability. The variability is, to some extent, the random outcome of complex nonlinear interactions among independently varying components of the climate system. However, at times, variability in precipitation and runoff in the Western U.S. is linked to the El-Ni–o/Southern Oscillation (ENSO) phenomenon. The alternating episodes of sea surface warming (El Ni–o) and cooling (La Ni–a) in the eastern and central tropical Pacific Ocean caused by ENSO modify storm tracks and, thus, the locations where droughts and floods are most likely to occur. The close link between climatic variations and water availability suggests that water resources will be sensitive to the regional effects of global climate change.
Western water management practices, storage infrastructure, and patterns of use are tuned to the expected range of variation in surface runoff and groundwater availability. Floods and droughts are part of this natural range of variation, although the probabilities of such extreme events may be difficult to discern from limited historical experience. Prospective climate change complicates long-term water resources planning because it will alter streamflow probability distributions and the characteristics of aquatic ecosystems in ways that are not yet entirely clear. The available evidence suggests that global warming may lead to substantial changes in mean annual streamflows, the seasonal distribution of flows, and the probabilities of extreme high or low flow conditions. Recent climate model studies project that significant warming may be apparent in this region within the next 50 to 100 years. Such warming will be accompanied by changes in precipitation, evaporation, and runoff, but those changes cannot yet be forecast reliably at the watershed scale. Runoff characteristics may change appreciably over the next several decades. In the near term, however, the effects of global warming are likely to be masked by ongoing year-to-year climatic variability.
Rapid population growth, increasing environmental concerns, and resulting changes in the character of water demands have led to increased competition for water even under normal flow conditions. These same changes contribute to increased vulnerability to hydrologic extremes. Under low flow conditions, the risk of shortages now falls on a growing set of competing uses and values, while residential, commercial, and industrial development in flood plains has increased the value of property at risk to extreme high flows. Environmental values are often quite vulnerable to hydrologic extremes. At the same time, efforts to preserve those values constrain traditional engineering approaches to managing variable streamflows. Thus, the significance of water resource variability is growing at a time when anticipated global climate change has increased our level of uncertainty regarding the future hydrologic characteristics of western river basins.
Effective design of long-term policy will require an understanding of the existing relationship of climate to water resources in the West, the nature of potential climate changes, the sources of uncertainty, and the prospects for resolving the uncertainties. This report provides an overview of these topics, placing the impacts of global climate change in the context of existing climatic variability. Effective policy also will require attention to developing options for responding to a wide range of possible changes in water availability or flood frequencies.
Current Climate and Water Resources of the West
The Western U.S. is remarkable for its variety of landscapes and local climates. Alpine meadows, dry open pine forests, and sagebrush semi-deserts often can be found in proximity to one another. This heterogeneity provides graphic evidence of the effects of differences in temperature, precipitation, and other climate variables on vegetation characteristics, primary productivity and water resource availability. Topography and marine influences interact to determine precipitation regimes across the West. The coastal ranges, the Cascades and the Sierra Nevadas, capture most of the precipitation and create downwind rain shadows. Other precipitation maxima occur over the Rockies and other high elevation areas. In addition to this spatial variability, precipitation and runoff also vary considerably over time, with drier areas generally affected by greater relative variability than wetter areas.
The Western States experience pronounced seasonal changes in precipitation, with winter storms supplying the bulk of the annual moisture in the West Coast States, while precipitation peaks in June on the Great Plains and in August in New Mexico and eastern Arizona. These seasonal precipitation patterns and the variability around the patterns are determined by changes in global atmospheric circulation that are partly predictable. In the winter, temperature and pressure differences between the northern polar region and the equator intensify, which, in turn, causes the polar jet stream to shift southward and increase in velocity.
This band of strong westerly winds in the upper atmosphere is often referred to as the "stormtrack" because storms form and migrate along its meandering path. The seasonal migration of the jet stream from its mean summer position at about 50degrees N latitude to its mean winter position of about 35degrees N latitude causes the number and course of Pacific storm systems entering North America to change in a recognizable seasonal pattern. However, the shape of the jet stream fluctuates chaotically between modes with relatively large or small waves. A pattern of strong alternating high and low pressure systems is present when there are deep waves in the jet stream. To some extent, jet stream waves, and thus the location of predominant patterns of high and low pressure, are affected by fixed geographical features such as coastlines and mountains. But the shape and location of the jet stream and the intensity of the high and low pressure systems that it carries eastward are dynamic phenomena that vary considerably over the course of a season as well as from year to year.
As pools of unusually warm or cool ocean water shift with the waxing and waning of El Ni–o and La Ni–a events, these sea-surface temperature anomalies exert an influence over the jet stream, thus altering the distribution of precipitation across Western North America. There is considerable interannual and interdecadal climate variability throughout the Western States. Rain-gage and selected stream-gage records can be used for the period beginning in the late 19th century to examine these long-term precipitation and runoff variations. Figure 5 displays the record of annual precipitation variability averaged over large sections of the Western States, as calculated from weather station records. In addition to substantial interannual variability, longer-term excursions into relatively wet or dry periods are evident. For example, the three decades from 1945-75 were relatively wet in the Northwest, with the exception of a couple of drought years. In the Southwest, that same period was relatively dry, with record dry conditions prevailing in 1956. The early 1980s were wet across the west, and unusually wet conditions in 1983 have been linked to the strong 1982-83 El Ni–o (Redmond and Koch, 1991). Other, less intense, El Ni–o events have had neither as strong nor as consistent an impact on annual Westwide precipitation. The influence of those other events is evident, however, in the precipitation records for particular seasons and locations, as will be discussed below. Relatively short instrumental records may not provide an adequate picture of the full range of climatic variability affecting western rivers.
A longer-term view is provided by the work of several researchers who have developed proxy records for precipitation and streamflow based on tree rings (Meko et al., 1991). There is no consistent pattern of association for the longer-term fluctuations between wet and dry conditions. For example, northern California experienced an extended dry period from 1918-37, during which time the Four Rivers Index dropped to 13.55 million acre-feet (Maf) from its long-term mean of 17.4 Maf. At the same time, conditions in the Upper Colorado River were much wetter than the long term mean of 13.5 Maf. On the other hand, the most severe extended drought in the Upper Colorado River Basin occurred during the period 1579-98, when average annual flow was only 10.95 Maf. That same period was among the driest in the northern California tree ring record (Meko et al., 1991). Role of ENSO
Efforts to understand the causes of these fluctuations between wet and dry conditions have recently focused on the ENSO phenomenon. The Southern Oscillation (the atmospheric part of ENSO) is evidenced by a pattern of inversely fluctuating atmospheric pressures at Darwin, Australia and Tahiti. Southern Oscillation Index (SOI) measures this pressure differential. When the SOI is negative and very low, an El Ni–o event develops. El Ni–o (the ocean part of ENSO) refers to a period of unusually warm sea surface temperatures in the eastern and/or central equatorial Pacific Ocean. Several analysts have attempted to discover consistent relationships between ENSO and precipitation or streamflow variations across the Western U.S. These efforts are complicated by the fact that no two El Ni–o location of precipitation anomalies depends on the strength of the event, the timing of its onset and decay, and the exact location of the pool of unusually warm ocean water.
While there are no simple, consistent relationships between ENSO and western streamflows, the SOI is statistically correlated with both streamflows and snowpacks in parts of the West. A study by Cayan and Webb (1992) examined the relationship of the SOI to snow water content (SWC) at 400 snow courses and to streamflow at 61 gaging sites across Western North America. They found that SWC on April 1 and December-August streamflow in large sections of the Northwest and Southwest are significantly correlated with the value of the SOI in preceding months. Specifically, they found that: Seasonal SWC and streamflow tend to be enhanced in the southwestern U.S. and diminished in the northwestern U.S. during the mature Northern Hemisphere winter El Ni–o phase of ENSO. Opposite behavior occurs during the La Ni–a phase of ENSO. (Cayan and Webb, 1992:29). This conclusion is consistent with the results of other analyses. For example, studies by Andrade and Sellers (1988) and Kahya and Dracup (1994) found that the Southwest tends to be wet in the fall-spring following development of an El Ni–o, while dry conditions prevail following La Ni–a summers.
In the Pacific Northwest, Redmond and Koch (1991) found a tendency for low streamflows in the water year following El Ni–o events, as defined by low spring-summer SOI, with the opposite tendency for high SOI (La Ni–a) years. Notable exceptions to that pattern were the wet winter of 1982-83 during the most intense El Ni–o of this century, and the dry winter of 1988-89 following a La Ni–a. Regarding the strength of these relationships, Cayan and Webb conclude: For SOI, correlations are positive (low SWC during winters of El Ni–o) over most of the Northwest and negative over a broad region of the Southwest. Strongest positive correlations, with magnitudes about 0.5, are found in patches over Idaho, Montana and Wyoming. Maximum negative correlations occur on a broad band over the Southwest, having values of approximately -0.4 (Cayan and Webb, 1992:36). Their analysis indicates that only 25 percent of the interannual variation in streamflow can be explained by ENSO in those small patches where the impact is most significant.1 ENSO explains considerably less of the streamflow variability elsewhere in the West. Because central and northern California constitute a transition zone between opposite ENSO signals in the Northwest and Southwest, there is no consistent relationship between the
SOI and snowpacks or streamflows in that region. Particularly strong El Ni–os, such as the 1982-83 and 1940-41 events, resulted in wet winters as far north as the Pacific Northwest because they pulled subtropical moisture much farther northward than usual. Other researchers have used the differences between El Ni–os to develop typologies of these events. Fu et al. (1986), for example, classifies the most common type of event as "type 1," in which the pool of warmest water appears in June-August of year 0, stretching eastward from Tahiti. For that type of event, Kahya and Dracup (1993) found that the subsequent winter tends to be very wet in southern California and wetter than normal in northern and central California, while the Pacific Northwest tends to be wet in the winter preceding such an event. A similar conclusion was reached by Schonher (1987), who also identified other types of El Ni–os which tended to be dry in northern or northern and central California.
During the past 20 years, El Ni–o events have occurred more frequently than during the previous decades of this century (figure 7). During the winter following onset of an El Ni–o, the Aleutian low pressure system tends to be larger and more intense than in other years, and a high pressure ridge tends to set up over western Canada. This pattern is associated with relatively warm conditions over Alaska and western Canada, extending into the Western U.S., and formation of a pool of unusually cold water in the north-central Pacific. These patterns can be seen in figure 8, which displays changes in average land surface air temperatures and sea surface temperatures between the periods 1955-74 and 1975-94. The period 1990-95 was particularly unusual because a negative SOI and very warm sea-surface temperatures in the central Pacific persisted throughout the period. The pool of warmest water was located much farther west during this "extended El Ni–o" than during classic "type 1" events. This resulted in climatic impacts rather different from those described above. Its unique character has also led to differing interpretations as to classification of the event (Glantz, 1996). Trenberth and Hoar (1996) estimate that such an extended event would have had a very small chance of occurring in the absence of anthropogenic climate change, and they speculate that the pattern may therefore be a manifestation of global warming. However, there is no scientific consensus regarding how ENSO would change in a warmer world.
Gerald Meehl of the National Center for Atmospheric Research notes that: Since there is still active debate about the mechanisms that produce ENSO, it becomes problematic to model future behavior of ENSO under conditions of global warming. Research has shown that there is likely to be a suite of ENSO mechanisms, and one or more in combination may work to produce what we observe and call ENSO. . . . Anything we say about future changes of ENSO must, at the very least, be tempered by our lack of ability not only to simulate present-day ENSO phenomena with models, but also to understand fully the observed behavior of ENSO events (Glantz, 1996:136).
During most of 1996, the SOI was positive but too weak to constitute a full-fledged La Ni–a, or cool event (National Oceanic and Atmospheric Administration [NOAA], 1997a). It does not appear, therefore, that ENSO played a significant role in the recent heavy rainfall and flooding in California and the Pacific Northwest. Rather, a National Weather Service assessment of the atmospheric circulation patterns that contributed to the flooding concludes that: "A large component of these circulation features reflects the normal highly variable nature typical of the wintertime atmospheric circulation" (NOAA, 1997b:3). While there is considerable evidence that ENSO affects streamflows in the Western States, it is not the only source of variability. Furthermore, changes in ENSO and its relationship to western streamflows under global warming remain unclear.
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