Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
El Niño and La Niña revision notes El Niño and La Niña are the opposite extremes of the ENSO cycle. Changes in air pressure and ocean temperatures in the Equatorial Pacific result in large-scale changes in global weather and climate patterns. Under normal conditions the air pressure at Darwin (in the west) is lower than at Tahiti (in the east). The ocean is warmer in the west and cooler in the east. The warm ocean phase, El Niño, occurs when there is higher air pressure in the western Pacific, i.e. a reversal in the air pressure and in ocean temperature The cold ocean phase, La Niña, occurs when there is lower air surface pressure than normal in the western Pacific. El Niño and La Niña episodes last about 9 - 12 months and occur about every 3 to 5 years. El Niño occurs more frequently than La Niña. Normal Conditions Low pressure in the east Wet in the east Higher pressure in the west Colder temperatures and ocean in the west Trade winds blow off shore in the east so cold, nutrient rich water upwells off the South American coast, resulting in productive fisheries and a more shallow thermocline. Trade winds push water over to west, which piles up in west. This results in a deeper surface layer and a deeper thermocline. El Niño Conditions Higher pressure in the east Dry in the east Lower pressure in the west Warmer temperatures and ocean in the west The trade winds weaken and may reverse direction. The normal flow of water away from South America decreases and ocean water piles up (relatively) off South America. The thermocline becomes deeper and upwelling decreases. The sea surface temperature increases to higher than normal in the Eastern Pacific. The rainfall is now more common in the Central Pacific while the Western Pacific becomes dry. Heavy rainfall and flooding occur over Peru and drought over Indonesia and Australia. The reduced up–welling, adversely affects fisheries off the coast of South America. The tropical cyclones in the South Pacific shift eastward. La Niña Conditions Even lower pressure in the east Wetter in the east Even higher pressure in the west Colder temperatures and ocean in the west Trade winds blow more strongly Even shallower thermocline and increased upwelling. Lower than normal sea surface temperatures. During El Niño, New Zealand has: stronger or more frequent winds from the west in summer drought in east coast areas and more rain in the west. In winter, the winds are more from the south, bringing colder conditions In spring and autumn south–westerly winds are more common. During La Niña, NZ has: More north–easterly winds occur Moist, rainy conditions to the north–east of the North Island Reduced rainfall to the south and south–west of the South Island. Some areas, such as central Otago and South Canterbury, may have a drought Warmer than normal temperatures