La Niña

La Niña has traditionally received less attention than the more well-known El Niño. For example, during the years 1997 and 1998, El Niño (pronounced el NEE-nyo) was the weather phenomenon most in the news. However, from late 1998 through the first half of 2000 El Niño was succeeded by the less widely publicized La Niña (la NEE-nya) phenomenon. La Niña is the cold-water counterpart to El Niño. El Niño is a period of unusual warming in the Pacific near Peru and Ecuador, while La Niña is a period of abnormally cool water temperatures in the same region. La Niña sometimes follows in the wake of El Niño. La Niña's influence is typically felt in the form of colder-than-usual winters in the northern United States and warmer-than-usual winters in the southern United States. La Niña also promotes the formation of Atlantic Ocean hurricanes that can affect Caribbean islands and the U.S. East Coast.

The term "la niña" is Spanish for "little girl." It was named for its relation to El Niño. Two other terms that are sometimes used to describe this cold-water phenomenon are El Viejo (el vee-AY-hoe), which means "the old man"; and "cold-phase ENSO event" (in contrast to El Niño, the warm phase of the El Niño/Southern Oscillation). According to guidelines set forth by the Japanese Meteorological Association, a La Niña year is one in which average sea temperatures along the equator in the Pacific are more than 1°F (0.5°C) colder than usual, and the water remains cold for at least six months.

La Niña sometimes, but not always, follows El Niño. Since 1975 there have been half as many La Niñas as there have been El Niños. On average, La Niña is present one year in every four. Sometimes La Niñas occur as infrequently as once every ten years. La Niña conditions typically last from nine to twelve months, but may persist for up to two years.

WORDS TO KNOW

air pressure:

the pressure exerted by the weight of air over a given area of Earth's surface. Also called atmospheric pressure or barometric pressure.

cold-phase ENSO (El Niño/Southern Oscillation):

another name for La Niña; colder-than-normal eastern Pacific waters.

convective zone:

the region of warm tropical water over which thunderstorms form; the ocean under the Intertropical Convergence Zone.

El Niño:

a term that means "the Christ child" or "little boy" in Spanish. A period of unusual warming of the Pacific Ocean off the coast of Peru and Ecuador. It usually starts around Christmas, which is how it got its name.

El Niño/Southern Oscillation (ENSO):

the simultaneous warming of the waters of the eastern Pacific and the accompanying shifts in air pressure over the eastern and western Pacific.

flooding:

the inundation of water onto land that is normally dry.

gale-force wind:

any wind whose sustained speed is between 39 and 54 mph (63 and 87 kph).

Intertropical Convergence Zone:

a belt of warm, rising, unstable air formed from the inward-flowing trade winds from north and south of the equator.

jet stream:

the world's fastest upper-air winds. Jet streams travel in a west-to-east direction, at speeds of 80 to 190 miles (130 to 300 kilometers) per hour, around 30,000 feet (9,150 meters) above the ground. Jet streams occur where the largest differences in air temperature and air pressure exist. In North America, jet streams are typically found over southern Canada and the northern United States, as well as over the southern United States and Mexico. The northern jet stream is called the polar jet stream, and the southern jet stream is called the subtropical jet stream.

La Niña:

Spanish for "little girl," a period of cooler-than-normal water temperatures in the eastern Pacific near the coast of Peru and Ecuador. It often follows an El Niño.

monsoon:

a name for seasonal winds that result in a rainy season during the summer on tropical continents, when the land becomes warmer than the sea beside it.

mud slide:

a landslide of mostly mud mixed with debris, often caused by heavy rains on steep land with sparse vegetation.

polar jet stream:

a North American jet stream, typically found over southern Canada or the northern United States.

precipitation:

water in any form, such as rain, snow, ice pellets, or hail, that falls to Earth's surface.

pressure gradient:

the difference in air pressure between a high and low pressure area relative to the distance separating them.

research buoy:

a tethered or drifting buoy placed in the open ocean capable of recording atmospheric and ocean conditions and transmitting them to a satellite.

subtropical jet stream:

a North American jet stream, typically found over the southern United States or northern Mexico.

thunderstorm:

a storm resulting from strong rising air currents; characterized by heavy rain or hail along with thunder and lightning.

trade winds:

an area near the equator of prevailing winds that blow from the northeast north of the equator and the southeast south of the equator.

upwelling:

cold water flowing toward the surface, often caused by prevailing winds or pressure differences.

warm-phase ENSO (El Niño/Southern Oscillation):

another name for El Niño; warmer-than-normal eastern Pacific waters.

A cooling of the waters

The main characteristic of La Niña is a cooling of the waters in the tropical Pacific, from the coast of South America to the central equatorial region. Normally, the Pacific Ocean temperature off the coast of South America is around 68°F (20°C). During El Niño years the water temperature may be 10°F (6°C) warmer than normal. In contrast, the temperature of coastal waters falls as much as 15°F (8°C) below normal during La Niña years.

La Niña's cold surface waters are produced by a strong upwelling—a churning up of cold water from below. The upwelling during La Niña is intensified by a strong easterly (from the east) trade wind, which is a prevailing wind near the equator. As the trade wind blows warm surface water westward across the ocean, cold water upwells to take its place.

The El Niño/La Niña connection

Like El Niño, La Niña is produced by shifts in water temperature and air pressure (also called atmospheric pressure) between the eastern and western portions of the southern tropical Pacific Ocean. While the shift in El Niño is toward warmer water and lower air pressure in the eastern Pacific, however, the shift in La Niña is toward colder water and higher air pressure in the eastern Pacific.

La Niña and El Niño can be thought of as extremes in the range of conditions that may exist in the tropical Pacific Ocean, or as book-ends of the El Niño/Southern Oscillation (ENSO).Whereas El Niño is considered a warm-phase ENSO, La Niña is considered a cold-phase ENSO.

In normal conditions, trade winds blow from the eastern Pacific (from the coast of South America), where pressure is higher, to the western Pacific, where pressure is lower. During El Niño, the pressure over the western Pacific rises and the pressure over the eastern Pacific drops, causing the trade winds to weaken or reverse course. During La Niña, the pressure gradient, or the difference between the high and low pressure areas related to the distance between them, tilts more steeply toward the west than usual. This is primarily because of the presence of very cold water in the eastern Pacific, which increases the air pressure there. This steepening of the pressure gradient causes trade winds to blow strongly toward the west.

Meteorologists' understanding of La Niña is not as great as that of El Niño. Less effort has been spent researching La Niña, in part, because La Niña is not as powerful or potentially destructive a force as El Niño. Whereas El Niño provokes unusual weather patterns, La Niña merely intensifies typical weather patterns.

La Niña, El Niño, or normal?

Defining the condition of the tropical Pacific Ocean is tricky business. While some meteorologists assert that the ocean undergoes periods of "normalcy," others argue that El Niño or La Niña is always present to some degree. Weather expert Kevin Trenberth of the National Center for Atmospheric Research stated that in the tropical Pacific for the years 1950–1997: conditions were normal 46 percent of the time; El Niño was present 31 percent of the time; and La Niña was present 23 percent of the time.

Scientists were driven to learn more about El Niño by the reduction of Peruvian fisheries in the 1970s. La Niña has no such obvious consequence. It was only in the 1980s, when the far-reaching impacts of La Niña (called teleconnections) were discovered, that research into La Niña began in earnest.

La Niña and the jet stream

During La Niña, the "obstacles" that divert the jet stream during El Niño are removed. That is to say, the towering thunderstorms (storms caused by rising air currents) that form off the coast of South America during El Niño years shift to the western-central Pacific and weaken, due to cooler waters. With the displacement of the convective zone (the region of warm water over which thunderstorms form) comes a weakening of the subtropical jet stream found over the southern United States and northern Mexico.

Generally, during La Niña the subtropical jet stream occupies a region farther north than it does during El Niño. The area of strongest jet stream activity during La Niña is the central Rocky Mountains. It is there that the most severe weather can be expected. The jet stream also passes over the Great Lakes in the east. The polar jet stream, typically over the northern United States and southern Canada, is also weakened and pushed farther north, over Alaska and northern Canada.

Effects of La Niña

The effects of El Niño and La Niña are felt most intensely in the winter, when temperature contrasts between northern and southern states are greatest and the jet stream more directly influences the weather. The warming and cooling of the waters that characterize El Niño and La Niña also peak during the Northern Hemisphere winter.

In many ways, La Niña's effects on the weather are the opposite of El Niño's. For instance, La Niña brings cold winters to the northern Great Plains states, the Pacific Northwest, the Great Lakes states, and Canada, and warmer-than-usual winters to the Southeast. El Niño, in contrast, often spells warmer-than-usual winters for the northern United States. and colder-than-usual winters for the southern United States.

Whereas El Niño brings flooding (the inundation of normally dry land) to California, the Southwest, eastward across the states bordering the Gulf of Mexico, and Florida, La Niña brings drier-than-usual conditions to those locations. Also in contrast to El Niño, La Niña brings dry weather to the Central Plains states and the Southeast, and lots of precipitation to the Pacific Northwest. While El Niño brings unusually heavy rains to Peru and Ecuador, and drought to Southeast Asia and Australia, La Niña brings drought to the South American coast and flooding to the western Pacific region.

Another contrast between the two phenomena is that while El Niño hinders development of Atlantic hurricanes, La Niña seems to promote hurricane formation in the region. El Niño's westerly winds (from west to east) blow in the opposite direction of low-altitude winds in the Atlantic Ocean, lopping the tops off thunderclouds before they can come together into hurricanes. La Niña's winds, in contrast, which blow strongly from east to west, encourage the development of Atlantic hurricanes.

The effects of La Niña on weather patterns can be described as an exaggeration of normal conditions. For instance, while the northern United States typically has cold winters, La Niña makes them colder; and while coastal Peru and Ecuador are typically dry, La Niña makes them drier. Indonesia typically receives monsoon rains, caused by seasonal winds in the summer months, but La Niña makes the rains heavier.

However, it is it is misleading to label a specific rainstorm, hurricane, or other weather occurrence a "La Niña event" or an "El Niño event." La Niña and El Niño influence the position and intensity of weather patterns, but they are only two among many other factors that influence local weather.

The 1998–2000 La Niña

In May 1998 a rapid cooling of the tropical Pacific waters began. This temperature drop signaled the end of El Niño and the start of La Niña. Over the next few months the cooling trend continued, after which temperatures varied. Ocean temperatures returned to normal at the end of June 1999, prompting some meteorologists to call a premature end to La Niña. The cooling resumed, however, leading to predictions that La Niña would continue through early 2000.

The first clue to the arrival of La Niña came in the fall of 1997. Scientists detected a drop in temperature of a large pool of ocean water in the western Pacific, 420 feet (128 meters) below the surface. Temperature readings over the next several months, taken by research buoys (floating devices containing weather instruments) indicated that the cool water was moving eastward and upward, toward the coast of South America. Throughout May and June 1998, temperatures along a 5,000-mile (8,000 kilometer) strip of coastal water dropped more than 15°F (8°C), to about 65°F (18.4°C).

Drought and wildfire

La Niña, as expected, produced dry conditions in the summers of 1998 and 1999 for the Southwest, the Southeast, the central Plains states, and even the mid-Atlantic states. In addition to drought, some of these areas faced extended heat waves. In many places, non-essential uses of water (such as watering lawns and washing cars) were banned. Perhaps the greatest consequence of the hot, dry conditions was the outbreak of wildfires. Hundreds of thousands of acres burned during these two summers.

A key reference to: The water cycle

The water cycle (also called the hydrologic cycle) is the continuous movement of water between the atmosphere and Earth's surface (oceans and landmasses). On one side of the equation is precipitation and on the other side is evaporation—the process by which liquid water at the surface converts to a gas and enters the atmosphere.

Eighty-five to ninety percent of the moisture that evaporates into the atmosphere comes from the oceans. The rest evaporates from the soil, lakes, and rivers that make up the continental land-masses. Plants lose water through tiny pores in the underside of their leaves in a process called transpiration.

Some of the moist air above oceans is carried over land by the wind. Clouds form and drop rain and snow on the ground. When precipitation hits the ground, it either sinks in or runs off, depending on the surface composition. On soft ground, most of the water sinks into the soil to be absorbed by plant roots or to seep down into underground aquifers, which are underground layers of spongy rock, gravel, or sand in which water collects.

Some of it runs off into streams and rivers. If the water strikes a hard surface, like rock or pavement, most of it runs directly into streams or man-made drains. Eventually this water also flows into rivers.

The oceans lose water in this portion of the cycle. More water evaporates from them than returns as precipitation. The oceans get this water back when the rivers empty their water back into the oceans. Thus the global water budget—the volume of water coming and going from the oceans, atmosphere, and continental land-masses—is kept in balance.

If the water cycle is kept in balance, that means that global precipitation levels remain fairly constant. So why do droughts occur? The answer is that rain and snow do not consistently fall in equal amounts in any given place. Moisture may evaporate from one place, travel through the atmosphere, and fall to the ground as rain in another. It is possible, then, for a given location to get lots of rainfall one year and almost no rainfall the next.

In Florida, more than 500,000 acres were consumed by fire in the summer of 1998. In the first half of 1999, more than 35,000 acres burned. Several homes were lost to the blaze. A state of emergency for the entire state was declared in April 1999 and the National Guard was brought in to assist firefighters.

In March 1999, fire blackened seventy-eight thousand acres of Nebraska prairie—an area the size of Rhode Island—and killed hundreds of cattle. That fire came one month after an unusual wintertime grass fire near North Platte, Nebraska, that burned fifteen thousand acres.

For the first half of 1999, the mid-Atlantic region (New Jersey, New York, Pennsylvania, Delaware, Maryland, and Washington, D.C.) had precious few drops of precipitation. That spring wildfires raged in Georgia, eastern Tennessee, and western North Carolina, prompting the evacuation of hundreds of residents.

The Southwest rebounded from its wet El Niño years with extraordinarily dry conditions in late 1998. The desert wildflowers that bloomed in abundance in the spring of 1998 were noticeably absent in 1999. Precipitation was so scarce in the mountains that Arizona ski resorts took the unusual step of closing for the 1998–1999 season. Even desert oases dried up in Arizona, prompting mountain lions and other desert-dwellers to enter populated areas and drink water out of chlorinated swimming pools.

In some parts of Texas the drought began as early as August 1998. By April 1999, a state of emergency had been announced for 170 of the state's 254 counties.

The wet zone

As predicted for La Niña, the Pacific Northwest received above average precipitation during the winter of 1998–99. In Washington State, Mount Baker logged more than 90 feet (27 meters) of snow, rivaling Mount Rainier's record for snowfall in a single season. Lowland communities in the Pacific Northwest and in the Sierra Nevada Mountains in California faced threats of flooding. Around Puget Sound, precipitation fell for ninety-one straight days during the first quarter of 1999.

In the early spring, the Midwest received heavy rains. The Midwest also experienced a series of tornadoes in early 1999; however, meteorologists are divided as to whether or not La Niña was responsible.

As predicted, India, China, Southeast Asia, and Australia experienced unusually heavy monsoon rains and flooding. In the summer of 1998 flood waters stood chest-high in parts of eastern India and Bangladesh. River flooding in China claimed the lives of 4,150 people.

In August 1999, several Asian countries were besieged by torrential downpours, and gale-force winds with speeds between 39 and 54 mph (63 and 87 kph). They also suffered from mud slides, landslides of mud caused by heavy rain, and flooding. Across South Korea, Thailand, the Philippines, and Vietnam tens of thousands of people were forced to evacuate their homes and at least fifty people perished in the floods.

An active hurricane season

A La Niña event began just in time for the 1998 hurricane season, which runs from June 1 through November 30. As predicted for a La Niña year, the Atlantic hurricane season was fast and furious. With a death toll of between twelve thousand and fifteen thousand, mostly in Central America and the Caribbean, 1998 was the deadliest Atlantic hurricane season 1780.

The year 1998 saw fourteen tropical storms, ten of which became hurricanes and three of which became major hurricanes. Seven of the 1998 storms made landfall in the United States (the third highest number on record), causing twenty-one deaths and more than 3.2 billion dollars in damage. The real story of the 1998 hurricane season was a pair of monster hurricanes, Georges and Mitch, that brought death and destruction to Central America.

A mild winter

Predictions of an extra-cold winter of 1998–99 for the northern United States turned out to be wrong. The polar jet stream remained farther north than expected, trapping polar air north of the Canadian border. As a result, northerners enjoyed a relatively mild winter. The southern portion of the United States, south of the Rockies in the west and New York in the east, experienced spring-like conditions. Record high temperatures were set in many places.

The winter of 1999–2000

In July 1999, meteorologists at the National Centers for Environmental Prediction, the National Oceanic and Atmospheric Administration, and other agencies predicted a continuing, but weakened, La Niña for the winter of 1999–2000. Forecasts included wet conditions for the Pacific Northwest and dry conditions for the South. The predictions were at least partly accurate. The winter proved to be one of the warmest and driest on record. It was driest in the Southwest and Southeast with Louisiana recording abnormally dry conditions. The Northwest was average to wet, with Montana and Wyoming experiencing above-average precipitation.

Weather experts also predicted a lively hurricane season for autumn 1999, anticipating at least three major storms. This prediction proved accurate. There were twelve named storms, eight hurricanes, and five intense hurricanes during the season.

Predictions for winter of 2006–2007

Scientists continue to try and predict the effects of El Niño and La Niña. In the fall of 2006, the Climate Prediction Center/National Centers for Environmental Prediction (a branch of the National Weather Service) predicted El Niño conditions developing for the 2006–2007 winter season. These conditions included warmer-than-normal temperatures for western and central Canada, and the western and northern United States. Wetter-than-average conditions were predicted for portions of the U.S. Gulf Coast and Florida, while drier-than-average conditions were expected in the Ohio River Valley and the Pacific Northwest. As of February 2007 scientists saw a return to ENSO-neutral conditions with the possible development of La Niña conditions.

[See AlsoClimate; El Niño; Flood; Forecasting; Hurricane; Weather: An Introduction ]

For More Information

BOOKS

D'Aleo, Joseph S. The Oryx Resource Guide to El Niño and La Niña. Phoenix, AZ: Oryx Press, 2002.

Philander, S. George. El Niño, La Niña, and the Southern Oscillation. San Diego, CA: Academic Press, 1990.

Rose, Sally. El Niño! And La Niña. New York: Simon and Schuster, 1999.

PERIODICALS

Trenberth, Kevin. "Uncertainty in Hurricanes and Global Warming." Science. (17 Jun 2005): pp. 1753-1754.

WEB SITES

"La Niña." National Aeronautic and Space Administration: Earth Observatory. 〈http://earthobservatory.nasa.gov/Library/LaNina/〉 (accessed March 8, 2007).

"La Niña." National Oceanic and Atmospheric Administration. 〈http://www.elnino.noaa.gov/lanino.html〉 (accessed March 9, 2007).