In this phenomenon of oscillation, E.N.S.O.,
the cold phase is called La Niña. The latter is the
opposite of El Niño
and this can be seen in the SST and atmospheric pressure
anomalies of the Pacific.
The waters of the Eastern Pacific are
cooler than normal and the waters of the Western Pacific
Atmospheric pressures are lower in the
West, but higher in the East.
During La Niña the thermocline descends to
the Western Pacific and rises to the
East.The trade winds
are stronger and coastal waters of Chile and Peru colder.
CAUSE OF CLASSIC LA NINA
The occurrence of this phenomenon depends
on the reflection of oceanic waves on the coasts of Indonesia
or South America. When Kelvin waves are reflected on the
coast of South America in Rossby waves in the West, the
thermocline rises to the Pacific East and descends to the
West. Then the temperature of the ocean surface varies.
The cold water of the thermocline goes up again, cools the
surface of the Eastern Pacific, causing instability of the
atmosphere-ocean coupling. The trade winds blow more strong
pushing the warm surface waters to Indonesia and Australia
allowing the upwelling of cold water on the surface.
These anomalies in the temperature of the
surface of the Pacific strengthen the depression of Indonesia
and Australia and the Eastern Pacific anticyclone.
The classic Nina is a phenomenon that is all the opposite
of El Niño classic and has therefore opposite consequences.
The warmer waters in Indonesia gradually warm up 84,2°F
(29°C) and the sea level rises of 40 cm on the Western
edges of the basin. Which amplifies precipitation southeast
Asia and eastern Australia, causing major floodings.
Monsoons are also abnormally high in India. South Africa
also faces more rain.
But the intertropical islands of the Central Pacific
and on the other side of the Pacific : Southern North America,
North of Mexico and California... experience periods of
This climatic phenomenon increases the number of hurricanes
in the Atlantic as it decreases the vertical wind shear.
Pacific forces and storm
tracks are different as jet
stream changes of latitude, they are less frequent.
La Niña from December
La Niña from June to August
Cool and rainy
Warm and rainy
Warm and dry
Cool and dry
Variation of the position of the Jet
stream during La Niña
Since 2007 a new type of La Niña
was discovered from analysis of a Japanese team that have
named it La Niña Modoki in Japanese. It is distinguished
from the classic La Niña by its specific impact on
the global atmospheric circulation.
Traditionally, the classic La Niña
is associated with the cooling in the eastern tropical Pacific
1+2 et 3). However, during La Niña Modoki the
anomaly of the sea surface temperature (SST) in the eastern
Pacific isn't affected by cooling but by warming just like
western equatorial Pacific, while a cold anomaly affects
the central equatorial Pacific (Niño
3.4). These zonal gradients of SST result in an anomaly
in circulation of two Walker cells on the tropical Pacific,
with a humid region in the Eastern and the Western Pacific.
The thermocline doesn't switch the same way as during the
classical La Niña because this one raises at the
Eastern Equatorial Pacific to sink at the West while during
its cousin the thermocline raises at the center of equatorial
Pacific Ocean and sinks at each end of the Pacific.
Comparison between the situation of Classic La Niña
La Niña Modoki (b)
During the classic La Niña, the West Coast of the
United States is dry but with La Niña Modoki it is
rather humid. While during the Classic La Niña there
is a significant increase in rainfall over the north and south
of Australia, that the Modoki event seems to lead to a large-scale
increase in the precipitations in the northwest and North
Australia. India also would be affected by more precipitations
with this second type of La Niña.
INDEX AND TECHNIQUES FORECASTS OF ENSO
Ocean surface temperature and atmospheric
pressure are constantly measured by moored buoys and satellites
around the Earth (Poseidon,
Jason...). From these data are calculated indexes
giving a more exact idea about the evolution of ENSO.
- With the anomaly of SST (sea surface temperature),
there are four indices that are calculated differently,
or rather in different locations.
El Niño 1+2 (0-10° South) (90°
El Niño 3 (5° North-5° South)
(150° West-90° West),
El Niño 4 (5° North-5° South)
(160° East-150 °West),
El Niño 3.4 (5° North-5° South)
It is the difference of the monthly SST compared to
the normal monthly average SST.
Example, if in December to (0-10° South) (90°
West - 80 ° West), the average of the SST is 22,32°C
then the index is -0,52 as the average in December is
23,84°C to (0-10° South) (90° West - 80°
When these indexes are positive it means that
there is El Niño and when they are negative it
means there is La Niña.
Here are the areas of the ocean where averages of
are measured to calculate the ENSO indexes
This graph was created and is updated according
- Then you can follow the evolution
of ENSO phases with the SOI which is an index of the
atmosphere, calculated with the atmospheric pressure
of Darwin (Northern Australia) and Papeete (Tahiti
AABF station managed by Météo France).
Indeed, when the index is negative, it is an El Niño
phase and when it is positive it is a La Niña
phase unlike other indexes.
This graph was created and is updated
- There is also the J.M.A, (Japan Meteorological Agency)
a new index based also on SST. It is applied on an average
of five months (4° North - 4° South) and (150 °
West - 90° West). The monthly data of this index range
from 1848 until now. When this index is positive it means
that there is El Niño and reverse when it is negative.
- Then comes the T.A.O. (Tropical Atmosphere Ocean)
an index created from more than 70 moored buoys covering
the equatorial Pacific between 8° North and 8° South.
They measure the atmospheric surface conditions (wind,
humidity, precipitation, radiative flux) and oceanic conditions
of the surface and subsurface up to 500 m of depth (the
temperature, currents, and salinity). Everything is organized
by the United States, France, the Japan and Taiwan.
Régnier helped me correct mistakes, please you to
Im not english speaker, some improprieties can appear
to english masters.
Could you help me reporting by mail any fault you read.
Thank you for all.