La Niña (//, Spanish pronunciation: [la ˈniɲa]) is a coupled ocean-atmosphere phenomenon that is the colder counterpart of El Niño, as part of the broader El Niño–Southern Oscillation (ENSO) climate pattern. The name La Niña originates from Spanish, meaning "the little girl", by analogy to El Niño meaning "the little boy". In the past it was also called an anti-El Niño, and El Viejo (meaning "the old man").
During a La Niña period, the sea surface temperature across the eastern equatorial part of the central Pacific Ocean will be lower than normal by 3 to 5 °C (5.4 to 9 °F). An appearance of La Niña persists for at least five months. It has extensive effects on the weather across the globe, particularly in North America, even affecting the Atlantic and Pacific hurricane seasons, in which more tropical cyclones occur in the Atlantic basin due to low wind shear and warmer sea surface temperatures, while reducing tropical cyclogenesis in the Pacific Ocean.
La Niña is a complex weather pattern that occurs every few years, as a result of variations in ocean temperatures in the equatorial band of the Pacific Ocean, The phenomenon occurs as strong winds blow warm water at the ocean’s surface away from South America, across the Pacific Ocean towards Indonesia. As this warm water moves west, cold water from the deep sea rises to the surface near South America; it is considered to be the cold phase of the broader El Niño–Southern Oscillation (ENSO) weather phenomenon, as well as the opposite of El Niño weather pattern. The movement of so much heat across a quarter of the planet, and particularly in the form of temperature at the ocean surface, can have a significant effect on weather across the entire planet.
La Niña events have occurred for hundreds of years and occurred on a regular basis, during the early parts of both the 17th and 19th centuries. Since the start of the 20th century, La Niña events have occurred during the following years:[a]
- 1903–04 1906–07 1909–11 1916–18 1924–25 1928–30 1938–39 1942–43 1949–51 1954–57 1964–65 1970–72 1973–76 1983–85 1988–89 1995–96 1998–2001 2005–06 2007–08 2008–09 2010–12 2016 2017–18 2020–21
Impacts on the global climate
La Niña impacts the global climate and disrupts normal weather patterns, which can lead to intense storms in some places and droughts in others.
Observations of La Niña events since 1950 show that impacts associated with La Niña events depend on what season it is. However, while certain events and impacts are expected to occur during these periods, it is not certain or guaranteed that they will occur.
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.
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 in China. In March 2008, La Niña caused a drop in sea surface temperatures over Southeast Asia by 2 °C (3.6 °F). It also caused heavy rains over Malaysia, the Philippines, and Indonesia.
Across most of the continent, El Niño and La Niña have more impact on climate variability than any other factor. La Niña is characterized by increased rainfall and cloud cover, especially across the east and north; snow cover is increased.
There is a strong correlation between the strength of La Niña and rainfall: The greater the sea surface temperature and Southern Oscillation difference from normal, the larger the rainfall change. There are also cooler daytime temperatures south of the tropics and fewer extreme highs, and warmer overnight temperatures in the tropics. There is less risk of frost, but increased risk of widespread flooding, tropical cyclones, and the monsoon season starts earlier.
La Niña causes mostly the opposite effects of El Niño, above-average precipitation across the northern Midwest, the northern Rockies, Northern California, and the Pacific Northwest's southern and eastern regions. Meanwhile, precipitation in the southwestern and southeastern states, as well as Southern California, is below average. This also allows for the development of many stronger-than-average hurricanes in the Atlantic and fewer in the Pacific.
The synoptic condition for Tehuantepecer winds 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–Bermuda high pressure system. Wind magnitude is weaker during La Niña years than El Niño years, due to the less frequent cold frontal incursions during La Niña winters, with its effects can last from a few hours to six days. Between 1942 and 1957, La Niña had an impact that caused isotope changes in the plants of Baja California.
During a time of La Niña, drought plagues the coastal regions of Peru and Chile. From December to February, northern Brazil is wetter than normal. La Niña causes higher than normal rainfall in the central Andes, which in turn causes catastrophic flooding on the Llanos de Mojos of Beni Department, Bolivia. Such flooding is documented from 1853, 1865, 1872, 1873, 1886, 1895, 1896, 1907, 1921, 1928, 1929 and 1931.
The ‘traditional’ or conventional La Niña is called an Eastern Pacific (EP) La Niña; it involves temperature anomalies in the eastern Pacific. However, aside from differences in diagnostic criteria,[a] non-traditional La Niñas were observed in the last two decades, in which the usual place of the temperature anomaly (Niño 1 and 2) is not affected, but rather an anomaly arises in the central Pacific (Niño 3.4). The phenomenon is called Central Pacific (CP) La Niña, dateline La Niña (because the anomaly arises near the dateline), or La Niña "Modoki" ("Modoki" is Japanese for "alternate / meta / similar-but-different"). These "flavors" of ENSO are in addition to EP and CP types, leading some scientists argue that ENSO is a continuum of phenomena – often with hybrid types.
The effects of the CP La Niña similarly contrast with the EP La Niña – it strongly tends to increase rainfall over northwestern Australia and northern Murray-Darling basin, rather than over the east as in a conventional La Niña. Also, La Niña Modoki increases the frequency of cyclonic storms over Bay of Bengal, but decreases the occurrence of severe storms in the Indian Ocean overall, with the Arabian Sea becoming severely non-conducive to tropical cyclone formation.
The recent discovery of ENSO Modoki has some scientists believing it to be linked to global warming. However, comprehensive satellite data go back only to 1979. Generally, there is no scientific consensus on how or if climate change may affect ENSO.
There is also a scientific debate on the very existence of this "new" ENSO. A number of studies dispute the reality of this statistical distinction or its increasing occurrence, or both, either arguing the reliable record is too short to detect such a distinction, finding no distinction or trend using other statistical approaches, or that other types should be distinguished, such as standard and extreme ENSO.
- 2010 Pakistan floods (attributed to La Niña)
- 2010–11 Queensland floods (attributed to La Niña)
- 2010–12 La Niña event
- 2010–13 Southern United States and Mexico drought (attributed to La Niña)
- 2011 East Africa drought (attributed to La Niña)
- El Niño–Southern Oscillation, the atmospheric component of La Niña–El Niño cycle
- Walker circulation
- Each forecast agency has a different criteria for what constitutes a La Niña event, which is tailored to their specific interests. For example, the Australian Bureau of Meteorology looks at the trade winds, SOI, weather models and sea surface temperatures in the Niño 3 and 3.4 regions before declaring that a La Niña event has started. However, the Japan Meteorological Agency declares that a La Niña event has started when the average five-month sea surface temperature deviation for the NINO 3 region is more than 0.5 °C (0.90 °F) cooler for six consecutive months or longer.
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