La Niña (pron.: //, Spanish pronunciation: [la ˈniɲa]) 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 little girl," analogous to El Niño meaning "the little 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 known for its potentially catastrophic impact on the weather along the Chilean, Peruvian, New Zealand, and Australian coasts, among others. It has extensive effects on the weather in North America, even affecting the Atlantic Hurricane Season. La Niña is often, though not always, preceded by an El Niño.
The results of La Niña are mostly the opposite of those of El Niño; for example, La Niña would cause a dry period in the Midwestern U.S., while El Niño would typically cause a wet period in that area. La Niña often causes drought conditions in the western Pacific; flooding in northern South America; mild wet summers in northern North America, and drought in the southeastern United States.
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 favorable note. However, 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 flooding that have affected Sri Lanka.
There was a strong La Niña episode during 1988–1989. La Niña also formed in 1995, and in 1999–2000, followed by neutral periods between 2000 and 2002. The La Niña which developed in mid-2007 and lasted until almost 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, and lasted until early 2011. It intensified again in the mid-2011 and was predicted to last at least until early 2012 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 Niña event is also a likely cause of a series of tornadoes of above-average severity that struck the Midwestern and Southern 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. 
In 2011, on a global scale, La Niña events helped keep the average global temperature below recent trends. As a result, 2011 tied with 1997 for the 11th warmest year on record. It was the second coolest year of the 21st century to date, and tied with the second warmest year of the 20th century. A relatively strong phase of La Niña opened the year, dissipated in the spring before re-emerging in October and lasted through the end of the year. When compared to previous La Niña years, the 2011 global surface temperature was the warmest observed. The 2011 globally-averaged precipitation over land was the second wettest year on record, behind 2010. Precipitation varied greatly across the globe. La Niña contributed to severe drought in East Africa and to Australia's third wettest year in its 112-year period of record. 
La Niña events between 1950 and 2011.
See also 
- Southern Oscillation, the atmospheric component of the La Niña–El Niño cycle
- Walker circulation
- 2010 Pakistan floods (attributed to La Niña)
- 2010–2011 Queensland floods (attributed to La Niña)
- 2011 Horn of Africa drought (attributed to La Niña)
- "ENSO Diagnostic Discussion". NOAA's Climate Prediction Center. Retrieved 10 January 2011.
- "El Niño Arrives; Expected to Persist through Winter 2009-10". Noaanews.noaa.gov. Retrieved 10 January 2011.
- "Ida now a coastal low assaulting the Mid-Atlantic". Eurekalert.org. 12 November 2009. Retrieved 10 January 2011.
- "La Niña is back". NOAA’s Climate Prediction Center. 8 September 2011. Retrieved 10 December 2011.
- "Why is Queensland flooded?". BBC News. 13 January 2011.
- "Tornadoes whipped up by wind, not climate: officials". www.physorg.com. Retrieved 30 April 2011.
- "NOAA: 2011 a year of climate extremes in the United States". Climate Prediction Center NOAA. Retrieved 22 January 2012.
- "Cold and warm episodes by season". Climate Prediction Center NOAA. Retrieved 12 January 2011.
- La Niña episodes in the Tropical Pacific
- Information page on El Niño–Southern Oscillation (ENSO)
- List of El Niño and La Niña events since 1950