FOUR
Why Is Climate So Variable in the West?
El Niño has taught two lessons that will endure. The first is that large-scale variability such as El Niño is not a disaster, anomaly, or cruel twist of fate; it is how Earth works. To mature and live harmoniously in the Earth system, human culture must adapt to Earth’s rhythms and use natural variability to its advantage.
MICHAEL GLANTZ, Currents of Change
CLIMATOLOGISTS IN THE AMERICAN WEST have been carefully recording daily observations of weather conditions for the past 150 years. They have also monitored ocean conditions over recent decades, providing insights into the critical interactions between the Pacific Ocean and its overlying atmosphere. In this way, climate scientists have assembled many seemingly unrelated observations into a larger picture that explains, at least in part, why climate in the American West changes from year to year, decade to decade, and perhaps over even longer timescales. This chapter provides an overview of the discoveries that begin to explain why climate is so variable in the West—at least over the past century and a half.
WHERE WESTERN CLIMATE PATTERNS START
To understand what controls climate in the American West, we look first to the region’s neighboring body of water, the Pacific Ocean, which is immense by any measure: it contains half the water on Earth and covers over one-quarter of the globe. In physical terms, the ocean is a vast heat engine as well as a source of water for the western United States. Although the ocean is 2.5 miles deep (on average), the temperature of the waters in just the top 300 feet or so is what affects climate in the West by altering large-scale patterns of atmospheric pressure and circulation, which are factors in the size and movement of rain-bearing storms.
The two pressure cells that most influence climate over the American West are the “Pacific High,” which extends from just east of Hawaii to the West Coast, and the “Aleutian Low,” which extends from the Gulf of Alaska to the Aleutian Islands in the North Pacific. The basic physics of our spinning planet result in the circulation of upper-level winds in a counterclockwise direction around the Aleutian Low and in a clockwise direction around the Pacific High. The Aleutian Low is strongest during the winter, when the Pacific High is weakened and displaced to the south. In the mid-latitudes—roughly midway between the equator and the North Pole—movement from west to east of air masses that form over the Pacific creates the prevailing westerly wind pattern over the West. In wintertime, most of the rain-bearing air masses bring moisture from the ocean across the northern coastal states (Washington, Oregon, and northern California).
Much of the moisture falls as the air masses are forced up the slopes of first the coastal mountains and then the Cascade or Sierra Nevada mountain ranges. In the higher elevations, this precipitation often falls as snow. In the northern Pacific Ocean, the Aleutian Low occasionally moves farther west, and thus a high-pressure cell may form over the eastern Pacific just off the West Coast, preventing storms from entering the West. This situation is often associated with drought.
During the summer, the Pacific High strengthens and moves farther north off the West Coast, deflecting storms into the Gulf of Alaska and Canada. California is positioned in just such a way that it lies under the eastern side of the Pacific High during the summer months, which effectively blocks storms from moving into the state. For this reason, California is one of the few regions in the world that receives virtually no summer rain.
In contrast, the southwestern region of North America—including Arizona, New Mexico, and Colorado—receives moisture in the summer during what is called the “North American Monsoon.” The monsoonal rains occur when moist air from the Gulf of Mexico, the Gulf of California, and the eastern Pacific Ocean moves northward into the southwestern United States, which has undergone intense solar heating during the months of July, August, and September. Monsoonal rainfall can account for as much as half of the annual precipitation in the U.S. Southwest, although it is highly variable since the region lies at the northern border of the monsoon (with the core located over northern Mexico).
Precipitation in the West is highly variable from year to year and decade to decade. Research over the past few decades has revealed that these seemingly random and unpredictable swings in climate are largely brought about by periodic changes in the Pacific Ocean and the atmosphere above it, producing phenomena known as the El Niño–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Together, the ENSO and the PDO are responsible for climate patterns over most of the western United States, including precipitation, wind patterns, and air temperature during various seasons. The ENSO arises from conditions in the tropical Pacific Ocean; the PDO is tied to conditions over portions of the northern Pacific Ocean. ENSO events are typically six to eighteen months in duration, whereas PDO phases last two to three decades or more.
THE EL NIÑO-SOUTHERN OSCILLATION (ENSO)
Conditions in the tropical Pacific Ocean and in the air above it play a major role in the year-to-year variability of climate in the North American West, and they are often responsible for the region’s climate extremes.
The earliest scientific observations leading to our present understanding of oceanic and atmospheric variability in the tropical Pacific were made in the early twentieth century by Sir Gilbert Walker, director of the Indian Meteorological Department. Walker’s expertise was in mathematical physics and statistics, not meteorology, but he had a passion for analyzing a wide array of seemingly unrelated meteorological data to explain such phenomena as droughts in India. He observed that there was a “swaying of pressure on a big scale backward and forward between the Pacific Ocean and the Indian Ocean” (p. 22). Walker called this seesawing surface pressure the Southern Oscillation, which is part of an east-west atmospheric cycle now known as the “Walker Circulation.”
Later in the twentieth century, Norwegian meteorologist Jacob Bjerknes, who later founded the Department of Meteorology at the University of California, Los Angeles, observed oceanic and atmospheric patterns in the tropical Pacific Ocean during the strong 1957–58 El Niño (and during subsequent events in 1963 and 1965). These conditions, characterized by anomalously warm ocean surface waters in the eastern tropical Pacific that occur in late December, have been recognized for centuries by Peruvian fishermen, who noticed significant reductions in their catch. The episodes studied by Bjerknes were also characterized by light easterly trade winds, heavy rainfall in the eastern Pacific, and the atmospheric pressure shifts described by Walker.
In the late 1960s, Bjerknes brought together all of these disparate pieces of information into a hypothesis that seemed to explain the entire ocean-atmosphere phenomenon. He proposed that, in typical years, the constant east-to-west (or easterly) trade winds push warm surface waters across the Pacific Ocean to the western end, where they pile up in a region called the “Pacific warm pool.” This “pool” of warm water is roughly four times the size of the continental United States, measuring 9,000 miles from east to west and 1,500 miles from north to south. High rates of evaporation off this body of warm water provide abundant rainfall for places such as Australia and Indonesia.
On the opposite side of the Pacific, just off the coasts of North and South America, the surface waters blown to the west by the trade winds are replaced by colder water from beneath the surface in a process known as “upwelling.” These cold, upwelled waters cool the overlying atmosphere, forming high pressure, resulting in very dry conditions in the adjacent land areas, including Ecuador, Peru, and Chile to the south and the American Southwest to the north.
El Niño Episodes
This vast, complex ocean-atmosphere system periodically stumbles, however, when trade winds mysteriously falter or even fail completely for months, producing the pattern known as El Niño. This phenomenon, which means “the child,” was named in South America, where it usually appears around Christmas and thus the celebration of the birth of the Christ child. During El Niño, the western Pacific warm pool—elevated several inches higher than the eastern Pacific—is released from the invisible force of the trade winds that hold it, and it flows eastward as a low-amplitude—but tremendously long—wave (called a “Kelvin wave”) across 8,000 miles of the Pacific Ocean toward the Americas at about 150 miles per day.
Upon reaching the eastern Pacific, this wave splits and travels along the coasts of North and South America, causing sea levels to rise by about six to ten inches—the amount equal to the height of the wave. Higher sea levels can lead to coastal flooding and erosion.
The wave of warm water also acts as a cap, suppressing the colder, nutrient-rich waters below. Cold water is denser and thus unable to rise above the warmer waters. These atypically warm surface waters pump water vapor, heat, and enormous amounts of energy into the atmosphere above, creating a column of warm, moist air. This column bulges into the atmosphere for six to ten miles, where it remains like a giant boulder in a stream—an obstacle that atmospheric winds and storm systems must move around—and so influences the strength and location of jet streams over the northern and southern Pacific Oceans.
The copious amounts of evaporation from this warm water also spawn large storms that are carried by westerly winds into northern Mexico and the southwestern United States. The deluges, floods, and mudslides experienced in the North American Southwest are often mirrored by opposite but equally tragic events in distant regions thousands of miles away, on the western side of the Pacific Ocean. There, the surrounding ocean waters are cooler than usual, with less evaporation and less rain. Droughts, forest fires, and dust storms often occur simultaneously in places such as Australia and Indonesia. The Pacific Northwest also experiences drier than normal conditions during an El Niño event.
La Niña Episodes
If El Niño conditions represent a disruption of what we think of as the normal climate state, La Niña episodes occur when “normal” conditions go into “overdrive.” The trade winds blow even harder across the Pacific Ocean, forming an even larger warm pool in the western Pacific and leading to an increased upwelling of cold surface waters in the eastern Pacific. In the western Pacific, evaporated water off this vast warm pool rises high into the atmosphere, forming towering clouds that drench the tropical regions over northern Australia and Indonesia. Across the Pacific, in Peru and in the North American Southwest, the unusually cool waters off the eastern Pacific bring severe drought.
ENSO and Precipitation in the West
Because the oceans provide the water and energy for most of the storms on Earth, the shifting of warm surface waters to the central and eastern Pacific during El Niño ultimately alters precipitation patterns across much of the globe by atmospheric teleconnections (or long-distance relationships between weather patterns). For example, droughts in Australia, Indonesia, southern Africa, southern India, Spain, and Portugal often occur at the same time as deluges and floods in Peru, the Mississippi River Basin, and Western Europe. In the American West, the Southwest normally experiences deluges and floods during El Niño while the climate in the Pacific Northwest is unusually dry (see figure 12A). The geographic transition between these two opposite climate responses often occurs in Central California, which can experience extreme conditions in either direction.
FIGURE 12. Maps of the western United States showing (A) the correlation between winter (November through April) precipitation and the EI Niño-Southern Oscillation, and (B) the correlation between winter precipitation and the Pacific Decadal Oscillation. (Maps courtesy of Dr. James Johnstone at the Joint Institute for the Study of the Atmosphere and Ocean, University of Washington.)
Together, El Niño, La Niña, and the Southern Oscillation are major players in the ENSO, that ocean/climate phenomenon of interrelated (and often destructive) shifting pressures, wind-ocean currents, and ocean surface temperatures. The Southern Oscillation Index (SOI) is a measure of the differences in atmospheric pressure between the western and eastern tropical Pacific Ocean (measured in Tahiti and in Darwin, Australia), and it reflects the state and strength of the system. When the SOI is negative, air pressure is below normal in Tahiti and above normal in Darwin (with warmer waters in the eastern Pacific)—typical of an El Niño year. During periods of negative SOI, precipitation is highest in the Southwest and lowest in the Pacific Northwest. A positive SOI brings the opposite climate patterns: dry in the American Southwest, wet in the Pacific Northwest.
The link between the ENSO and precipitation in the western United States arises, in part, from the location of the jet stream, which is a corridor of fast-flowing air occurring at the boundary between cold air to the north and warmer air to the south. Low pressure systems tend to form beneath the jet stream and move along with it, bringing wet storms from the Pacific Ocean into the western United States in the winter. During El Niño events, the warm surface waters in the eastern Pacific lead to a greater temperature contrast farther south, pulling the jet stream southward and bringing more storms into the Southwest. In contrast, during La Niña events, the eastern Pacific Ocean becomes even cooler than normal, reducing the temperature contrast and therefore reducing the likelihood of a jet stream into this region, decreasing the occurrence of storms and precipitation in the Southwest.
The 1997–1998 El Niño: Biggest on Record
El Niño events recur every two to seven years, at varying degrees of severity. The 1997–98 El Niño was the most heavily monitored and closely observed climate event in history, and it taught climate scientists much of what they know about the ENSO today. At the start of this event, unusually warm waters began moving eastward across the equatorial Pacific during the spring of 1997, alerting major oceanographic and atmospheric research institutions that a massive El Niño event was on its way. Researchers worldwide watched as the wave of warm ocean waters made a rapid journey eastward across the Pacific, warming oceanic islands and coral reefs all along the way and reaching the west coast of South America by August of that year.
Oceanographers and climatologists had already deployed an array of buoys across the tropical Pacific Ocean after the monstrous 1982–83 El Niño event had taken them by surprise. These buoys monitor water temperature between the surface and a depth of 1,600 feet, wind speed and direction, air temperature and humidity, and water movement (currents). The data are automatically relayed back to computers via the Argos satellite system and are used to predict an upcoming El Niño event several months in advance, which is the amount of time it takes for tropical ocean currents to respond to changes in the trade winds.
In 1997, scientists were predicting deluges in Ecuador and Peru, California, the Southwest, and throughout the southern United States, and they expected droughts in the American Midwest, Brazil, Australia, and Indonesia. Their predictions were spot on, although the actual impacts were significantly greater than anticipated. Storms in November 1997 generated massive flooding in southern Ecuador that left over three thousand people homeless in minutes. Floodwaters also swept through coastal cities and agricultural lands in Peru. By December, an area of warm water greater than the size of the United States covered the eastern Pacific, heating the overlying air from the eastern Pacific to Central America and even into the Atlantic from Cuba to Libya, imposing its own weather patterns. The Northern Hemisphere jet stream shifted in February 1998, bringing warm tropical air northward and warmer than average temperatures into North America, Europe, and eastern Asia.
Half a world away, rice paddies in the Philippines were dry and dying, with more than $20 billion in lost crops, food shortages, and several hundred deaths. Drought led to catastrophic wildfires in Indonesia because the normal monsoon rains never came, and the fires filled the air with thick clouds of smoke, causing rampant respiratory illnesses. These disasters occurred despite an early warning system and programs that gave advance notice of droughts, floods, and potential crop failures to affected regions of the world. The 1982–83 and 1997–98 El Niño events combined caused $120 billion in damages and 24,000 deaths worldwide.
THE PACIFIC DECADAL OSCILLATION (PDO)
Scientists have been monitoring the ocean and atmosphere over several decades and have detected long-term climate patterns originating in the North Pacific Ocean. One of these patterns was discovered when fisheries scientists Robert Francis and Steven Hare began analyzing cycles in salmon populations off the coast of Alaska in the early 1990s. When they compared the salmon population sizes with climatic and oceanographic data over a period of several decades, they realized that the two cycles appeared to be tracking each other.
Climatologist Nate Mantua and colleagues at the University of Washington further analyzed the data and correlated fluctuations in salmon productivity in the area between Alaska and the Pacific Northwest with changing ocean conditions during the twentieth century. The team observed that changes in wind direction during wintertime influence the temperature of surface waters along the Pacific Coast of North America. When the direction of the winds causes increased upwelling of cool, nutrient-rich waters, the enhanced growth of phytoplankton and krill allows salmon populations to increase. During periods of decreased upwelling, sea-surface temperatures rise and nutrient levels are lower, causing krill and phytoplankton to decrease, leaving less food for salmon and thus reducing their populations.
Mantua and his colleagues analyzed the oceanographic and fisheries data and found that these cycles of sea-surface temperature and salmon populations lasted decades. They discovered that sea-surface temperatures in the northeast Pacific were warmer than average from 1925 to 1947 and from 1977 to 1999, and that they were cooler than average from 1947 to 1977 and after about 1999. The team called these broad oceanographic cycles the Pacific Decadal Oscillation, or PDO. The warm and cold phases each lasted two to three decades.
The PDO is an elegant demonstration of the linkages between climate, oceans, and marine life, and it also helps explain the enigmatic shifts that occur every few decades in the West’s climate as measured by rainfall and streamflow across the region. During a positive (warm) phase of the PDO, surface waters of the central North Pacific are anomalously cold while surface waters along the northwestern coast of North America are unusually warm. There is also warming of tropical surface waters in the eastern Pacific. This PDO phase brings higher than average winter precipitation to the American Southwest, with increased risk of flooding, while lower than average winter precipitation occurs in the Pacific Northwest. The patterns reverse during a negative (cool) PDO phase, with cooling of surface waters along the northwestern coast of North America and in the tropical eastern Pacific. During this phase, average winter precipitation is lower in the American Southwest and higher in the Pacific Northwest (see figure 12B).
Two full PDO cycles have occurred in the past century: cool, or negative, regimes occurred from 1890 to 1924 and from 1947 to 1976; warm, or positive, regimes occurred from 1925 to 1946 and from 1977 to 1998. Regime shifts—transitions between positive and negative phases of the PDO—occur every few decades and can also have dramatic effects on the climate. For instance, the winter of 1976–77 marked the transition from a cool to a warm phase of the PDO, and, during that year, extreme drought had societal impacts beyond temporary water restrictions. It was the driest year on record in California, the Pacific Northwest, and other regions of the North American West. In the Pacific Northwest, the human population and agriculture had expanded during the wetter, cool-phase PDO between 1945 and 1976. The 1977 drought introduced the first of several dry decades in the region—and the first of many conflicts between farmers, fishermen, and other water users accustomed to wetter times.
Some of the deepest droughts and most catastrophic floods are caused by the interactions between the PDO and the ENSO. When the PDO is in a positive (warm) phase, El Niño events bring more rain to the American Southwest and less to the Pacific Northwest, as was the case during the period between 1977 and 1999. In contrast, during a negative (cool) PDO phase, La Niña events bring drier conditions to the American Southwest and wetter conditions to the Pacific Northwest, such as during the period from 1945 to 1976.
A negative (cool) PDO regime began in 1999, marked by a deep drought in the Southwest. But this time, the cold-phase PDO lasted only four years, flipping back to a warm phase from 2002 to 2005. It then returned to a cool phase again, where it has remained until the present time (2013). These recent fluctuations in the PDO are not typical for the twentieth century and are currently under investigation by atmosphere and ocean scientists to assess whether they are possibly related to global warming.
ATMOSPHERIC RIVER STORMS
Water Resources
Scientists at the U.S. Geological Survey and elsewhere have found that water vapor from the tropical Pacific Ocean is carried to higher latitudes through narrow corridors of concentrated moisture, dubbed “atmospheric rivers.” Satellite technology over the past decade has allowed researchers to observe and image these storms. They discovered currents in the lower atmosphere (about one mile high, 250 miles wide, and extending thousands of miles) carrying water vapor and warm air from the eastern North Pacific to the West Coast. Once these atmospheric rivers hit the mountain slopes, they are forced upward and cooled, releasing copious amounts of precipitation along the Coast Ranges, Sierra Nevada, and Cascades.
Atmospheric rivers are actually a type of cyclone that forms at higher latitudes than hurricanes. Although they lack a cyclone’s characteristic circular pattern, they carry hurricane-strength winds and a similar amount of rainfall. Unlike hurricanes, which obtain their energy from surface ocean heat content, these atmospheric rivers derive their energy from the contrasts in temperature between the tropics and the poles. Atmospheric river storms contribute between 30 and 50 percent of the total amount of precipitation that falls each year in California, so they are now recognized as a crucial source of water for the state. They also contribute a significant amount (10 to 40 percent) of the total annual precipitation in other western states, including Oregon, Washington, Nevada, and Idaho.
Floods
Atmospheric river storms are a double-edged sword: they provide critical water resources to the region, but they also produce the largest and most destructive floods along the West Coast. Climatologist Mike Dettinger of the U.S. Geological Survey and his colleagues have studied the past sixty years of climate and flood history in California and have shown that the years with the largest “Pineapple Express” storms (a type of atmospheric river storm that draws heat and vapor from the tropics near Hawaii and transports it to the West Coast) correlate with the largest floods. Dettinger also observed that these storms are tied to both the tropical Pacific (ENSO events) and the North Pacific (PDO events). His analysis reveals that, during El Niño when the PDO is in a positive (warm) phase, a higher proportion of precipitation in south-central California and western Washington comes from Pineapple Express storms.
Atmospheric river storms bring high volumes of precipitation—the equivalent of ten to fifteen Mississippi Rivers—from the tropics to the mid-latitudes over relatively short periods of time, and, because they are warm, more of that precipitation falls as rain high in the mountains rather than as snow, leading to severe floods. On the West Coast, atmospheric river storms occurred during the stormiest years on record—1861–62, 1997, and 2006—and other stormy years in between (1937, 1955, 1964, 1969, 1986, and 1994). The relationship between unusually warm storms and severe flooding was observed as early as 1900 by John Muir, who wrote: “The Sierra Rivers are flooded every spring by the melting of the snow as regularly as the famous old Nile. Strange to say, the greatest floods occur in winter, when one would suppose all the wild waters would be muffled and chained in frost and snow. . . . But at rare intervals, warm rains and warm winds invade the mountains, and push back the snow line from 2000 ft to 8000 ft, or even higher, and then come the big floods” (chap. 11).
Atmospheric river storms are likely responsible for the 1861–62 floods, according to scientists at the U.S. Geological Survey. But just how common are these storms and resulting floods? Was this a freak event that is unlikely to occur again, or is it something for which we should prepare? Millions of current westerners have never even heard of those floods, much less have any idea about the potential for another, similar event.
Only by examining the climatic history of the West preserved in its sediments, trees, landforms, and lakes can we begin to understand whether these large flood events have been repeated over centuries or millennia, and, on the opposite end of the climate spectrum, how large and frequent droughts were in the past. In the next few chapters, we will explore the evidence revealing past climate change in the West, including the startling evidence that the 1861–62 floods and the twentieth-century droughts may not have been the largest that nature can deliver.