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Questions and answers on
la Niña and current extreme weather

What is La Niña??

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 around Christmas), which is characterized by unusually warm ocean surface
temperatures. The two events are strongly coupled with changes in atmospheric pressure and the
associated large-scale circulation patterns, and are considered to be the opposite phases of air-
sea interactions over the region collectively referred to as “El Niño/Southern Oscillation (ENSO”.
They disrupt the normal patterns of tropical precipitation and atmospheric circulation, and have
widespread impacts on climate in many parts of the world accompanied by the associated climate-
related risks.

El Niño or La Niña are known to occur once in 2 to 7 years, and 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 one of the most important factors leading
to anomalous seasonal climate in many parts of the world, it is however difficult to explicitly
label individual extreme weather events as being directly caused by El Niño or La Niña without
considering the influence of other factors in determining net impacts.

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Is La Niña linked to the recent spate of extreme weather events around the world?

The Bureau of Meteorology in Australia has linked the recent heavy rains and flooding in
Queensland to the current La Niña and said that 2010 was the third wettest year on record in
Australia because of the phenomenon. It was the wettest December on record for Queensland and
for eastern Australia as a whole. The heavy late November and December rainfall followed
a very wet July to October for Australia, meaning many catchments were already wet. It was
Australia’s wettest July to October on record and also the wettest July to December on record.
Previous strong La Niña events, such as those of 1974 and 1955, have also been associated with
widespread and severe flooding in eastern Australia.

Heavy rains in Indonesia, the Philippines and Thailand are also typical of La Niña. The flooding
in Sri Lanka has occurred within the November to February rainy season (linked to the north east
Indian Monsoon). Considering that historically La Niña was associated with a relatively weaker
winter monsoon over South Asia, the current floods in Sri Lanka might be considered to be
rather atypical of La Niña impacts and could be due to other regional factors. However a La Niña
influence on the flooding should not be ruled out.

La Niña usually leads to increased rainfall in North Eastern Brazil, Colombia and other northern
parts of South America and is associated with rainfall deficiency in Uruguay and parts of Argentina.
Drier than normal conditions are generally observed along coastal Ecuador and North Western
Peru.

However, it is unclear with the current knowledge that la Niña is the primary cause of the recent
torrential rains and mudslides which have caused such heavy fatalities in South Eastern Brazil.
More research is being conducted into the impact of La Niña in different climatic zones and
different months in South Eastern Brazil. The South American monsoon is active in December to
February in this region and this can lead to heavy rains and thunderstorms. The intensity of these
rains can also be affected by factors such as the heating of the nearby tropical Atlantic Ocean
which can increase humidity in the atmosphere and hence precipitation.

La Niña episodes feature a very wave-like jet stream flow over the United States and Canada
in the northern winter, with colder and stormier than average conditions across the North, and
warmer and less stormy conditions across the South.

La Niña events are generally associated with increased rainfall in southern Africa, although they
are not the only contributing factors. La Niña is associated with rainfall deficiency in equatorial
eastern Africa and the current drought in Somalia and northern Kenya is believed to be due to La
Niña influence.

Is it normal to swing from El Niño to La Niña as we did in 2010?

During El Niño events, sea temperatures at the surface in the central and eastern tropical Pacific
Ocean become substantially higher than normal. In contrast, during La Niña events, the sea
surface temperatures in these regions become lower than normal. A transition between El Niño
and La Niña is not unusual as both are part of the same phenomenon called the El Niño/Southern
Oscillation or ENSO, which swings between unusually warm and cold conditions over eastern and
central tropical Pacific. However, the warm and cold phases need not necessarily follow each
other in quick succession, and are quite often separated by extended periods (from a few months
to a couple of years) of neutral situations, i.e., neither El Niño nor La Niña. There have also been
instances when an El Niño was followed by another El Niño, and La Niña followed by another La
Niña, separated by neutral conditions of a few months.

Is La Niña a new phenomenon?

The world’s climate has always been influenced by the interaction between the Earth’s atmosphere
and the oceans and phenomena like El Niño and La Niña are a natural part of it, historically
documented for centuries. However, they are more talked about now than in the past because
research has increased our understanding of these phenomena and their worldwide climatic
impacts.

How will climate change impact on El Niño and La Niña?

In the context of climate change, the Intergovernmental Panel on Climate Change (IPCC) in its
Fourth Assessment Report said that "there is no consistent indication at this time of discernible
changes in projected ENSO amplitude or frequency in the 21st century." However, this does
not preclude possible changes in the extremes associated with them or otherwise. The IPCC’s
Assessment Report concluded that "it is very likely that hot extremes, heat waves and heavy
precipitation events will continue to become more frequent."

Further research is being conducted to conclusively establish whether or not climate change will
lead to more frequent and intense El Niño and/or La Niña events.

How are El Niño/La Niña monitored and predicted?

The meteorological and oceanographic data that allow El Niño and La Niña episodes to be
monitored and forecast are drawn from national and international observing systems. The
exchange and processing of the data are carried out under programmes coordinated by the World
Meteorological Organization.

The forecasting of Pacific Ocean developments is undertaken in a number of ways. Complex
dynamical models project the evolution of the tropical Pacific Ocean from its currently observed
state. Statistical forecast models can also capture some of the precursors of such developments.
Expert analysis of the current situation adds further value, especially in interpreting the implications

of the evolving situation below the ocean surface. Best forecast methods incorporate the effects of
ocean-atmosphere interactions within the climate system. Several global centres around the world
constantly monitor the ENSO situation, and regularly prepare and disseminate their diagnostics
and predictions. WMO facilitates consensus among the various centres and prepares an El Niño/
La Niña Update, issued on a quasi-regular basis (approximately once in three months) through
a collaborative effort with the International Research Institute for Climate and Society (IRI) as a
contribution to the United Nations Inter-Agency Task Force on Natural Disaster Reduction. It is
based on contributions from the leading centres around the world dealing with this phenomenon.

 

 

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