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La Nina

As we head towards summer, weather forecasters are predicting another severe La Nina effect for Australia, especially in tropical areas of north western and north eastern parts of Australia.

What is La Nina?

La Niña is a coupled ocean-atmosphere phenomenon that is the counterpart of El Niño as part of the broader El Niño-Southern Oscillation climate pattern. During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean will be lower than normal by 3–5 °C. In the United States, an episode of La Niña is defined as a period of at least 5 months of La Niña conditions. The name La Niña originates from Spanish, meaning “the girl,” analogous to El Niño meaning “the boy.”

La Niña, sometimes informally called “anti-El Niño”, is the opposite of El Niño, where the latter corresponds instead to a higher sea surface temperature by a deviation of at least 0.5 °C, and its effects are often the reverse of those of El Niño. El Niño is famous due to its potentially catastrophic impact on the weather along both the Chilean, Peruvian and Australian coasts, among others. La Niña is often preceded by a strong El Niño.

Effects

Regional impacts of La Niña
The results of La Niña are mostly the opposite of those of El Niño; for example, El Niño would cause a wet period in the Midwestern U.S., while La Niña would typically cause a dry period in that area. At the other side of the Pacific La Niña can cause heavy rains. For India, an El Niño is often a cause for concern because of its adverse impact on the south-west monsoon; this happened in 2009. A La Niña, on the other hand, is often beneficial for the monsoon, especially in the latter half. The La Niña that appeared in the Pacific in 2010 probably helped 2010’s south-west monsoon end on a favourable note. But then, it also contributed to the deluge in Australia, which resulted in one of that country’s worst natural disasters with large parts of Queensland either under water from floods of unusual proportions or being battered by tropical cyclones, including that of category 5 Tropical Cyclone Yasi. It wreaked similar havoc in south-eastern Brazil and played a part in the heavy rains and consequent flooding that have affected Sri Lanka.

Recent occurrences
There was a strong La Niña episode during 1988–1989. La Niña also formed in 1995, and in 1999–2000. A minor La Niña occurred 2000–2001. The La Niña which developed in mid 2007 and lasted until early 2009, was a moderate one. NOAA confirmed that a moderate La Niña developed in their November El Niño/Southern Oscillation Diagnostic Discussion, and that it would likely continue into 2008. According to NOAA, “Expected La Niña impacts during November – January include a continuation of above-average precipitation over Indonesia and below-average precipitation over the central equatorial Pacific. For the contiguous United States, potential impacts include above average precipitation in the Northern Rockies, Northern California, and in southern and eastern regions of the Pacific Northwest. Below-average precipitation is expected across the southern tier, particularly in the southwestern and southeastern states.

However, an El Niño returned in May–June 2009 and lasted until April 2010. The effects of El Niño in 2009 were already being seen in the fall of 2009 as the remnants of Tropical Storm Ida strengthened into a powerful coastal storm.

A new La Niña episode developed quite quickly in the eastern and central tropical Pacific in mid-2010,[4] and lasted at least until early 2011.[1] This La Niña, combined with record-high ocean temperatures in the north-eastern Indian Ocean, has been a large factor in the 2010–2011 Queensland floods, and the quartet of recent heavy snowstorms in North America starting with the December 2010 North American blizzard. The same La Nina event is also a likely cause of a series of tornadoes of above-average severity that struck the Midwestern and Northwestern United States in the spring of 2011, and is currently a major factor in the drought conditions persisting in the South Central states including Texas, Oklahoma and Arkansas.

El Nino

What is El Nino?
El Niño is defined by prolonged differences in Pacific Ocean surface temperatures when compared with the average value. The accepted definition is a warming or cooling of at least 0.5 °C (0.9 °F) averaged over the east-central tropical Pacific Ocean. Typically, this anomaly happens at irregular intervals of 2–7 years and lasts nine months to two years. The average period length is 5 years. When this warming or cooling occurs for only seven to nine months, it is classified as El Niño/La Niña “conditions”; when it occurs for more than that period, it is classified as El Niño/La Niña “episodes”.

The first signs of an El Niño are:

1. Rise in surface pressure over the Indian Ocean, Indonesia, and Australia
2. Fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean
3. Trade winds in the south Pacific weaken or head east
4. Warm air rises near Peru, causing rain in the northern Peruvian deserts
5. 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.

El Niño’s warm rush of nutrient-poor tropical water, heated by its eastward passage in the Equatorial Current, replaces the cold, nutrient-rich surface water of the Humboldt Current. When El Niño conditions last for many months, extensive ocean warming and the reduction in Easterly Trade winds limits upwelling of cold nutrient-rich deep water and its economic impact to local fishing for an international market can be serious.

Early stages and characteristics of El Niño

5-day running mean of MJO. Note how it moves eastward with time.
Although its causes are still being investigated, El Niño events begin when trade winds, part of the Walker circulation, falter for many months. A series of Kelvin waves—relatively warm subsurface waves of water a few centimetres high and hundreds of kilometres wide—cross the Pacific along the equator and create a pool of warm water near South America, where ocean temperatures are normally cold due to upwelling. The weakening of the winds can also create twin cyclones, another sign of a future El Niño.The Pacific Ocean is a heat reservoir that drives global wind patterns, and the resulting change in its temperature alters weather on a global scale. Rainfall shifts from the western Pacific toward the Americas, while Indonesia and India become drier.

Jacob Bjerknes in 1969 helped toward an understanding of ENSO, by suggesting that an anomalously warm spot in the eastern Pacific can weaken the east-west temperature difference, disrupting trade winds that push warm water to the west. The result is increasingly warm water toward the east. Several mechanisms have been proposed through which warmth builds up in equatorial Pacific surface waters, and is then dispersed to lower depths by an El Niño event. The resulting cooler area then has to “recharge” warmth for several years before another event can take place.

While not a direct cause of El Niño, the Madden-Julian Oscillation, or MJO, propagates rainfall anomalies eastward around the global tropics in a cycle of 30–60 days, and may influence the speed of development and intensity of El Niño and La Niña in several ways. For example, westerly flows between MJO-induced areas of low pressure may cause cyclonic circulations north and south of the equator. When the circulations intensify, the westerly winds within the equatorial Pacific can further increase and shift eastward, playing a role in El Niño development. Madden-Julian activity can also produce eastward-propagating oceanic Kelvin waves, which may in turn be influenced by a developing El Niño, leading to a positive feedback loop.

Southern Oscillation

Normal Pacific pattern. Equatorial winds gather warm water pool toward west. Cold water upwells along South American coast. (NOAA / PMEL / TAO)
The Southern Oscillation is the atmospheric component of El Niño. This component is an oscillation in surface air pressure between the tropical eastern and the western Pacific Ocean waters. The strength of the Southern Oscillation is measured by the Southern Oscillation Index (SOI). The SOI is computed from fluctuations in the surface air pressure difference between Tahiti and Darwin, Australia.[17] El Niño episodes are associated with negative values of the SOI, meaning that the pressure difference between Tahiti and Darwin is relatively small.

Low atmospheric pressure tends to occur over warm water and high pressure occurs over cold water, in part because deep convection over the warm water acts to transport air. El Niño episodes are defined as sustained warming of the central and eastern tropical Pacific Ocean. This results in a decrease in the strength of the Pacific trade winds, and a reduction in rainfall over eastern and northern Australia.

Walker circulation

El Niño Conditions. Warm water pool approaches South American coast. Absence of cold upwelling increases warming.

La Niña Conditions. Warm water is farther west than usual.
During non-El Niño conditions, the Walker circulation is seen at the surface as easterly trade winds that move water and air warmed by the sun toward the west. This also creates ocean upwelling off the coasts of Peru and Ecuador and brings nutrient-rich cold water to the surface, increasing fishing stocks. The western side of the equatorial Pacific is characterized by warm, wet low pressure weather as the collected moisture is dumped in the form of typhoons and thunderstorms. The ocean is some 60 centimetres (24 in) higher in the western Pacific as the result of this motion.
Effects of ENSO’s warm phase (El Niño)

South America
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 and dry weather months April–October along the coasts of northern Peru and Ecuador, causing major flooding whenever the event is strong or extreme. 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. This leads to fish kills offshore Peru.
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. 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.
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.
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.
North America

Regional impacts of warm ENSO episodes (El Niño).
See also: Effects of the El Niño-Southern Oscillation in the United States
Winters, during the El Niño effect, are warmer and drier than average in the Northwest, Northmidwest, and Northmideast United States, and therefore those regions experience reduced snowfalls. Meanwhile, significantly wetter winters are present in northwest Mexico and the southwest United States including central and southern California, while both cooler and wetter than average winters in northeast Mexico and the southeast United States (including the Tidewater region of Virginia) occur during the El Niño phase of the oscillation.
In Canada, both warmer and drier winters (due to forcing of the Polar Jet further north) over much of the country occur, although less variation from normal is seen in the Maritime Provinces. The following summer is warmer and sometimes drier creating a more active than average forest fire season over Central/Eastern Canada.[citation needed] Some believed that the ice-storm in January 1998, which devastated parts of Southern Ontario and Southern Quebec, was caused or accentuated by El Niño’s warming effects. El Niño warmed Vancouver for the 2010 Winter Olympics, such that the area experienced a subtropical-like winter during the games.

Summers, during the El Niño effect, are wetter than average in the Northwest, Northmidwest, Northmideast, and mountain regions of the United States.
El Niño is credited with suppressing hurricanes and made the 2009 hurricane season the least active in twelve years. El Niño is also associated with increased wave-caused coastal erosion along the United States Pacific Coast.

There is some evidence that El Niño activity is correlated with incidence of red tides off the Pacific coast of California.

Tropical cyclones
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. When the subtropical ridge position shifts due to El Niño, so will the preferred tropical cyclone tracks. 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. During El Niño years, Guam’s chance of a tropical cyclone impact is one-third of the long term average. The tropical Atlantic ocean experiences depressed activity due to increased vertical wind shear across the region during El Niño years.

Elsewhere
In Africa, East Africa Рincluding Kenya, Tanzania, and the White Nile basin Рexperiences, in the long rains from March to May, wetter-than-normal conditions. There are also drier than normal conditions from December to February in south-central Africa, mainly in Zambia, Zimbabwe, Mozambique, and Botswana. Direct effects of El Ni̱o resulting in drier conditions occur in parts of Southeast Asia and Northern Australia, increasing bush fires, worsening haze, and decreasing air quality dramatically. Drier-than-normal conditions are also in general observed in Queensland, inland Victoria, inland New South Wales, and eastern Tasmania from June to August.
Many ENSO linkages exist in the high southern latitudes around Antarctica . 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 .
El Niño’s effects on Europe are not entirely clear, but it is not nearly as affected as at least large parts of other continents. There is some evidence that an El Niño may cause a wetter, cloudier winter in Northern Europe and a milder, drier winter in the Mediterranean Sea region. The El Niño winter of 2006/2007 was unusually mild in Europe, and the Alps recorded very little snow coverage that season.
In most recent times, Singapore experienced the driest February in 2010 since records begins in 1869. With only 6.3 millimetres of rain fell in the month and temperatures hitting as high as 35 degrees Celsius on 26 February. The years 1968 and 2005 had the next driest Februaries, when 8.4 mm of rain fell.

Effects of ENSO’s cool phase (La Niña)

La Niña is the name for the cold phase of ENSO, during which the cold pool in the eastern Pacific intensifies and the trade winds strengthen. The name La Niña originates from Spanish, meaning “the girl”, analogous to El Niño meaning “the boy”. It has also in the past been called anti-El Niño, and El Viejo (meaning “the old man”).

Africa
La Niña results in wetter-than-normal conditions in Southern Africa from December to February, and drier-than-normal conditions over equatorial East Africa over the same period.

Asia
During La Niña years, the formation of tropical cyclones, along with the subtropical ridge position, shifts westward across the western Pacific ocean, which increases the landfall threat to China. In March 2008, La Niña caused a drop in sea surface temperatures over Southeast Asia by an amount of 2 °C. It also caused heavy rains over Malaysia, Philippines, and Indonesia.

South America
During a time of La Niña, drought plagues the coastal regions of Peru and Chile.[40] From December to February, northern Brazil is wetter than normal.
North America

Regional impacts of La Niña.
La Niña causes mostly the opposite effects of El Niño. La Niña causes above-average precipitation across the North Midwest, the Northern Rockies, Northern California, and the Pacific Northwest’s southern and eastern regions. Meanwhile there is below-average precipitation in the southwestern and southeastern states.

La Niñas occurred in 1904, 1908, 1910, 1916, 1924, 1928, 1938, 1950, 1955, 1964, 1970, 1973, 1975, 1988, 1995.

In Canada, La Niña will in general cause a cooler, snowier winter, such as the near record-breaking amounts of snow recorded in the La Niña winter of 2007/2008 in Eastern Canada.

Recent occurrences
There was a strong La Niña episode during 1988–1989. La Niña also formed in 1995 and from 1998–2000, and a minor one from 2000–2001. In recent times, an occurrence of El Niño started in September 2006 and lasted until early 2007. From June 2007 on, data indicated a moderate La Niña event, which strengthened in early 2008 and weakened by early 2009; the 2007–2008 La Niña event was the strongest since the 1988–1989 event. The strength of the La Niña made the 2008 Atlantic hurricane season one of the most active since 1944; there were 16 named storms of at least 39 mph (63 km/h), eight of which became 74 mph (119 km/h) or greater hurricanes.

According to NOAA, El Niño conditions were in place in the equatorial Pacific Ocean starting June 2009, peaking in January–February. Positive SST anomalies (El Niño) lasted until May 2010. Since then, SST anomalies have been negative (La Niña) and expected to stay negative for the next northern winter.

The 2010-2011 La Niña was one of the strongest ever observed. The effect on Eastern Australia was devastating.

Remote influence on tropical Atlantic Ocean
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. About half of El Niño events persist sufficiently into the spring months for the Western Hemisphere Warm Pool (WHWP) to become unusually large in summer. 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.

Cases of El Niño-type events in both oceans simultaneously have been linked to severe famines related to the extended failure of monsoon rains.

ENSO and global warming
During the last several decades the number of El Niño events increased, and the number of La Niña events decreased. The question is whether this is a random fluctuation or a normal instance of variation for that phenomenon or the result of global climate changes toward global warming.

The studies of historical data show that the recent El Niño variation is most likely linked to global warming. For example, one of the most recent results is that, even after subtracting the positive influence of decadal variation, shown to be possibly present in the ENSO trend, the amplitude of the ENSO variability in the observed data still increases, by as much as 60% in the last 50 years.

It is not certain what exact changes will happen to ENSO in the future: Different models make different predictions (cf.) It may be that the observed phenomenon of more frequent and stronger El Niño events occurs only in the initial phase of the global warming, and then (e.g., after the lower layers of the ocean get warmer as well), El Niño will become weaker than it was. It may also be that the stabilizing and destabilizing forces influencing the phenomenon will eventually compensate for each other. More research is needed to provide a better answer to that question, but the current results do not completely exclude the possibility of dramatic changes. The ENSO is considered to be a potential tipping element in Earth’s climate.

El Niño “Modoki” and Central-Pacific El Niño

Map showing Nino3.4 and other index regions

Map of Atlantic major hurricanes during post-“Modoki” seasons, including 1987, 1992, 1995, 2003 and 2005.
The traditional Niño, also called Eastern Pacific (EP) El Niño, involves temperature anomalies in the Eastern Pacific. However, in the last two decades non-traditional El Niños were observed, in which the usual place of the temperature anomaly (Nino 1 and 2) is not affected, but an anomaly arises in the central Pacific (Nino 3.4).[60] The phenomenon is called Central Pacific (CP) El Niño, “dateline” El Niño (because the anomaly arises near the dateline), or El Niño “Modoki” (Modoki is Japanese for “similar, but different”).

The effects of the CP El Niño are different from those of the traditional EP El Niño — e.g., the new El Niño leads to more hurricanes more frequently making landfall in the Atlantic. The recent discovery of El Niño Modoki has some scientists believing it to be linked to global warming. However, Satellite data goes back only to 1979. More research must be done to find the correlation and study past.

El Niño episodes.
The first recorded El Niño that originated in the central Pacific and moved toward the east was in 1986.
A joint study by the National Aeronautics and Space Administration and the National Oceanic and Atmospheric Administration concluded that climate change may contribute to stronger El Niños. El Niño “Modoki” events occurred in 1991-92, 1994–95, 2002–03, 2004–05, and 2009-10. The strongest such Central Pacific El Niño event known occurred in 2009-2010.

Health and social impacts of El Niño
Extreme weather conditions related to the El Ni̱o cycle correlate with changes in the incidence of epidemic diseases. For example, the El Ni̱o cycle is associated with increased risks of some of the diseases transmitted by mosquitoes, such as malaria, dengue, and Rift Valley fever. Cycles of malaria in India, Venezuela, and Colombia have now been linked to El Ni̱o. Outbreaks of another mosquito-transmitted disease, Australian encephalitis ( Murray Valley encephalitis РMVE), occur in temperate south-east Australia after heavy rainfall and flooding, which are associated with La Ni̱a events. A severe outbreak of Rift Valley fever occurred after extreme rainfall in north-eastern Kenya and southern Somalia during the 1997-98 El Ni̱o.

NSO may be linked to civil conflicts. Scientists at the Earth Institute of Columbia University have analyzed data from 1950 to 2004 and suggest that ENSO may have had a role in 21% of all civil conflicts since 1950, with the risk of annual civil conflict doubling from 3% to 6% in countries affected by ENSO during El Niño years relative to La Niña years.
Cultural history and pre-historic information

Average equatorial Pacific temperatures
ENSO conditions have occurred at two- to seven year intervals for at least the past 300 years, but most of them have been weak. There is also evidence for strong El Niño events during the early Holocene epoch 10,000 years ago.

El Niño affected pre-Columbian Incas and may have led to the demise of the Moche and other pre-Columbian Peruvian cultures. A recent study suggests that a strong El-Niño effect between 1789–93 caused poor crop yields in Europe, which in turn helped touch off the French Revolution. The extreme weather produced by El Niño in 1876–77 gave rise to the most deadly famines of the 19th century.

An early recorded mention of the term “El Niño” to refer to climate occurs in 1892, when Captain Camilo Carrillo told the Geographical society congress in Lima that Peruvian sailors named the warm northerly current “El Niño” because it was most noticeable around Christmas. The phenomenon had long been of interest because of its effects on the guano industry and other enterprises that depend on biological productivity of the sea.
Charles Todd, in 1893, suggested that droughts in India and Australia tended to occur at the same time; Norman Lockyer noted the same in 1904. An El Niño connection with flooding was reported in 1895 by Pezet and Eguiguren. In 1924, Gilbert Walker (for whom the Walker circulation is named) coined the term “Southern Oscillation”.

The major 1982–83 El Niño led to an upsurge of interest from the scientific community. The period from 1990–1994 was unusual in that El Niños have rarely occurred in such rapid succession. An especially intense El Niño event in 1998 caused an estimated 16% of the world’s reef systems to die. The event temporarily warmed air temperature by 1.5 °C, compared to the usual increase of 0.25 °C associated with El Niño events. Since then, mass coral bleaching has become common worldwide, with all regions having suffered “severe bleaching”.

Major ENSO events were recorded in the years 1790–93, 1828, 1876–78, 1891, 1925–26, 1972–73, 1982–83, and 1997–98, with 2010–2011 being one of the strongest ever. (Ref. Wikipedia.org)