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El Niņo and La Niņa
Background Information


El Niņo and La Niņa refer respectively to the warming and cooling of surface waters over the central and eastern equatorial
Pacific Ocean which affect the atmospheric circulation worldwide. (Click this link to jump to the definition of El Niņo and La Niņa event)
 
El Niņo usually peaks around Christmas, hence the name of the phenomenon (Spanish for "the little boy" or "the Christ Child").  It recurs every few years, lasting 12 months or so.  La Niņa, "the little girl" in Spanish, occurs less frequently but lasts longer compared with El Niņo.

Under normal (i.e. without El Niņo and La Niņa) condition, trade winds over the equatorial Pacific pile up warm surface water to the east of Indonesia.  Warm water drives air to rise.  Part of the air mass moves east aloft and descends near South America to form a feedback loop which is called the Walker Circulation (Figure 1).


Figure 1

 

(Source: National Oceanic and Atmospheric Administration, USA)

Figure 1          Normal atmospheric and oceanic conditions


During El Niņo, trade winds over the equatorial Pacific are weaker than normal.  Warm surface water over the western equatorial Pacific retreats east, causing sea surface temperature over the central and eastern equatorial Pacific to become higher than normal.  Convection over the equatorial Pacific shifts east and hence affects the Walker Circulation (Figure 2).


Figure 2

 

(Source: National Oceanic and Atmospheric Administration, USA)

Figure 2          Atmospheric and oceanic conditions during El Niņo


During La Niņa, trade winds over the equatorial Pacific are stronger than normal.  Warm surface water of the ocean is pushed further west.  Convection over the ocean and the Walker Circulation shift accordingly (Figure 3).


Figure 3

 

(Source: National Oceanic and Atmospheric Administration, USA)

Figure 3          Atmospheric and oceanic conditions during La Niņa



Another phenomenon, the Southern Oscillation, is closely related to El Niņo and La Niņa.  The Southern Oscillation is an east-west balancing movement of air masses between the Pacific and the Indo-Australian areas.  It is roughly synchronised and associated with typical wind patterns of El Niņo and La Niņa.  The oscillation is measured by the Southern Oscillation Index (SOI), the difference between sea level atmospheric pressures at 
Tahiti and Darwin.


El Niņo and La Niņa are the oceanic components while the Southern Oscillation is the atmospheric counterpart.  This combination gives rise to the term El Niņo-Southern Oscillation (ENSO).  In general, negative values of the SOI are associated with El Niņo while positive values with La Niņa. ENSO conditions are commonly defined with reference to the sea surface temperatures in the Niņo regions (Figure 4).


Figure 4

 

Figure 4 Graphical depiction of the four Niņo regions and the location of Tahiti and Darwin for SOI




Definition of El Niņo and La Niņa Event

The Observatory defines El Niņo (La Niņa) event based on the sea surface temperature (SST) anomaly index of the composite Niņo region, i.e. Niņo 1 to 4 region (see Figure 4).  An El Niņo (A La Niņa) event is established when

(i) the SST anomaly index maintains at +0.5°C or above (-0.5°C or below) for at least 5 to 6 months; or

(ii) the SST anomaly index remains at +0.5°C or above (-0.5°C or below) for 5 consecutive months and the sum of anomaly reaches +4.0°C or above (-4.0°C or below).




Last revision date: <21 Dec 2012>