What Is La Niña, And Will It Cause Serious Climate Disruption?
By Steve Connor
03 March, 2007
The IndependentWhy are we asking this question now?
American scientists have warned that a weather phenomenon in the South Pacific known as "El Niño", which results in warm Pacific Ocean currents, is dying out and will be replaced by its opposite number, "La Niña". The US National Oceanic and Atmospheric Administration (NOAA) said that cooler-than-normal water temperatures have developed at the surface in the east-central equatorial Pacific.
It means that La Niña conditions are likely to develop this spring and are set to strengthen over the next few months and could last for up to two years. This naturally occurring phenomenon is likely to reach peak intensity in December to February and then begin to weaken next March to May.
Does this have any wider implications?
One of the greatest concerns is that La Niña is associated with an increase in Atlantic hurricanes. It can also cause drier-than-usual conditions in the southern United States, which experienced a serious drought during the last La Niña some seven years ago. However, Australia and Indonesia, which often have droughts during El Niño years, can be wetter during a La Niña phase.
Conrad Lautenbacher, NOAA's administrator, warned that the next La Niña may mean a stormier yet drier time ahead for the United States: "Although other scientific factors affect the frequency of hurricanes, there tends to be a greater-than-normal number of Atlantic hurricanes and fewer-than-normal number of eastern Pacific hurricanes during La Niña events," he said. "During the winter, usual La Niña impacts include drier and warmer-than-average conditions over the southern United States."
How often does La Niña occur?
On average, El Niño and La Niña occur every three to five years. However, the actual interval between them has varied from two to seven years. La Niñas began this century with the first in 1903, followed by events in 1906, 1909, 1916, 1924, 1928, 1938, 1950, 1954, 1964, 1970, 1973, 1975, 1988, and 1995. The last La Niña began in 1998 and ended in 2001. These events typically continued into the following spring, although they can last longer. Since 1975, La Niñas have been only half as frequent as El Niños.
What causes La Niña?
Cooler-than-normal subsurface water in the tropical Pacific builds up, as eastern-moving winds and waves help to bring cold, deep water to the surface through a complex set of events that are still under study. Sea surface temperatures can fall to at much as 4C below normal. The warm body of water associated with El Niño - which expands to cover the tropics during this phase of the Pacific cycle - begins to shrink to the west during La Niña.
What is the difference between La Niña and El Niño?
Both terms refer to large-scale changes in sea-surface temperature across the central and eastern tropical Pacific. Usually, sea-surface readings off South America's west coast range from 15C to 20C, while they exceed 26C in the "warm pool" located in the central and western Pacific. This warm pool expands to cover the tropics during El Niño but shrinks to a small enclave in the west during La Niña.
La Niña events sometimes follow on the heels of El Niño conditions, although not always. Sometimes there is a "neutral" phase that is neither one nor the other. The scientific name for the phenomenon is the El Niño/Southern Oscillation (ENSO), and it describes the coupled ocean-atmosphere process that includes both El Niño and La Niña.
Does global warming influence La Niña or El Niño?
Scientists are still not sure about this. They want to know whether a warmer world is likely to produce more El Niños and La Niñas or cause more intense cycles to occur. According to NOAA: "At this time we can't preclude the possibility of links but it would be too early to definitely say there is a link." What is pretty clear, however, is that global warming is likely to increase the risk of extreme weather events associated with La Niña and El Niño. In other words, global warming is likely to make matters worse.
What is the origin of the name 'La Niña'?
Fishermen off the west coast of South America recognised the periodic appearance of warm water in their fishing grounds, which they called El Niño, which means "The Little Boy" or "Christ child" in Spanish. They called it this because the warm body of water usually appeared around Christmas time.
When scientists recognised that there was an opposite event in the southern Pacific oscillation, they dubbed it La Niña, which means "The Little Girl" in Spanish. La Niña is sometimes called El Viejo (Old Man), anti-El Niño, or simply "a cold event" or "a cold episode". There has been a confusing range of uses for the terms El Niño, La Niña and ENSO by both the scientific community and the general public.
Are there any environmental benefits of La Niña?
Coral bleaching - when coral reefs eject the microbes that give them their distinctive colours - appears to be worse during an El Niño phase, when Pacific Ocean temperatures get warmer. Coral bleaching was particularly pronounced during 1997-98 because a very strong El Niño occurred that year. The increase of sea temperatures caused by that El Niño was superimposed on the sea temperature warming trend observed in some parts of the Pacific and Indian Oceans, caused by global warming. Coral seem very sensitive to rising sea temperatures, so the cooler phase created by La Niña should in theory benefit corals, albeit temporarily given that ocean temperatures globally are tending to rise.
Is it true that La Niña and El Niño can affect the spin of the Earth?
Yes, but the effect is very small. El Niño results in a decrease in the earth's rotation rate, an increase in the length of day. La Niña tends to have the opposite effect.
El Niño is associated with a weakening of the tropical Pacific trade winds, and with a strengthening of the mid-latitude westerlies both at the surface and at higher altitudes. To balance these changes in atmospheric winds, the earth's rotation rate decreases in order to conserve total angular momentum of the earth-atmosphere system. Conservation of angular momentum is a basic physical principle which operates, for example, when a ballerina brings her arms closer to her body to spin faster. The change, however, is only about 1 millisecond at the peak of a strong El Niño. There are 86,400 seconds in a day, so this change represents one part in 100 million. Such a change will have little effect on normal activities on a human scale, such as flying an aeroplane.
© 2007 Independent News and Media Limited
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