Posted on 4 December 2011 by John Hartz

The following is a reprint of a news release isssued by the World Meteorological Organization (WMO) on Nov 17, 2011.

La Niña conditions have re-emerged in the tropical Pacific since August 2011, according to the latest Update from the World Meteorological Organization. This La Niña is expected to persist through the end of this year and into early 2012, possibly strengthening to moderate intensity. However, it is likely to be considerably weaker than the recent episode that was linked to flooding and drought in different parts of the world.

The Update is based on input from climate prediction centres and experts around the world and is an authoritative source of information on a phenomenon which has such a widespread impact on weather and climate – and lives and livelihoods – around the globe.

La Niña is characterized by unusually cool ocean surface temperatures in the central and eastern tropical Pacific. It is the opposite of El Niño, which is marked by unusually warm ocean surface temperatures. Both are strongly coupled to the atmospheric circulation in the tropics and are major – but not the only - determinants of the seasonal and year-to-year fluctuations of our climate.

The present La Niña follows closely behind the moderate to strong La Niña that started in September 2010 and ended with neutral conditions established in May 2011, when ocean temperatures, tropical rainfall patterns, and atmospheric winds over the equatorial Pacific Ocean returned to near the long-term average.

The neutral conditions subsequently gave way to a re-emergence of La Niña. By the end of October the La Niña had slowly strengthened to a weak-to-moderate level.

El Niño ruled out before April 2012

Historical precedents and the latest outputs from forecast models suggest that peak intensity of this La Niña will be reached in late 2011 or early 2012, and that it is very unlikely to reach conditions as strong as those of the 2010-11 La Niña event,, taking into account forecasts from a large number of computer models.

The Update points to a return to a neutral state during March to May 2012. However, given the uncertainty in the range of varying model outputs, close monitoring is required for firmer signs of the peak strength and likely duration of this event.

Development of El Niño is considered highly unlikely prior to the typical transition period of March to May.

It is always important to recognize that several other factors influence seasonal climatic patterns in addition to El Niño and La Niña. One noteworthy aspect is the current warmer than normal sea surface temperatures in the western equatorial Indian Ocean, which is not commonly observed during La Niña events. This requires careful monitoring, along with conditions in the tropical eastern Indian Ocean, as these can influence surrounding continental climate patterns.

The El Niño/La Niña Update is a consensus-based product prepared by WMO in close collaboration with the International Research Institute for Climate and Society (IRI), USA.

Background

La Niña, a Spanish term that literally translates to ”girl child”, is characterized by unusually cold ocean surface temperatures in the central and eastern tropical Pacific. It is the opposite of El Niño (“boy child”, traditionally used by Peruvians to refer to the Christ Child as the phenomenon was often observed in December). El Niño/La Niña events are known to occur once in 2 to 7 years, and once established, last for typically 9 to 12 months and occasionally for two years. However, the outcomes of each event are never exactly the same.

Though El Niño and La Niña are considered to be among the most important factors influencing regional climate patterns in many parts of the world, other factors – such as the conditions over the Indian Ocean, Atlantic, Eurasian snow cover, etc. – also are known to have an influence and so should be adequately taken into consideration.

The following list indicates some typical impacts that are associated with La Niña. This is, however, not an exhaustive account and it is important to remember that no two La Niñas are the same.

Australia: La Niña periods are usually, but not always, associated with above normal rainfall during the second half of the year across large parts of Australia, most notably eastern and northern regions. Daytime temperatures are typically cooler than average and tropical cyclone risk for northern Australia increases during the cyclone season (November-April). Rainfall is also affected by local factors such as the temperature of the oceans surrounding Australia’s northern coasts. These temperatures are currently cooler than they were at the same time in 2010, suggesting a weaker influence on Australian rainfall, The moderate to strong La Niña event of 2010-2011 was linked to heavy rains and flooding in the eastern states of Queensland and Victoria in December 2010 and January 2011.

Asia: During La Niña conditions, major parts of the Philippines experience near normal to above normal rainfall conditions particularly over the eastern sections of the country.  Above normal rain in Indonesia and Thailand is also typical of La Nina.

South America: La Niña often leads to increased rainfall in North Eastern Brazil, Venezuela, Colombia and northern Ecuador in the October to December period. The 2010-2011 event was linked to torrential rainfall, mudslides and floods in Colombia and Venezuela.

It is often linked to below normal rainfall on the coast of Ecuador, the Bolivian Plains, Central and South Chile, northern Argentina and Uruguay in October to December.

North America: La Niña often features drier than normal conditions in Southwestern parts of the United States in late summer through the subsequent winter. Drier than normal conditions also typically occur in the Central Plains in the autumn and in the Southeast in the winter. In contrast, the Pacific Northwest is more likely to be wetter than normal in the late fall and early winter with the presence of a well-established La Niña. Additionally, on average La Niña winters are warmer than normal in the Southeast and colder than normal in the Northwest. The ‘wild card’ is the lesser-known and less predictable Arctic Oscillation that could produce dramatic short-term swings in temperatures this winter

Africa: La Niña events are generally associated with increased rainfall in southern African countries including Angola, southern Democratic Republic of Congo, Lesotho, Malawi, Mozambique, Namibia, South Africa, Swaziland, Zambia and Zimbabwe during the southern hemisphere summer, as well as some parts of West Africa. La Niña is not the only contributing factor. La Niña is associated with rainfall deficiency in equatorial eastern Africa and the 2011 drought in Somalia and northern Kenya was exacerbated by La Niña’s influence. Its impact is not uniform, and some parts of the Greater Horn of Africa receive greater than normal rainfall during La Niña episodes.