El Niño and La Niña
El Niño and La Niña refer to weather patterns that occur periodically across the Pacific Ocean. They are both characterised by variable levels of winds and rainfall in different parts of New Zealand.
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El Niño is a natural feature of the global climate system. Originally it was the name given to the periodic development of unusually warm ocean waters along the tropical South American coast and out along the Equator to the dateline, but now it is more generally used to describe the whole "El Niño – Southern Oscillation (ENSO) phenomenon", the major systematic global climate fluctuation that occurs at the time of an "ocean warming" event. El Niño and La Niña refer to opposite extremes of the ENSO cycle, when major changes in the Pacific atmospheric and oceanic circulation occur.
When neither El Niño nor La Niña are present, (usually referred to as "neutral" or normal conditions), trade winds blow westward across the Pacific, piling up warm surface water so that Indonesian sea levels are about 50 cm higher than those in Ecuador. Cool, nutrient–rich sea water "wells up" off the South American coast, supporting marine ecosystems and fisheries. Relatively cold sea temperatures also extend along the equator from South America towards the central Pacific. High rainfall occurs in the rising air over the warmest water to the west, whereas the colder east Pacific is relatively dry.
During El Niño events, the trade winds weaken, leading to a rise in sea surface temperature in the eastern equatorial Pacific and a reduction of "up–welling" off South America. Heavy rainfall and flooding occur over Peru, and drought over Indonesia and Australia. The supplies of nutrient rich water off the South American coast are cut off due to the reduced up–welling, adversely affecting fisheries in that region. In the tropical South Pacific the pattern of occurrence of tropical cyclones shifts eastward, so there are more cyclones than normal in areas such as the Cook Islands and French Polynesia.
During La Niña events, the trade winds strengthen, and the pattern is a more intense version of the "normal conditions", with an even colder tongue of sea surface temperatures in the eastern equatorial Pacific.
It was the atmospheric part of ENSO – (called the 'Southern Oscillation' or 'SO'), that first attracted the attention of scientists. Sir Gilbert Walker documented and named the SO in the 1930s. It is one of several persistent patterns of high and low pressures around the globe.
The clearest sign of the SO is the inverse relationship between surface air pressure at two sites: Darwin, Australia, and the South Pacific island of Tahiti. Over periods of a month or longer, higher pressure than normal at one site is almost always concurrent with lower pressure at the other, and vice versa. The pattern reverses every few years. It represents a "see–saw", a mass of air oscillating back and forth across the International Date Line in the tropics and subtropics.
The Southern Oscillation Index (SOI) measures how abnormal the pressure difference between Tahiti and Darwin is. Very low values of this index correspond to El Niño conditions (red), while very high SOI values coincide with La Niña episodes (black). For example, the large El Niño in 1997/98 is evident in the figure below, as is the moderate La Niña in 1998/99.
During El Niño, New Zealand tends to experience stronger or more frequent winds from the west in summer, typically leading to drought in east coast areas and more rain in the west. In winter, the winds tend to be more from the south, bringing colder conditions to both the land and the surrounding ocean. In spring and autumn south–westerly winds are more common.
La Niña events have different impacts on New Zealand's climate. More north–easterly winds are characteristic, which tend to bring moist, rainy conditions to the north–east of the North Island, and reduced rainfall to the south and south–west of the South Island. Therefore, some areas, such as central Otago and South Canterbury, can experience drought in both El Niño and La Niña. Warmer than normal temperatures typically occur over much of the country during La Niña, although there are regional and seasonal exceptions.
Although ENSO events have an important influence on New Zealand's climate, it accounts for less than 25% of the year to year variance in seasonal rainfall and temperature at most New Zealand measurement sites. East coast droughts may be common during El Niño events, but they can also happen in non El Niño years (for example, the severe 1988–89 drought). Also, serious east coast droughts do not occur in every El Niño. However, the probabilities of the climate variations discussed above happening in association with ENSO events are sufficient to warrant management actions and planning to be taken when an El Niño or La Niña is expected or in progress.