At the end of every year, the cold, northward strong Peruvian current is replaced by a warm, southward weak countercurrent. This occurrence was orginally named 'El Nino', after Christ as it usually appeared during the Christmas season. The warm weak southward countercurrents typically last for a few weeks before it is replaced by the cold strong northward Peruvian current. However, this weak warm southward countercurrent becomes stronger every 3 to 7 years, and along with this current is the accumulation of warm surafce water in the Eastern Pacific. Scientists these days use the term 'El Nino' specifically referring to the episodes of ocean warming that occurs every few years only.
The first observation of something related would be by Sir Gilbert Walker, who noticed an irregular oscillation between atomspheric pressure in Darwin, northern Australia, and Tahiti, a central Pacific equatorial Island. Tahiti usually has higher atmospheric pressure than Darwin, but every time, towards the end of the year, where El Nino occurs, the barometric pressure drops over large portions of the South eastern Pacific, whereas in the Western Pacific, near Indonesia and Northern Australia, the pressure increases. Then, when the currents change, the pressure difference between the two regions change back to normal, with the Eastern Pacific having higher pressure while the Western Pacific turned to having lower pressure.
From Wikipedia:
El Niño /ɛl ˈniːnjoʊ/ (Spanish pronunciation: [el ˈniɲo]) is the warm phase of the El Niño Southern Oscillation (commonly called ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (between approximately the International Date Line and 120°W), including off the Pacific coast of South America. El Niño Southern Oscillation refers to the cycle of warm and cold temperatures, as measured by sea surface temperature, SST, of the tropical central and eastern Pacific Ocean. El Niño is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. The cool phase of ENSO is called "La Niña" with SST in the eastern Pacific below average and air pressures high in the eastern and low in western Pacific. The ENSO cycle, both El Niño and La Niña, cause global changes of both temperatures and rainfall.[2][3]
Originally the term El Niño applied to an annual weak warm ocean current that ran southwards along the coast of Peru and Ecuador at about Christmas time.[7] However, over time the term has evolved and now refers to the warm and negative phase of the El Niño Southern Oscillation and is the warming of the ocean surface or above-average sea surface temperatures in either the central and eastern tropical Pacific Ocean.[8][9] This warming causes a shift in the atmospheric circulation with rainfall becoming reduced over Indonesia and Australia, while rainfall and tropical cyclone formation increases over the tropical Pacific Ocean.[10] The low-level surface trade winds, which normally blow from east to west along the equator, either weaken or start blowing from the other direction.[9]
Typically, this anomaly happens at irregular intervals of two to seven years, and lasts nine months to two years.[24] The average period length is five years. When this warming occurs for seven to nine months, it is classified as El Niño "conditions"; when its duration is longer, it is classified as an El Niño "episode".[25]
Effects on the global climate[edit]
El Nino affects the global climate and disrupts normal weather patterns, which as a result can lead to intense storms in some places and droughts in others.[60][61]
Tropical cyclones[edit]
Most tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving into the main belt of the Westerlies.[62] Areas west of Japan and Korea tend to experience much fewer September–November tropical cyclone impacts during El Niño and neutral years. During El Niño years, the break in the subtropical ridge tends to lie near 130°E, which would favor the Japanese archipelago.[63]
Within the Atlantic Ocean vertical wind shear is increased, which inhibits tropical cyclone genesis and intensification, by causing the westerly winds in the atmosphere to be stronger.[64] The atmosphere over the Atlantic Ocean can also be drier and more stable during El Niño events, which can also inhibit tropical cyclone genesis and intensification.[64] (Recall: Air is drier means unsaturated and thus conditional stability due to environmental lapse rate being between the dry adiabatic lapse rate and the moist adiabatic lapse rate. Additionally, air may also be absolutely stable because the environmental lapse rate is below the moist adiabatic lapse rate.) Within the Eastern Pacific basin: El Niño events contribute to decreased easterly vertical wind shear and favours above-normal hurricane activity.[65] However, the impacts of the ENSO state in this region can vary and are strongly influenced by background climate patterns.[65] The Western Pacific basin experiences a change in the location of where tropical cyclones form during El Niño events, without a major change in how many develop each year.[64]As a result of this change Micronesia is more likely to be affected by several tropical cyclones, while China has a decreased risk of being affected by several tropical cyclones.[citation needed] A change in the location of where tropical cyclones form also occurs within the Southern Pacific Ocean between 135°E and 120°W, with tropical cyclones more likely to occur within the Southern Pacific basin than the Australian region.[10][64] As a result of this change tropical cyclones are 50% less likely to make landfall on Queensland, while the risk of a tropical cyclone is elevated for island nations like Niue, French Polynesia, Tonga, Tuvalu and the Cook Islands.[10][66][67]
Remote influence on tropical Atlantic Ocean[edit]
A study of climate records has shown that El Niño events in the equatorial Pacific are generally associated with a warm tropical North Atlantic in the following spring and summer.[68] About half of El Niño events persist sufficiently into the spring months for the Western Hemisphere Warm Pool to become unusually large in summer.[69] Occasionally, El Niño's effect on the Atlantic Walker circulation over South America strengthens the easterly trade winds in the western equatorial Atlantic region. As a result, an unusual cooling may occur in the eastern equatorial Atlantic in spring and summer following El Niño peaks in winter.[70] Cases of El Niño-type events in both oceans simultaneously have been linked to severe famines related to the extended failure of monsoon rains.[22]
Antarctica[edit]
Many ENSO linkages exist in the high southern latitudes around Antarctica.[71] Specifically, El Niño conditions result in high-pressure anomalies over the Amundsen and Bellingshausen Seas, causing reduced sea ice and increased poleward heat fluxes in these sectors, as well as the Ross Sea. The Weddell Sea, conversely, tends to become colder with more sea ice during El Niño. The exact opposite heating and atmospheric pressure anomalies occur during La Niña.[72] This pattern of variability is known as the Antarctic dipole mode, although the Antarctic response to ENSO forcing is not ubiquitous.[72]
Regional impacts[edit]
Observations of El Niño events since 1950, show that impacts associated with El Niño events depend on what season it is.[73] However, while certain events and impacts are expected to occur during events, it is not certain or guaranteed that they will occur.[73] The impacts that generally do occur during most El Niño events include below-average rainfall over Indonesia and northern South America, while above average rainfall occurs in southeastern South America, eastern equatorial Africa, and the southern United States.[73]
Australia and the Southern Pacific[edit]
During El Niño events, the shift in rainfall away from the Western Pacific, can mean that rainfall across Australia is reduced.[10] Over the southern part of the continent, warmer than average temperatures can be recorded as weather systems are more mobile and fewer blocking areas of high pressure occur.[10] The onset of the Indo-Australian Monsoon in tropical Australia is delayed by two to six weeks, which as a consequence means that rainfall is reduced over the northern tropics.[10] The risk of a significant bushfire season in south-eastern Australia is higher following an El Niño event, especially when it is combined with a positive Indian Ocean Dipole event.[10] During an El Niño event, New Zealand tends to experience stronger or more frequent westerly winds during their summer, which leads to an elevated risk of drier than normal conditions along the east coast.[74] There is more rain than usual though on New Zealand's West Coast, because of the barrier effect of the North Island mountain ranges and the Southern Alps.[74]
Fiji generally experiences drier than normal conditions during an El Niño, which can lead to drought becoming established over the Islands.[75] However, the main impacts on the island nation is felt about a year after the event becomes established.[75] Within the Samoan Islands, below average rainfall and higher than normal temperatures are recorded during El Niño events, which can lead to droughts and forest fires on the islands.[76] Other impacts include a decrease in the sea level, possibility of coral bleaching in the marine environment and an increased risk of a tropical cyclone affecting Samoa.[76]
Africa[edit]
In Africa, East Africa — including Kenya, Tanzania, and the White Nile basin — experiences, in the long rains from March to May, wetter-than-normal conditions. Conditions are also drier than normal from December to February in south-central Africa, mainly in Zambia, Zimbabwe, Mozambique, and Botswana.
Asia[edit]
As warm water spreads from the west Pacific and the Indian Ocean to the east Pacific, it takes the rain with it, causing extensive drought in the western Pacific and rainfall in the normally dry eastern Pacific. Singapore experienced the driest February in 2014 since records began in 1869, with only 6.3 mm of rain falling in the month and temperatures hitting as high as 35 °C on 26 February. The years 1968 and 2005 had the next driest Februaries, when 8.4 mm of rain fell.[77]
Europe[edit]
El Niño's effects on Europe are controversial, complex and difficult to analyse, as it is one of several factors that influence the weather over the continent and other factors can overwhelm the signal.[78][79]
North America[edit]
Over North America, the main temperature and precipitation impacts of El Niño, generally occur in the six months between October and March.[80][81] In particular the majority of Canada generally has a milder than normal winters and springs, with the exception of eastern Canada where no significant impacts occur.[82] Within the United States, the impacts generally observed during the six-month period include; wetter-than-average conditions along the Gulf Coast between Texas and Florida, while drier conditions are observed in Hawaii, the Ohio Valley, Pacific Northwest and the Rocky Mountains.[80] Over California and the South-Western United States, there is a weak relationship between El Nino and above-average precipitation, as it strongly depends on the strength of the El Niño event amongst other factors.[80]
The synoptic condition for the Tehuantepecer is associated with high-pressure system forming in Sierra Madre of Mexico in the wake of an advancing cold front, which causes winds to accelerate through the Isthmus of Tehuantepec. Tehuantepecers primarily occur during the cold season months for the region in the wake of cold fronts, between October and February, with a summer maximum in July caused by the westward extension of the Azores High. Wind magnitude is greater during El Niño years than during La Niña years, due to the more frequent cold frontal incursions during El Niño winters.[83] Its effects can last from a few hours to six days.[84]
South America[edit]
Because El Niño's warm pool feeds thunderstorms above, it creates increased rainfall across the east-central and eastern Pacific Ocean, including several portions of the South American west coast. The effects of El Niño in South America are direct and stronger than in North America. An El Niño is associated with warm and very wet weather months in April–October along the coasts of northern Peru and Ecuador, causing major flooding whenever the event is strong or extreme.[85] The effects during the months of February, March, and April may become critical. Along the west coast of South America, El Niño reduces the upwelling of cold, nutrient-rich water that sustains large fish populations, which in turn sustain abundant sea birds, whose droppings support the fertilizer industry. The reduction in upwelling leads to fish kills off the shore of Peru.[86]
The local fishing industry along the affected coastline can suffer during long-lasting El Niño events. The world's largest fishery collapsed due to overfishing during the 1972 El Niño Peruvian anchoveta reduction. During the 1982–83 event, jack mackerel and anchoveta populations were reduced, scallops increased in warmer water, but hake followed cooler water down the continental slope, while shrimp and sardines moved southward, so some catches decreased while others increased.[87] Horse mackerel have increased in the region during warm events. Shifting locations and types of fish due to changing conditions provide challenges for fishing industries. Peruvian sardines have moved during El Niño events to Chilean areas. Other conditions provide further complications, such as the government of Chile in 1991 creating restrictions on the fishing areas for self-employed fishermen and industrial fleets.[citation needed]
The ENSO variability may contribute to the great success of small, fast-growing species along the Peruvian coast, as periods of low population removes predators in the area. Similar effects benefit migratory birds that travel each spring from predator-rich tropical areas to distant winter-stressed nesting areas.[citation needed]
Southern Brazil and northern Argentina also experience wetter than normal conditions, but mainly during the spring and early summer. Central Chile receives a mild winter with large rainfall, and the Peruvian-Bolivian Altiplano is sometimes exposed to unusual winter snowfall events. Drier and hotter weather occurs in parts of the Amazon River Basin, Colombia, and Central America.[citation needed]
