El NiÑo and La NiÑa phenomena

El Niño and La Niña are the names given to changes in the winds, atmospheric pressure , and seawater that occur in the Pacific Ocean near the equator. El Niño and La Niña are opposite phases of a back and forth cycle in the Pacific Ocean and the atmosphere above it. Unlike winter and summer, however, El Niño and La Niña do not change with the regularity of the seasons ; instead, they repeat on average about every three or four years. They are the extremes in a vast repeating cycle called the Southern Oscillation, El Niño being the warm extreme and La Niña the cold extreme.

Although El Niño and La Niña take place in a small portion of the Pacific, the changes caused by Southern Oscillation can affect the weather in large parts of Asia , Africa , Indonesia and North and South America . Scientists have only recently become aware of the far-reaching effects of the Southern Oscillation on the world's weather. An El Niño during 1982–83 was associated with record snowfall in parts of the Rocky Mountains, flooding in the southern United States, and heavy rain storms in southern California, which brought about floods and mud slides.

The name El Niño comes from Peruvian fishermen. They noticed that near the end of each year, the seawater off the South American coast became warmer, which made fishing much poorer. Because the change appeared each year close to Christmas, the fishermen dubbed it El Niño, Spanish for "the boy child" referring to the Christ child. Every few years, the changes brought with El Niño were particularly strong or long lasting. During these strong El Niños, the warmer sea waters nearly wiped out fishing and brought significant

changes in weather. For example, normally dry areas on shore could receive abundant rain, turning deserts into lush grasslands for as long as these strong El Niños lasted. In the 1950s and 60s it was found that strong El Niños were associated with increased sea surface temperatures throughout the eastern tropical Pacific. In recent years, these strong El Niños have been recognized as not just a local change in the sea, but as one half of a vast atmospheric-oceanic cycle.

The other half of the repeating cycle has been named La Niña, or the girl child. This phase of the Southern Oscillation is also sometimes called El Viejo, or the old man.

The Southern Oscillation was detected in the early 1920s by Sir Gilbert Walker. He was trying to understand the variations in the summer monsoons (rainy seasons) of India by studying the way atmospheric pressure changed over the Pacific Ocean. Based on meteorologists' previous pressure observations from many stations in the southern Pacific and Indian oceans , Walker established that over the years, atmospheric pressure seesawed back and forth across the ocean. In some years, pressure was highest over northern Australia and lowest over the southeastern Pacific, near the island of Tahiti. In other years, the pattern was reversed. The two pressure patterns had specific weather patterns associated with each, and the change from one phase to the other could mean the shift from rainfall to drought , or from good harvests to famine. In the late 1960s, Jacob Bjerknes, a professor at the University of California, first proposed that the Southern Oscillation and the strong El Niño sea warming were two aspects of the same vast atmosphere-ocean cycle.

El Niño and the Southern Oscillation (often referred to as ENSO) take place in the tropics, a part of the world dominated by prevailing winds, called the trade winds. Near the equator in the tropical Pacific, these easterly (east to west) winds blow day in and day out and tend to pull the surface water of the ocean along with them. This pulls the warm surface water westward, where it collects on the western edge of the ocean basin, the area that includes Indonesia, eastern Australia and many Pacific Islands. The warm waters literally pile up in these areas, where the sea level is about 16 inches (40 cm) higher than in the eastern Pacific.

Meanwhile, along the coast of South America colder water from the ocean depths rises to the top, since the warmer water has been blown westward. The result is called upwelling, and it occurs along much of the coasts of South and North America . Upwelling has two important consequences. The cold deep waters tend to have more nutrients than surface water; these nutrients are essential to phytoplankton, the tiny plants of the sea that provide food for many other types of sea life. Thus, upwelling zones are very productive for fish and the animals (and people) that depend on fish for food. The second result of upwelling cold water is that it cools the air above it. Cool air is denser than warm air, and cool air in the atmosphere cannot begin rising to form clouds and thunderstorms. As a result, the areas near upwelling zones tend to be arid (desert-like) because rain clouds rarely form.

The warmer water that builds up in the western Pacific warms the air above it. This warm moist air frequently rises to form clouds, which eventually produce rainfall. When the trade winds are blowing the warm water their way, the lands along the western Pacific enjoy abundant rainfall. Many rain

forests are found in these areas, such as those of Borneo and New Guinea.

The pattern of winds described above is the La Niña phase of the Southern Oscillation. It sets up the areas of high and low atmospheric pressure observed by Walker and others: in the west, warm air rising produces low pressure, while farther east the cooler, denser air leads to areas of high pressure.

The atmosphere and the ocean form a system that is coupled, that is, they respond to each other. Changes in the ocean will cause a response in the winds above it, and vice versa. For reasons not yet fully understood, the coupled atmosphere-ocean system of the La Niña phase begins to change, slowly developing the characteristics of El Niño phase. The trade winds weaken somewhat, so that they pull less warm water to the western edge of the Pacific. This causes far-reaching changes. Fewer rain clouds form over the lands along the western Pacific. The lush rain forests dry out and become fuel for forest fires. The area of heavy rain shifts to the mid-southern Pacific, where formerly desert island are soaked day after day. In the eastern Pacific, the surface water becomes warmer, since it is no longer being driven westward. Ocean upwelling is weakened, so the water near the surface soon runs low on nutrients, which support the ocean food chain. Many species of fish are driven elsewhere to find food; in severe El Niño years fish populations may be almost completely wiped out. Bird species that depend on fish must look elsewhere, and the human fishing population face economic hardship. At the same time, the warmer waters offshore encourage the development of clouds and thunderstorms. Normally dry areas in western South America, such as Peru

and Ecuador, may experience torrential rains and flooding during the El Niño phase.

While its effects have long been noted in the tropical Pacific, El Niño is now being studied for its impact on weather around the world. The altered pattern of winds and ocean temperatures during an El Niño is believed to change the high-level winds, called the jet streams, that steer storms over North and South America. El Niños have been linked with milder winters in western Canada and the northern United States, as most severe storms are steered northward to Alaska. As Californians saw in 1982-83, El Niño can cause extremely wet winters along the west coast, bringing torrential rains to the lowlands and heavy snow packs to the mountains. The jet streams altered by El Niño can also contribute to storm development over the Gulf of Mexico , which bring heavy rains to the southeastern United States. Similar changes occur in countries of South America, such as Chile and Argentina, while droughts may affect Bolivia and parts of Central America.

El Niño also appears to affect monsoons, which are annual shifts in the prevailing winds that bring on rainy seasons. The rains of the monsoon are critical for agriculture in India, Southeast Asia and portions of Africa; when the monsoon fails, millions of people are at risk of starvation. At present it appears that while El Niños do not always determine monsoons, they are associated with weakened monsoons in India and southeastern Africa, while tending to strengthen those in eastern Africa.

In general, the effects of El Niño are reversed during the La Niña extremes of the Southern Oscillation cycle. During the 1999 La Niña episode, for example, the central and north-eastern

United States experienced record snowfall and sub-zero temperatures, while rainfall increased in the Pacific Northwest and a record number of tornadoes plagued the southern states. Not all El Niños and La Niñas have equally strong effects on the global climate because every El Niño and La Niña event is of a different magnitude and duration.

The widespread weather impacts of the two extreme phases of the Southern Oscillation make their understanding and prediction a high priority for atmospheric scientists. Researchers have developed computer models of the Southern Oscillation that mimic the behavior of the real atmosphere-ocean system. These computer simulations require the input of mountains of data about sea and wind conditions in the equatorial Pacific. The measurements are provided by a large and growing network of instruments. Ocean buoys, permanently moored in place across the Pacific, constantly relay information on water temperature , wind, and air pressure to weather prediction stations around the world. The buoys are augmented by surface ships, island weather stations, and Earth observing satellites.

Even with mounting data and improving computer models, El Niño, La Niña and the Southern Oscillation remain difficult to predict. However, the Southern Oscillation models are now being used in several countries to help prepare for the next El Niño. Countries most affected by the variations in El Niño, such as Peru, Australia and India, have begun to use El Niño prediction to improve agricultural planning.

See also Air masses and fronts; Atmospheric circulation; Ocean circulation and currents