3st PART









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 are warmer.

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.


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 pronounced droughts.

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 to February

La Niña from June to August

Cool and rainy Warm and rainy
Warm and dry Cool and dry
Cool Dry
Rainy Warm



Variation of the position of the Jet stream during La Niña

Click here to see an animation of the evolution of the snow
cover from August to July during the years with La Niña
(1973/74, 1975/76 and 1988/89). Red tones indicate less
snow cover, and the blue tones indicate more snow cover.


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 (Niño 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 (a) and 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.



  • E.N.S.O. index

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.

  1. - 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° West-80° West),
    • 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) (170-120° West).
    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° West).

    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 SST
are measured to calculate the ENSO indexes

This graph was created and is updated according to NOAA data

  1. - 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 according to NOAA data

  2. - 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.

    This graph was created and is updated according to
    Center for Ocean-Atmospheric Prediction Studies (COAPS)

  3. - 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.
 NORMAL                                       EL NINO                                     LA NINA
Click here to see the reconstitution of the surface characteristics
and subsurface oceanic from the TAO array and data from sea level according to the TOPEX / POSEIDON satellite : The red color
represents water with a temperature higher than 86°F (30°C), the
blue is water with a temperature below 46,4°F (8°C)

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