Environmental Health Center
Extreme weather — floods and droughts, tornadoes and hurricanes, and other wild weather-induced events — is staking its claim to be “story of the decade.” And perhaps also tobe the story of the first decade of the coming century.
When weather moves to page one of the paper or to the opening piece on the evening news broadcasts, few question that it becomes a “hard news” story of great interest to print and broadcast audiences.
“Normal” weather is reported as weather … it’s the unusual weather that moves it into the team-reporting hard-news category. And that has been happening a lot in the past year or so.
Weather and climate scientists’ understanding and skill in forecasting weather extremes have improved markedly over the past decade. Communicated effectively, those forecasts, with mitigation and advance planning, can help save lives, property, and billions of dollars in insurance and repair costs.
Fewer extreme weather events today need be explained as random “freaks of nature,” thanks to improvements in understanding of weather cycles such as El Niño and La Niña.
Despite the leaps in understanding and reporting on such weather cycles, forecasters are quick to point to continuing uncertainties and uncharted territory still requiring better understanding. Potentially significant worldwide warming in coming decades and centuries will add a major new element of uncertainty to the weather forecaster’s domain, and local and regional impacts may be especially unpredictable.
While uncertainties about climate variability and change will persist, broadcast meteorologists and their colleagues in the meteorology field increasingly can understand how weather extremes may affect different regions … and how their viewers and audiences might best anticipate and manage risks from those extremes.
Hurricane Bonnie, which devastated parts of North Carolina in August, was only the first in an unusual series of destructive Atlantic hurricanes, each one coming virtually on the heels of the last: Danielle, Earl, Georges, Mitch. At one point there were four hurricanes grinding away in the Atlantic at the same time. By the time the season ended in November, it had become one of the deadliest in over 200 years, killing some 10,000 people in eight countries and causing billions of dollars in damage.
Coming a year after the relative calm of the 1997 Atlantic hurricane season, the La Niña-influenced 1998 hurricane season was a busy one for broadcast meteorologists, and one of the most deadly and damaging on record. While there may be no scientific way to link a specific hurricane to a particular cause such as La Niña, the 1995-96 La Niña brought to America’s eastern shoreline one of the most active and destructive hurricane seasons in a century — 19 named storms, 11 of which were hurricanes. (Opal and Fran stand in the record books among the 10 most damaging hurricanes since the United States began keeping records.)
The 1998 Atlantic season, between June 1 and November 30, spawned 14 tropical cyclones (the average is 10), of which ten became hurricanes (the average is six). There were $3.2 billion in insured damages and 21 deaths in the United States alone.
Just as El Niños suppress conditions favorable to the formation of hurricanes along the eastern United States, La Niñas create conditions conducive to them. But what, exactly, is La Niña?
The phase when the temperature of the sea surface in the eastern tropical Pacific is warmer than “normal” is called El Niño. The opposite phase, when the sea surface temperature there is colder than normal, is called La Niña. Climatologists call the whole cycle the El Niño/Southern Oscillation (ENSO). 1
The Earth absorbs a major portion of the sun’s radiant energy in the tropical Pacific, where surface waters are warmed. In a “normal” year, trade winds blow steadily from the East (an area of higher pressure) to the West (an area of lower pressure). The pressure difference and wind push the warm surface layer of the ocean westward, piling warm water up in the West Pacific. (The sea surface is actually higher in the West than in the East in a normal year, and also dramatically warmer.)
On the East side of the Pacific, off the coast of Peru, for example, cold water normally wells up from the deep to replace the warm surface water.
In El Niño years, by comparison, the pressure difference eases, and the trade winds relax. The upwelling of cold water in the eastern Pacific subsides. The area of warm surface water extends much farther east.
In “normal” years, the warmer water in the western Pacific spawns tropical rainclouds there. In El Niño years, the warmer water, and consequently the rain, move eastward into the open Pacific. Then, without the rainfall, places like Indonesia may experience serious drought.
Over the warm regions of the tropical Pacific, the immense tropical rainclouds towering toward the stratosphere are large enough to influence the global circulation of the atmosphere — to push off course the jet streams that circle the globe. As a result of changes in global circulation, normal weather patterns can be disrupted far beyond the tropical Pacific. These distant effects are called “teleconnections.”
The ENSO actually can swing beyond the “normal” state to a state opposite to that of El Niño, with the trade winds amplified and the eastern Pacific colder than normal. This phenomenon is often referred to as La Niña. In a La Niña year, many regions inclined toward drought during an El Niño are instead prone to more rain — and those more likely to see rain during an El Niño tend toward dryness.
Some scientists estimate that there are fewer than half as many hurricanes in the Atlantic during an El Niño than during “normal” conditions and more hurricanes than normal during La Niña conditions.2 Nowadays, scientists prefer to call El Niño the “warm phase” and La Niña the “cold phase.”
Both El Niños and La Niñas vary in intensity from weak to strong. The intervals at which El Niños return are not exactly regular, but vary from two to seven years (see Figure 1). Sometimes an El Niño subsides into a “normal” pattern. At other times it gives way to a La Niña. During the 1980s, ENSO followed a fairly regular four-year cycle (warm in 1982-83, cold in 1984-85, warm in 1986-87, and cold in 1988-89).
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| Source: Higgins, R.W., J.-K.E. Schemm, W. Shi, and A. Leetmaa, 1999: Extreme precipitation events in the western United States related to tropical forcing, J. Climate, 12, (in press) |
Indonesia, during the 1997-98 El Niño drought episode, was ravaged by forest fires. But that country’s officials feared that torrential La Niña rains on Indonesia’s charred and devegetated lands could produce flash floods, serious soil erosion, and an ashy brew of runoff toxic enough to kill fish and damage ecosystems.3
In the United States, La Niña tends to bring colder than normal winter temperatures to upper Midwest states like Minnesota, North Dakota, and Wisconsin. At the same time, La Niña brings warmer than normal winter temperatures to the southern states, a thin band along much of the Pacific coast, and, during some months, portions of the Great Lakes and New England areas (see Figure 2).4
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| Source: National Oceanic and Atmospheric Administration's Climate Prediction Center |
La Niña tends also to bring wetter winter weather to much of the northern tier to which it brings colder weather — and to bring drier winter weather to the southern tier (see Figure 3).
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| Source: National Oceanic and Atmospheric Administration's Climate Prediction Center |
There are important exceptions to the above generalization. For one thing, different parts of the country feel the effects of La Niña at different times. During the fall the drought effects tend to reach deep into the nation’s heartland, including some normally arid states like Iowa, Missouri, Nebraska, South Dakota, and Utah. Drought effects may be especially intense in already arid southwestern states like Arizona, New Mexico, and Texas. Effects may also differ depending on whether the La Niña is strong (pronounced), moderate, or weak.
In some cases, the effects of La Niña are not the opposite of the effects of El Niño. Californians were plagued by excess rain through most of the El Niño winter of 1997-98. But if Northern Californians think La Niña will bring respite, they could be wrong. Bill Mork, the state’s climatologist, says some La Niña winters have caused even more problems with flooding.5
Few people on the U.S. Eastern Seaboard have forgotten the blizzard of January 6, 1996, in which “some of the heaviest snowfall amounts of the late 20th century blanketed the urban corridor from Washington, D.C., to Boston,”6 according to the NOAA’s National Weather Service. The deepest snowfall, some 48 inches, fell at Snowshoe, West Virginia,7 but 31 inches fell on Philadelphia. The National Weather Service attributed some 80-90 deaths to the storm,8 and said it “brought most private and government activity to a halt for the better part of a week.”9 The National Weather Service cited insurance losses of more than $500 million, but this did not include public cleanup costs and the economic impact of lost business.
Adding insult to injury, heavy floods plagued communities across Pennsylvania about 10 days later, when a quick snowmelt was aggravated by several inches of rain and ice jams in rivers.10
While the origins of this classic winter storm, in the end, boil down to the right combination of moisture and cold air, some National Weather Service analysts saw the fingerprints of La Niña on it. They noted that the 1995-96 winter featured unusual patterns of jet stream circulation that were associated with a newly born La Niña.11
Even as it suppresses tropical storms in the Atlantic, El Niño seems to promote them in the eastern Pacific.14 El Niño got the blame for a monster hurricane named Linda which menaced the California coast in September 1997 — one of the strongest ever seen off the West Coast.15 Hurricanes in those waters are rare, and not likely to be seen during La Niña conditions.
La Niña may also bring more tornadoes to the United States. Some researchers have found a correlation between ENSO events, as measured by sea surface temperature in the Pacific, and tornado occurrences in the eastern two-thirds of the United States.16 Those results suggest that El Niños reduce tornado activity in the Great Plains and Florida, while La Niñas increase tornado activity in the Ohio and Tennessee River Valleys. But victims of the lethal series of tornadoes that raked Florida February 22-23, 1998, during an El Niño episode, may take little comfort from those averages. Forty-one people died17 in this event, which was also blamed on El Niño. Clearly there is more to learn, and more research needed.
Of course, any answer must keep in mind that we can only really talk about averages and tendencies. Your mileage may vary, as they say.
While climate scientists have made huge strides in understanding the ENSO phenomenon in the last two decades, remaining uncertainties are a reminder that there is still much more research to be done. Predictions of the ENSO cycle have only recently become possible. They have gotten better, and with work, they will get better still.
Climatologists today have an arsenal of scientific instruments needed to observe ENSO in fine detail. The decade-long Tropical Ocean Global Atmosphere (TOGA) program (which ended in 1994) left in place the Tropical Atmosphere Ocean (TAO) Array of about 70 moored buoys with measuring instruments spanning the equatorial Pacific. Today’s satellites, like the NOAA GOES series and NASA’s TOPEX/Poseidon, can measure many environmental variables, including the changes in sea-surface temperature and sea level that mark the ENSO cycle. Computer models have also gotten better at simulating the coupled ocean-atmosphere system that produces ENSO.
ENSO research goes forward on many fronts, carried out by university researchers, international programs, and U.S. government agencies. Much of the daily observation and prediction of the ENSO cycle is carried out by units of the National Oceanic and Atmospheric Administration (NOAA). NOAA’s Office of Global Programs supports basic research into the physical processes that underlie climate. Many NOAA units are involved in collecting the basic data needed to observe the ENSO cycle, as is the National Aeronautics and Space Administration, which flies the observation satellites. Finally, NOAA units like the Climate Prediction Center turn observations into predictions that can be used by the many people and industries affected by the ENSO cycle.
A key question involves the frequency and severity of El Niños and La Niñas. Scientists increasingly are debating the proposition that global greenhouse warming may be causing more frequent and intense El Niños (the ENSO warm phase) and less frequent La Niñas (the cold phase).
Kevin Trenberth and Timothy Hoar, both of the National Center for Atmospheric Research, note18 that the El Niño episode that lasted from 1990 to 1995 was uncommonly long (see Figure 1). (Some scientists think this was actually two moderate El Niños with a weak La Niña in between.) They calculate that such an event is likely to occur only once every 1,000 to 3,000 years. The rarity of such an event leads them to question whether global warming may be changing the normal ENSO pattern. One possible hitch in their approach, however, is that they can work with only a 100-year record of solid data. They must use statistical techniques to simulate ENSO’s behavior over the longer period.
The old shibboleth about the weather’s always changing no doubt will remain true long into the future. What won’t change is the public’s appetite for timely and insightful weather reporting and forecasting, and that’s an area where journalists join with the meteorology community in finding better and better ways to meet evolving information needs.
A prize-winning compendium of information and links on ENSO, from many other Web sources. Lots of graphics. See the “Impacts” section for many leads on the potential effects of a La Niña on different U.S. localities.
Good collection of accessible references to all sorts of ENSO effects. Some for general audience. Also has a list of other bibliographies and an “El Niño Resource Center” with links to many other sources.
A round-up of ENSO-related information from all parts of NOAA. Includes links to current research and the latest forecasts.
Everything you wanted to know about hurricanes — and more — in fairly plain English from acknowledged experts.
This page lists all the ENSO forecasts. They don’t always agree. The forecasts are useful for following a La Niña as it develops. The official U.S. forecast is from NOAA’s Climate Prediction Center, National Centers for Environmental Prediction, National Weather Service.
This interactive page gives maps plotting the risks of extremes of temperature or precipitation in different regions of the United States according to type of ENSO event and the time of year. Good for localizing impacts.
This consortium of university programs, funded by the National Science Foundation, posts a page of links to information on La Niña impacts, with special emphasis on the West and California.
A broad collection of authoritative and up-to-date information about ENSO and other climate variations.
Detailed maps showing the historical temperature and precipitation variations during La Niñas for all times of year, plotted out to a sub-state level of detail.
Three-month climate “outlooks” (forecasts) showing the probability ranges for weather extremes in the United States, rendered in easily readable color maps.
2. James J. O’Brien, Todd S. Richards, and Alan C. Davis, “The Effect Of El Niño on U.S. Landfalling Hurricanes,” Center for Ocean Atmospheric Prediction Studies, Florida State University, Tallahassee, http://www.coaps.fsu.edu:80/~richards/paper.html. Christopher W. Landsea, “FAQ: Hurricanes, Typhoons, and Tropical Cyclones, Part F: Tropical Cyclone Forecasting,” Version 2.6 (13 January, 1998), by NOAA AOML/Hurricane Research Division, http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqF.html#F2.
3. J. Madeline Nash, Cover story, “Fire and Rain, Hell May Have Fury Like La Niña,” Time Asia, April 20, 1998, Vol. 151, No. 15, http://www.pathfinder.com/time/asia/magazine/1998/ 980420/cover1.html.
4.This is based on maps at http://nic.fb4.noaa.gov:80/products/analysis_monitoring/ensostuff/laNiña/index.aspx.
5. Edie Lau, Sacramento Bee, November 3, 1997, p. A1.
6. National Weather Service summary, no title, Eastern Region, Pittsburgh office, http://www.nws.noaa.gov/er/pit/bliz96.htm.
7. National Climatic Data Center, NOAA, “The Blizzard of 96!,” http://www.ncdc.noaa.gov/publications/blizzard96.html.
8. National Weather Service, http://nws.noaa.gov/om/images/96dis.gif.
9. National Weather Service summary, Eastern Region, Pittsburgh office, no title, http://www.nws.noaa.gov/er/pit/bliz96.htm.
10. U.S. Geological Survey, U.S. Department of the Interior, “Statewide Floods in Pennsylvania, January 1996,” Fact Sheet FS-103-96, http://water.usgs.gov/public/wid/FS_103-96/FS_103-96.html.
11.Climate Prediction Center, National Weather Service, NOAA, Special Climate Summary 96/1, http://nic.fb4.noaa.gov:80/products/special_summaries/96_1/.
12. James J. O’Brien, Todd S. Richards, and Alan C. Davis, “The Effect of El Niño on U.S. Landfalling Hurricanes,” Center for Ocean Atmospheric Prediction Studies, Florida State University, http://www.coaps.fsu.edu:80/~richards/paper.html.
13. W. M. Gray, “Atlantic Seasonal Hurricane Frequency. Part I: El Niño and 30 mb Quasi-Biennial Oscillation Influences,” Monthly Weather Review, 112 (1984), 1649-1668.
14. Pacific Marine Environmental Laboratory, NOAA, “Frequently Asked Questions,” http://www.pmel.noaa.gov/toga-tao/el-nino/faq.html#hurricanes.
15. Charles Petit, “Monster Storm Heads Toward South State; El Niño Blamed for Rare West Coast Hurricane,” San Francisco Chronicle, September 13, 1997.
16. Mark C. Bove, “Impacts Of ENSO On United States Tornadic Activity,” Center For Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee, http://www.coaps.fsu.edu/~bove/tornado/main.html.
17. National Climatic Data Center, NOAA, http://www.ncdc.noaa.gov/pub/data/special/febstorm.html#FLOR.
18. Kevin Trenberth and Timothy Hoar, National Center for Atmospheric Research, Geophysical Research Letters, January 1, 1998.
| June 23, 2000 | | Disclaimer/Policy |
