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La Niña reappears: still weak, but expected to slightly strengthen

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.


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.

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Comments 1 to 5:

  1. Thank you for the overview, particularly of how La Nina affects locations around the world. We are enjoying a wonderfully mild spring by comparison with the record hot weather we've had recently in springs this past decade or so (down here in south eastern Australia). The average maximum temperature for November was 26.8C, still 1.5C above the long term average (25.3C), but it has been much hotter in the recent past. Having rains has fuelled so much vegetative growth and gardens are looking luxuriant. The downside is that the extra growth provides fuel for fires. (Hopefully not too many more this summer.) I'm enjoying it while we can. This part of the world is getting hotter and drier overall, so this interval of rain and mild weather makes it a good time to establish 'green curtains' to insulate the house and make other medium term preparations before the next hot and dry spell.
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  2. Over here in western europe we are also having a very hot summer. Yesterday we had a 'climate manifestation' in Brussel, 14°, while its normally somewhere between 0 and 5°C...
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  3. Are the current Rhine river levels low (revealing the WWII bomb this week) because of water extraction or because of a lack of rain?? In the South East of England rivers are low and there is a threat of drought next year. Apart from that, it is a warm winter, with insects, birds and plants getting confused. We have had bees and other insects out and about.
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  4. The weekly detailed discussions indicate just how much information we gather on this phenomenon. This La Nina appears to be quite similar to the rebound La Nina in 2009. After a stronger winter 2008 La Nina ended, we reentered a weaker La Nina fall of 2008. This ended in the spring of 2009 with the development of an El Nino by summer. That rapid transition was of interest to me because of the extraordinary glacier melt in the Pacific Northwest of North America that summer.
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  5. @mspelto , thanks for the weekly detail. I've been reading this blog for a few weeks, but just signed up. I follow the courses, destruction, and other data of La Nina, and other storms, particularly since 2008. I've collected data, that may be of use to you guys. I don't mind you sharing with with your community. My particular interest is climate patterns, deviations, etc. Anyhow, this is a great site, and I'm not a skeptic, just have an extremely open mind. :) Data talks, opinions walk. Which is why I love this place.
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