Here comes El Niño—but what exactly is it?

December 2, 2014 | It’s been a fascinating journey, watching how scientific and public views of El Niño have evolved over the last several decades. Sometimes you still see El Niño portrayed as a devastating creature, guaranteed to wreak havoc on everything in its supposed path. But the picture has gradually grown more sophisticated as scientists have learned more about the workings of this vast, complex phenomenon and how to communicate its impacts effectively. How did we get from there to here? A few landmarks stand out in the last 50 years of research and communication on El Niño, La Niña, and the El Niño–Southern Oscillation (ENSO).  (For quick definitions of each of these, check out our Weather-Maker Patterns Glossary. For frequentlly asked questions, see the El Niño, La Niña & ENSO FAQ) Linking atmosphere, ocean, and distant weather This diagram shows the Walker Circulation, a vast loop of air above the equatorial Pacific Ocean. (Illustration by Gabriel Vecchi, NOAA/GFDL.) Scientists in the mid-20th century were aware of the tropical Pacific Ocean's Walker Circulation (see diagram), as well as a vast see-saw in atmospheric pressure—the Southern Oscillation—that modified this prototypical pattern. And people living near the coast of Peru and Ecuador knew that the cold upwelling that nourished their fish-packed coastal waters sometimes disappeared for months at a time, especially around Christmas (thus the local name “El Niño,” or “the Christ Child”). These two phenomena were assumed to be unrelated until Jacob Bjerknes published a landmark paper in 1966 (see PDF). The concepts began to mesh further after a major El Niño event in 1972–73, when scientists began to draw firmer links between the oceanic warming of El Niño (which extended much further into the tropical Pacific than researchers had realized) and various atmospheric features that occurred across much of the globe (dubbed (“teleconnections”). The term El Niño–Southern Oscillation (ENSO) soon came into vogue. Pioneering social scientists such as Michael “Mickey” Glantz (then at NCAR, now at the University of Colorado Boulder) delved into the implications of El Niño for people on the ground and the potential usefulness of El Niño predictions for saving lives and livelihoods (see PDF, 1977). Building a detection system Extreme drought across Australia, fostered by a strong El Niño event, fed this wall of dust that swept across Melbourne on February 8, 1983. (Photo by Trevor Farrar, courtesy Australian Bureau of Meteorology and phys.org) During the northern winter of 1982–83,  the most intense El Niño recorded up to that time brought devastating winter storms to much of California and severe drought and wildfire to Australia, among other widespread impacts. Scientists recognized this event only after it had formed, due to limited observations, primitive computer models, the presence of simultaneous effects from a 1982 eruption of Mexico’s El Chichón, and an incomplete recognition of how El Niño development could vary from one event to another. The next decade saw a concerted effort by the United States, Japan, and other nations to deploy instrumented buoys as part of the Tropical Ocean and Global Atmosphere program (TOGA). Since these buoys can detect the eastward spread of warm water months before an El Niño event hits maturity, they serve as an invaluable early-warning tool. (Unfortunately, it’s been a struggle at times to keep the buoys fully funded and maintained. In early 2014, Nature reported that “nearly half of the buoys in the Tropical Atmosphere Ocean array have failed because of delayed maintenance.”) Dozens of instrumented buoys were moored throughout the equatorial Pacific Ocean in the early 1990s, as part of what’s now called the TAO/TRITON array. The array gathers surface meteorological and oceanographic data and records ocean temperature to a depth of about 1,650 feet. (Photo courtesy NOAA.) Computer models also leapt forward in the 1980s, confirming the ocean-atmosphere linkage and pointing the way toward El Niño forecasts. A regional model developed by Mark Cane and Stephen Zebiak (Lamont-Doherty Earth Observatory) generated some of the first model-based ENSO predictions starting in the late 1980s. Other scientists began to simulate ENSO within global climate models, a task with its own challenges (some of which are still being tackled, as steady progress continues). A landmark field campaign hits the Pacific TOGA mounted one of the largest-ever atmospheric research projects, the Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). Aircraft and ships canvassed the western Pacific tropics for months, and the resulting analyses showed how tightly the ocean and atmosphere components of ENSO were linked. The study is often cited in research papers to this day. El Niño of the century California's Russian River saw damaging floods  in March 1998 in the wake of heavy rain associated with that winter's intense El Niño event. (Photo by Dave Gatley, FEMA.) A massive El Niño, even stronger than the 1982–83 event, took hold in 1997–98. This time, thanks to the TOGA buoy network and more sophisticated satellites, forecasters saw the oceanic precursors well in advance and sounded the alarm ahead of the biggest impacts. These included record rains in California, severe drought in Indonesia, and a deadly tornado outbreak in Florida. The early warning not only helped decision makers charged with public safety get ready: it also piqued the interest of the public, many of whom hadn’t heard the term El Niño before. That label’s pithiness and catchy flair surely helped. Journalists had a field day with “El Niño,” whereas any headline writer would struggle with “the warm phase of the El Niño–Southern Oscillation.” This event’s high-water mark in pop culture came on October 25, 1997, as Chris Farley strode onto Saturday Night Live wearing a pro-wrestling outfit and bellowing, “I am El Niño!” La Niña takes center stage Sea surface temperature and height anomalies in the eastern Pacific Ocean show the influence of expansion and warming during El Niño (November 1997, top) and cooling during La Niña (November 1998, bottom). (Images courtesy NASA.) In surprisingly short order, the intense El Niño conditions of early 1998 segued into a strong La Niña event (1998–99). Over the preceding years, researchers hadn’t been in total agreement on a name for the cool-water counterpart to El Niño—some had proposed “El Viejo” (the old man”), “anti–El Niño,” or “cold event”—but by 1998, La Niña (“the young girl”) had begun to stick. That summer, NCAR’s Glantz brought a diverse group of scientists, journalists, and policy experts to Boulder for the first-ever La Niña Summit (see PDF for executive summary, United Nations University Press for collected papers). Thus far, the 21st century has been a quiet century for El Niño. The last 15 years have been more dominated by La Niña events, part of a Pacific-wide rearrangement of ocean circulation that tends to shift modes every 20–30 years. This phenomenon is referred to as the Pacific Decadal Oscillation or the Interdecadal Pacific Oscillation, depending on which part of the ocean is analyzed. It’s not yet clear what causes the PDO to switch modes from negative (cool sea surface in the central North Pacific) to positive (warm sea surface), or vice versa. But evidence is mounting that the world’s oceans tend to retain more heat deeper down, well below the surface, during the PDO’s negative phase—apparently a big reason why Earth’s atmosphere hasn’t warmed much since the late 1990s. Forecasts, busts, and probabilities Today, a number of computer models around the world use current ocean temperatures and atmospheric conditions to project the state of ENSO, looking a year or more into the future. Forecasters examine multi-model ensembles, scrutinizing where various models agree or disagree, in order to issue El Niño and La Niña forecasts. Among the leading sources of regular ENSO forecasts are NOAA’s Climate Prediction Center, the International Research Institute for Climate and Society at Columbia University, and the Australian Bureau of Meteorology. (Note that the Australian BOM uses a higher threshold for El Niño development than the U.S. definition.) El Niño hype returned in a big way early in 2014, when a huge surge of warm water spread eastward across the tropical Pacific. Some computer models suggested that a major El Niño event could develop by later in the year. But by the autumn of 2014, El Niño hadn’t yet materialized, a no-show that was widely interpreted as a forecast “bust.” Only in November did the signs of an approaching El Niño become more insistent, with sea surface temperatures in the Pacific’s Niño3.4 measurement region finally pushing well above the threshold of 0.5°C above average. Was the forecast really so bad? Northern spring is a notoriously difficult time to predict ENSO evolution. Moreover, the models and forecasters were never in lockstep agreement about what would happen later in 2014. The model consensus posted by the International Research Institute in April (in partnership with NOAA) wasn’t particularly dire, though some models did end up overshooting the mark. As conditions and model solutions evolved, NOAA's predicted likelihood that an El Niño would form by summer 2014 rose from 50% in the March outlook (PDF) to "exceeding 50%" in April (PDF), 65% in May (PDF), and 80% in June (PDF), with that June forecast applying to fall/winter 2014–15 rather than summer 2014. Update: December 2, 2014 | The above paragraph has been revised to incorporate a fuller range of NOAA products issued in early 2014. Lessons from 2014 As of mid-November 2014, the various prediction models examined by the International Research Institute for Climate and Society (IRI) tended to produce a weak to moderate El Niño for winter 2014-15, but with a fair bit of spread among the model solutions. (Image courtesy IRI.) The “missing” El Niño event of mid-2014 reminds tells us that we need to carefully manage our expectations about El Niño prediction. Once the oceanic and atmospheric fingerprints of El Niño begin to take shape in northern fall, as they're doing right now, then we can look more confidently to the impacts likely to occur the following winter, when El Niño events typically peak. However, this isn’t really predicting El Niño itself so much as “nowcasting” the emergence of El Niño’s oceanic warming and predicting the resulting impacts we’re likely to see. The oceanic shift itself remains much more difficult to forecast. The dance between atmosphere and ocean that nurtures and sustains El Niño is simply too complex for any model to capture with total accuracy. That’s why it’s so important to keep probabilities in mind. We can’t expect ironclad certainty on whether or when the eastern Pacific will warm into an El Niño event, but we can use multiple models to help gauge the odds. Similarly, and crucially, since each El Niño is different and since other weather and climate variables are always in the mix, the presence of El Niño doesn’t guarantee that Los Angeles will be doused with midwinter rain or Seattle will be dry and mild. However, it does shift the likelihoods in those directions. In that sense, we might want to think of El Niño less like an appealing buffoon from a comedy sketch—and more like a shrewd oddsmaker from a densely plotted thriller. Dive Deeper El Niño, La Niña & ENSO FAQ (NCAR/UCAR Backgrounder, December 2014) The "Almost" El Niño of 2014 (UCARConnect, December 2014) Writer/contact:Bob Henson, NCAR/UCAR Communications          

California dryin’

Bob Henson • January 13, 2014 | The first precipitation measurements in what’s now downtown San Francisco began in July 1849, a year before statehood. Now extending almost 165 years, the local rainfall database is one of the nation’s longest—and never has it shown a year as dry as 2013. From the Mexican border to the Pacific Northwest, the past 12 months have left records lying in the dust. It’s been the most parched calendar year in the weather annals of two state capitals (Salem, OR, and Sacramento, CA) as well as Los Angeles and other locations in between. The table below shows how far the new records dipped not only below average but below the prior records.   City             New record     Old record      Average     Records begin   Los Angeles, CA     3.60”       4.08” (1953)     14.93”     1877  San Francisco, CA   5.59”       9.00” (1917)     23.65”     1849  Shasta Dam, CA     16.61”      27.99” (1976)     62.72”     1943  Eugene, OR         21.19”      23.56” (1944)     46.10”     1890 San Francisco’s rainfall is prone to sharp swings from one calendar year to the next, but the dry conditions of 2013 are well below anything experienced since precipitation records began there in 1849. The recording site has moved several times within the downtown area during this period (see history). Golden Gate Weather Services maintains a comprehensive website with various San Francisco rainfall statistics. (Graphic ©UCAR, data courtesy Jan Null. This image is freely available for media & nonprofit use.)   Given this magnitude, it’s impressive how little everyday life in California has been affected by the growing drought. Despite pleas from lawmakers, the state has yet to declare a drought emergency, and Angelenos aren’t yet being asked to take any special measures. That’s largely due to the peculiarities of water supply and delivery across the Southwest. It’s a complex system involving both water and energy, and NCAR scientists are working to unravel how its future may evolve in a climate where rainfall trends are uncertain. Update – January 17: California governor Jerry Brown officially declared a drought emergency today. Two kinds of years As impressive as the 2013 rainfall records are, those who watch California weather will be even more focused on what happens over the next several months. Most of the state’s rainfall comes in the winter—roughly a third of the water used in California is drawn from the Sierra Nevada’s vast snowpack—so it’s the fall-through-spring totals that make or break things in the dry heat of summer. That’s why experts look to the water year (July–June), rather than the calendar year, as the crucial index of California precipitation. That said, the 2013–14 water year is off to a rotten start. It’s telling that a 900-acre fire destroyed dozens of homes in the Big Sur area only a few days before Christmas. The Sierra’s snowpack water content was 84 percent below average as of January 10. This week appears likely to bring record warmth to much of California, along with strong, dry winds and high fire danger in some areas. Rather than bringing wet storms from the Pacific into California, the jet stream at the 500-millibar level (about 18,000 feet) arced far to the north during most of December 2013, as shown in this map of conditions averaged for the month. (Original graphic by NOAA/ESRL Physical Sciences Division, courtesy John Monteverdi, San Francisco State University.) The culprit: a powerful, persistent ridge of upper-level high pressure has prevailed off the U.S. West Coast for more than a year now. This pattern—the strongest and longest-lasting of its type in decades, according to meteorologist/blogger Daniel Swain—tends to steer wet, mild Pacific storms far to the north. Fairbanks, Alaska, got its third freezing rain event of the autumn in early December, a time when the average low temperature there is around –12°F. East of this high pressure zone, which Swain has dubbed "The Ridiculously Resilient Ridge," the jet stream has dipped well south into the central and eastern U.S., bringing some of the most intense winter chill that's been observed in recent years (including the subzero air mass that invaded the region last week). Right now, both the short-range and seasonal outlooks remain worrisome. The most recent model guidance from the National Weather Service's Global Forecast System (GFS), which looks ahead 16 days, suggests little if any rain for much of California into the last week of January. And the latest seasonal outlook from NOAA indicates that the drought is likely to persist throughout most of the Southwest through March. As Swain puts it, "each day that passes without meaningful precipitation is another day when our long-term deficits grow measurably larger." A rainless ridge dominates the southwest United States, including California, in the 384-hour forecast produced by the NWS Global Forecast System at 06Z (1:00 a.m. EST) on January 13. The graphic above shows cumulative precipitation in inches predicted by the model for the entire 16-day period ending on January 29. (Image courtesy NOAA/NWS National Centers for Environmental Prediction.) Still, things can change quickly this time of year, notes Jan Null (Golden Gate Weather Services). Now a consulting meteorologist, Null is a former lead forecaster at the Bay Area’s National Weather Service office. He’s followed regional rainfall for a long time, having completed his master’s thesis at San José State University on San Francisco’s precipitation climatology. Null points out that, as opposed to the frequent but lighter rains of a Seattle winter, California tends to gain most of its precipitation in a few big events, which often occur from midwinter to early spring. “We still have almost six months to go,” says Null. “There are terms in the water community such as Fabulous February, Miracle March, and Awesome April. All of these have been used when we’ve turned things around after the first half of the water year.” Focusing too much on the calendar year data, Null adds, “is like taking the last half of one 49ers game, combining it with the first half of the next game, and saying, ‘We won!’ ” A nexus of moisture and power Water and energy are intertwined in many ways across the U.S. Southwest, including California. Some of those connections stand out more clearly in the harsh light of a drought. Yet there’s been little quantitative study of the interplay between the two variables as climate change unfolds. Using the Southwest as a first case study, NCAR’s David Yates and colleagues have developed a modeling system that takes on this challenge. It tracks how energy and water resources interact as population and water demands grow and as climate swings in and out of periods of drought. The "bathtub ring" around Lake Mead (top, photographed in 2007), an important water source for California, reveals the effect of low levels since the early 2000s. After reaching a record low in 2010–11, the reservoir made a brief recovery before dipping back down over the last two years. As of January 13,  the reservoir's water level was at 48% of capacity ("full pool"). (Wikimedia Commons photo by Waycool27; chart courtesy Lake Mead Water Database.) Yates and NCAR colleague Kathleen Miller recently published a summary of the project, which is supported by the NOAA Sectoral Applications Research Program. It incorporates two models, both developed for resource managers by the Stockholm Environment Institute in collaboration with NCAR: The Water Evaluation and Planning (WEAP) system The Long-range Energy Alternatives Planning (LEAP) system There’s no shortage of complexity when trying to accurately represent and predict the flows and interrelationships of water and energy across the Southwest. California's vast cities and farms draw most of their water from the yearly Sierra snowpack and the Colorado River system, including Lake Mead (pictured at right). It takes a lot of energy to move water hundreds of miles from those locations to where it’s needed. In fact, an estimated 19% of California’s electricity use and 32% of its natural gas consumption are related to water use. And there are complex regulations dictating who gets first rights to each drop. Yates and colleagues are now analyzing results from a WEAP-LEAP integration that examines how a ten-year-long drought—similar to the driest decade in a paleoclimatic simulation of the period 1456 to 1500—might affect the California of the 2020s. Among other things, a drop in available hydropower could force the state to consume more fossil fuel, which could butt up against increasingly strict guidelines for greenhouse gas emissions. Reduced hydropower would also cut into the very energy that’s used to pump water from distant reservoirs to farms and cities. Desalination is drawing interest as a way to wring potable water from the oceans that lie next door to California’s biggest cities. However, it also requires major power. A WEAP-LEAP analysis of this topic assumes that about 5% of Southern California’s water could be produced by desalination by the year 2049. In that scenario, greenhouse gas emissions are slightly higher than for “business as usual,” and the region would still need to import large amounts of water due to growing demands on top of increased year-to-year variability. Long-term outlooks, short-term implications Climate models for the 21st century continue to disagree on whether total precipitation will increase or decrease this century across the Southwest, as noted in a 2013 regional report serving as input for this year’s pending U.S. National Climate Assessment.  The risk of long-term precipitation decline appears greater the further south you go in the region, and short-term variability is expected to remain larger than any longer-term shift. In the most recent weekly U.S. Drought Monitor, based on data through January 7, 2013, about 28% of California is categorized as being in "extreme drought," and more than 98% of the state is experiencing some level of drought. (Analysis by Matthew Rosencrans, NOAA Climate Prediction Center.) There’s higher confidence that rising temperatures will tend to pull more moisture out of the system when drought happens to strike. Warmer conditions will tend to reduce the average snowpack and advance the timing of spring melt across the Sierras. Similar effects are expected in the snowy, high-altitude origins of the Colorado River. The NCAR-led Colorado Headwaters project has been working to simulate how water supplies downstream might change as the century unfolds.  As for the current drought, Yates is watching it closely. After two consecutive water years on the dry side, a severe drought this year could bring increasing trouble. Los Angeles is high on the hydrological pecking order, so the city is projecting ample water supply into at least 2015, but some Central Valley farmers may have to let fields go fallow this summer. Many other complications could ensue if the drought extends into midyear, including an increased risk of major wildland fires. Groundwater often serves as a backup during California drought, but increased pumping is making that resource more scarce over time—and the pumping itself uses energy. “It’ll be interesting to see how agricultural irrigators turn to groundwater and what kind of impact that has on electricity use in California,” says Yates. “If there’s a hot summer,I wouldn’t be surprised to see strains on the electrical system.”

Drought and tourism

July 8, 2013 | A young tech worker in a Virginia office suite may be daydreaming of skiing Colorado’s famed fluffy powder, while a family in Pennsylvania, weary of gray winter skies, plans a summer river float past Rocky Mountain vistas. These are the sorts of visitors who sustain Colorado’s tourism industry, which brought in $14.6 billion in 2010 alone—accounting for roughly 19% of the state’s total economy. But Colorado’s four-season activities are vulnerable to the vagaries of climate. Drier ski slopes, reduced river flows, and increased wildfires can potentially discourage tourists to the state. Should local officials and business leaders do more to plan for the potential impacts of drought on tourism? NCAR scientist Olga Wilhelmi worked with collaborators at the University of Colorado at Denver, the Colorado Water Conservation Board, and the Mountain Studies Institute on a pilot project in southwestern Colorado to study the issue. After compiling a detailed list of local officials, businesses, nonprofits, and other stakeholders, the researchers conducted surveys to assess both the level of drought awareness among those stakeholders and the extent to which they planned for drier conditions that would affect their businesses and communities. The researchers also conducted two focus group discussions in Durango and did follow-up interviews. Wilhelmi and her colleagues found that stakeholders often did not view drought as a business concern. However, communities can take advantage of opportunities to vary offerings during times of drought, such as promoting summer recreational activities if there is a shortened winter, and shifting from water-based sports, such as rafting, boating, or fishing, to other types of sports, such as mountain biking. “Our findings suggest a distinct opportunity for communities in Colorado, and perhaps communities in neighboring states, to become more resilient to the economic impacts of climate,” Wilhelmi says. “This requires a lot of planning but can be well worthwhile given the importance of tourism to local economies.” She notes that climate projections indicate an increased risk for future drought across much of the Southwest, making it even more important for local officials and businesses to plan for dry conditions.  

Dry and drier

Bob Henson • August 6, 2012 | Whether you’re looking at the next few weeks or the next few decades, many parts of the United States are likely to face the silent but devastating impacts of drought. New research, and corrections to an earlier study, help bring this point home. In 2010, NCAR scientist Aiguo Dai (who joins the faculty of the University of Albany, State University of New York, this fall) reviewed the state of knowledge on how drought is expected to evolve as Earth’s climate warms. His 2010 paper in Wiley Interdisciplinary Reviews: Climate Change (WIRES) included a global portrait of 20th century drought, based on observations, and a projection for the 21st century, based on 22 climate models. The resulting maps show much of the globe at an increasing risk of extreme drought conditions over the coming century, if we continue to emit heat-trapping greenhouse gases. Projections averaged across a suite of climate models show a progressively increasing drought risk across much of the globe, as measured by the Palmer Drought Severity Index (PDSI). These maps have been revised from earlier versions. For details and a larger-format version, see the associated news release. This outlook is based on decadal averages of soil moisture content and the Palmer Drought Severity Index (PDSI), a widely accepted measure of drought severity. The PDSI is calculated from precipitation, temperature, radiation, and other factors that influence soil moisture. For 2030–39, Dai’s maps (see right) show much of the central U.S. experiencing drier soils than in 2000–09. The drought risk expands further in the 2060s, and by the 2090s, most of southern Europe and about half of the United States is gripped by extreme drought. These projections are not meant to be literal forecasts for each decade, since there is no way to know exactly how natural variability will play out from decade to decade or how quickly greenhouse gas emissions will increase. New models, similar picture Dai has now strengthened the case by showing that climate models successfully replicate the effects of El Niño and La Niña on global wetness and drought, as well as a worldwide tendency toward drier soils over the last several decades. Dai’s new study, which appeared on August 5 in the online version of Nature Climate Change, also includes fresh projections for the 21st century that draw on new climate runs and scenarios developed for the next major assessment from the Intergovernmental Panel on Climate Change (IPCC). Natural variations will shape the course of drying, Dai points out. Models don’t yet capture the important swings of the Interdecadal Pacific Oscillation (IPO), which affects ocean temperatures as well as North American climate. The oscillation, whose causes are poorly understood, has been in a cold (negative) phase since around 1999. As Dai outlined in another recent paper, this tends to foster drought across the western and central United States. Each phase of the IPO typically lasts 20–30 years, which suggests that the dry conditions that have prevailed across much of the western United States for much of the last decade may last for another 10 to 20 years. After that, one might look for some long-term drought relief, as the IPO could swing back to a warm phase, during which western and central U.S. states typically see above-normal precipitation. However, Dai’s new work stresses that the drying effect of human-produced greenhouse gases should overwhelm natural variability by later this century. “The U.S. may never again return to the relatively wet conditions experienced from 1977 to 1999,” he says. How quickly will we dry out? As noted in an update to this 2010 news release, Dai recently discovered an error in the way that he had extended 21st century projections from the year 2000 onward. He has since recalculated the projections and produced new maps, and a formal correction has been submitted to WIRES. Quantitatively, the error in Dai’s original study is significant, overstating the severity of drought in some regions by a factor of two. However, the corrected maps displayed here continue to show that large parts of the world will continue to move toward increased drought risk, albeit at a slower pace—with the intensity in many locations roughly half as much as indicated in the original maps. The drought projections for the 2090s generated for Dai’s newest paper with the latest IPCC models also show a similar spatial pattern with a reduced intensity. Dai emphasizes that the ultimate magnitude of the drying depends on the rate of future greenhouse gas emissions, while the spatial patterns would not be as directly affected by the rate. For both of his recent papers, Dai employed relatively moderate emission scenarios. While the strength of expected drying is less than in earlier projections, the overall message from Dai’s work is unchanged: drought is likely to become an increasingly widespread and serious threat as this century unfolds. That general conclusion is shared by other recent studies, including a special report on climate extremes issued in late 2011 by the Intergovernmental Panel on Climate Change. In this aerial photo, looking south, smoke from the Flagstaff Fire is visible about two miles (threee kilometers) southwest of the NCAR Mesa Laboratory on June 28, 2012. The building was evacuated for more than a day as near-record heat and tinder-dry conditions fueled the fire. (Photo © Marijke Unger, NCAR Computational & Information Systems Laboratory.) Scientists are also going a step further, pairing global models with fire models to directly evaluate fire risk. A paper published early this year by scientists at the Max Planck Institute for Meteorology; Cornell; University of California, Irvine; and NCAR found that global fire emissions will rise substantially by 2100, with North America showing a strong trend toward increased emissions. And a major study involving 16 global models, published this month in the journal Ecosphere, found that “substantial and rapid shifts are projected for future fire activity across vast portions of the globe.” Drought and heat More drought and fire don’t necessarily imply less rain and snow. Some regions are projected to see drought risk going up even without any drop in overall precipitation. Why the mismatch? It’s largely because the key drought-boosting variable looming in the future isn’t less rain and snow—it’s warmer air. Moisture evaporates from the ground more readily when the atmosphere is warmer, and temperatures rise more quickly when soils are dry. These processes work together to help intensify drought in the absence of rain-making weather features. This dance of drought is playing out right now across much of the United States. A generally dry spring left soils parched, and a blistering summer has intensified the drying across much of the central United States. The weekly U.S. Drought Monitor issued on August 2 (see map below) shows severe, extreme, or exceptional drought now covering most of the area from Nevada to Illinois and from Nebraska to Oklahoma, as well as central Georgia. Some hope for this autumn is evident in NOAA’s most recent seasonal drought outlook, issued on July 19. While drought is projected to persist across the Midwest and Great Plains, improvements are foreseen for the Southwest due to continued periods of showers and storms already being produced by an active North American Monsoon pattern. And most of the world’s longer-range computer models are pointing to the likelihood of an El Niño developing by year’s end. A strong El Niño would boost the odds of above-average precipitation during the winter months across many of the areas now experiencing drought. One important caveat: El Niño doesn’t guarantee that moist conditions will stage a comeback—it simply hikes the odds of that outcome. In the meantime, the nation will likely continue to experience a taste of the longer-term drought conditions that may become increasingly common later this century. The U.S. Drought Monitor issued August 2, 2012, for the week ending July 31 shows a patchwork of intensifying drought over much of the nation. (Image courtesy U.S. Drought Monitor.)

Linking Russia’s heat and Pakistan’s rain

January 4, 2012 | New research led by NCAR scientist Thomas Galarneau provides an in-depth analysis of two extreme weather events whose connection may come as a surprise: Russia’s intense heat wave in summer 2010 and the heavy rains that occurred simultaneously in Pakistan.   The heat wave, which was centered over eastern Europe and western Russia, brought with it severe drought conditions and intense wildfires. At the same time, northern Pakistan was being inundated by episodic bursts of heavy rainfall. Both of these weather events reached extreme intensities on July 28–29, when maximum temperatures in Moscow exceeded 95°F (35°C) and 48-hour rainfall totals measured in parts of Pakistan exceeded about 16 inches (40 centimeters). For the study, which is slated for publication in Monthly Weather Review, Galarneau and colleagues used temperature and rainfall data from surface stations in both Russia and Pakistan, data from satellites and radiosondes, and numerical model analysis fields and ensemble forecasts, to produce a multi-scale analysis of the two events. The results show that a large-scale, stagnant weather pattern known as a “block” developed over western Russia, dividing the jet stream and preventing the normal progression of weather systems from west to east. Energy dispersed downstream from this block, creating a trough (elongated region of low atmospheric pressure) northwest of Pakistan and a ridge (elongated region of high pressure) above the Tibetan Plateau. The pattern was further amplified in response to enhanced convective outflow on the large-scale associated with the active phase of the Madden-Julian Oscillation. The interaction between these extratropical and tropical processes facilitated the formation of an intense upper-level jet stream over northern Pakistan. These conditions also contributed to enhanced southeasterly winds at low levels over northern India and Pakistan, bringing tropical moisture unusually far west from the Bay of Bengal into the area, leading to exceptional rainfall. The intense upper-level jet stream, ridge over the Tibetan Plateau, and deep easterly flow over northern India were features that were absent during previous, less-intense rain events over northern Pakistan in 2010 and were therefore likely key factors in the extreme nature of the late July event.

Climate change: Drought may threaten much of globe within decades

Update - July 3, 2012 This news release has been revised to reflect a miscalculation in the original study that inadvertently resulted when simulations of historical drought were combined with simulations of future drought. The revised maps, below, indicate that drought levels on the Palmer Drought Severity Index may reach -10 in certain regions, whereas the levels reached -20 on the original maps. Similarly, upper-latitude areas become less moist than previously projected. Large portions of the globe are still expected to experience dryness that is extreme if not unprecedented. For many regions, the corrected data show the movement toward drought taking place about three decades slower than originally projected. BOULDER—The United States and many other heavily populated countries face a growing threat of severe and prolonged drought in coming decades, according to a new study by National Center for Atmospheric Research (NCAR) scientist Aiguo Dai. The detailed analysis concludes that warming temperatures associated with climate change will likely create increasingly dry soil conditions across much of the globe in the next 30 years, possibly reaching a scale in some regions by the end of the century that has rarely if ever been observed in modern times. Using an ensemble of 22 computer climate models and a comprehensive index of drought conditions, as well as analyses of previously published studies, the paper finds much of the Western Hemisphere, along with large parts of Eurasia and Africa, may be at threat of extreme drought this century. In contrast, higher-latitude regions from Alaska to Scandinavia are likely to become more moist. Dai cautioned that the findings are based on the best current projections of greenhouse gas emissions. What actually happens in coming decades will depend on many factors, including actual future emissions of greenhouse gases as well as natural climate cycles such as El Niño. The new findings appear this week as part of a longer review article in Wiley Interdisciplinary Reviews: Climate Change. The study was supported by the National Science Foundation, NCAR’s sponsor. “We are facing the possibility of widespread drought in the coming decades, but this has yet to be fully recognized by both the public and the climate change research community,” Dai says. “If the projections in this study come even close to being realized, the consequences for society worldwide will be enormous.” While regional climate projections are less certain than those for the globe as a whole, Dai’s study indicates that most of the United States, especially the West, will be significantly drier by the 2030s. Large parts of the nation may face an increasing risk of extreme drought during the century. Other countries and continents that could face significant drying include: Much of Latin America, including large sections of Mexico and Brazil Regions bordering the Mediterranean Sea, which could become especially dry Large parts of Southwest Asia Much of Africa, especially in western and southern regions The study also finds that drought risk can be expected to decrease this century across much of Northern Europe, Russia, Canada, and Alaska, as well as some areas in the Southern Hemisphere. However, the globe’s land areas should be drier overall. “The increased wetness over the northern, sparsely populated high latitudes can't match the drying over the more densely populated temperate and tropical areas,” Dai says. A climate change expert not associated with the study, Richard Seager of Columbia University’s Lamont Doherty Earth Observatory, adds: “As Dai emphasizes here, vast swaths of the subtropics and the midlatitude continents face a future with drier soils and less surface water as a result of reducing rainfall and increasing evaporation driven by a warming atmosphere. The term 'global warming' does not do justice to the climatic changes the world will experience in coming decades.  Some of the worst disruptions we face will involve water, not just temperature.” A portrait of worsening drought Previous climate studies have indicated that global warming will probably alter precipitation patterns as the subtropics expand. The 2007 assessment by the Intergovernmental Panel on Climate Change (IPCC) concluded that subtropical  areas will likely have precipitation declines, with high-latitude areas getting more precipitation. In addition, previous studies by Dai have indicated that climate change may already be having a drying effect on parts of the world. In a much-cited 2004 study, he and colleagues found that the percentage of Earth’s land area stricken by serious drought more than doubled from the 1970s to the early 2000s. Last year, he headed up a research team that found that some of the world’s major rivers are losing water. In his new study, Dai turned from rain and snow amounts to drought itself, and posed a basic question: how will climate change affect future droughts? If rainfall runs short by a given amount, it may or may not produce drought conditions, depending on how warm it is, how quickly the moisture evaporates, and other factors. Droughts are complex events that can be associated with significantly reduced precipitation, dry soils that fail to sustain crops, and reduced levels in reservoirs and other bodies of water that can imperil drinking supplies. A common measure called the Palmer Drought Severity Index (PDSI) classifies the strength of a drought by tracking precipitation and evaporation over time and comparing them to the usual variability one would expect at a given location. Dai turned to results from the 22 computer models used by the IPCC in its 2007 report to gather projections about temperature, precipitation, humidity, wind speed, and Earth’s radiative balance, based on current projections of greenhouse gas emissions. He then fed the information into the Palmer model to calculate the PDSI. A reading of +0.5 to -0.5 on the index indicates normal conditions, while a reading at or below -4 indicates extreme drought. The index ranges from +10 to -10 for current climate conditions, although readings below -6 are exceedingly rare, even during short periods in small areas. By the 2030s, the results indicated that some regions in the United States and overseas could experience particularly severe conditions, with average decadal readings potentially dropping to -2 to -4 in much of the central and western United States as well as several regions overseas, and -4 or lower in parts of the Mediterranean. By the end of the century, many populated areas, including parts of the United States and much of hte Mediterranean and Africa, could face readings in the range of -4 to -10. Such decadal averages would be almost unprecedented. Dai cautions that global climate models remain inconsistent in capturing precipitation changes and other atmospheric factors, especially at the regional scale. However, the 2007 IPCC models were in stronger agreement on high- and low-latitude precipitation than those used in previous reports, says Dai. There are also uncertainties in how well the Palmer index captures the range of conditions that future climate may produce. The index could be overestimating drought intensity in the more extreme cases, says Dai. On the other hand, the index may be underestimating the loss of soil moisture should rain and snow fall in shorter, heavier bursts and run off more quickly. Such precipitation trends have already been diagnosed in the United States and several other areas over recent years, says Dai. “The fact that the current drought index may not work for the 21st century climate is itself a troubling sign,” Dai says. About the article Title: Drought under global warming: a review Author: Aiguo Dai Publication: Wiley Interdisciplinary Reviews: Climate Change      Future drought. These four maps illustrate the potential for future drought worldwide over the decades indicated, based on current projections of future greenhouse gas emissions. These maps are not intended as forecasts, since the actual course of projected greenhouse gas emissions as well as natural climate variations could alter the drought patterns.The maps use a common measure, the Palmer Drought Severity Index, which assigns positive numbers when conditions are unusually wet for a particular region, and negative numbers when conditions are unusually dry. A reading of -4 or below is considered extreme drought. Regions that are blue or green will likely be at lower risk of drought, while those in the red and purple spectrum could face more unusually extreme drought conditions. Update: The above maps were uploaded to this article in June 2012. (Courtesy Wiley Interdisciplinary Reviews. This image is freely available for media & nonprofit use*.)
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