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El NiⳈnd La NiⳈ

Background Information



El NiⳈnd La NiⳈefer 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 NiⳈnd La NiⳈvent
)
 
El NiⳈsually 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 NiⳈ"the little girl" in Spanish, occurs less frequently but lasts longer compared with El NiⳈ

Under normal atmospheric and oceanic conditions (i.e. without El NiⳈnd La NiⳈalso known as the ENSO-neutral state), 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 NiⳈtrade 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 NiⳈ


During La NiⳈtrade 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 NiⳈ


Another phenomenon, the Southern Oscillation, is closely related to El NiⳈnd La NiⳈnbsp; 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 NiⳈnd La NiⳈnbsp; The oscillation is measured by the Southern Oscillation Index (SOI), the difference between sea level atmospheric pressures at 
Tahiti and Darwin.


El NiⳈnd La NiⳈre the oceanic components while the Southern Oscillation is the atmospheric counterpart.  This combination gives rise to the term El NiⳈouthern Oscillation (ENSO).  In general, negative values of the SOI are associated with El NiⳈhile positive values with La NiⳈENSO conditions are commonly defined with reference to the sea surface temperatures in the NiⳈegions (Figure 4).


Figure 4

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



Definition of El NiⳈnd La NiⳈvent

The Observatory defines El NiⳈLa NiⳈevent based on the sea surface temperature (SST) anomaly index of the composite NiⳈegion, i.e. NiⳈ to 4 region (see Figure 4).  An El NiⳈA La NiⳈevent is established when

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

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





Last revision date: <14 Jan 2016>