history

normal situation

El Niņo

Kelvin and Rossby waves

The delayed oscillator model

El Nino indexes

 

 

 

El Nino


           

            To explain the large sea surface temperature anomalies in the Central and the Eastern Pacific during El Niņo years, Wyrtki (1975) suggested that the triggering mechanism of El Niņo was a positive trade winds anomaly  to build up an excess of water in the western Pacific the year prior El Niņo. This assumption was driven by the idea that the amount of warm water accumulated on the western equatorial part of the Pacific would be released to the eastern equatorial part with the relaxation of the trade winds occurring during El Niņo years. He thought that the warm waters could be carried by the Equatorial Undercurrent (eastward current between 50 to 300m depth), the North Equatorial Counter Current (eastward surface current) and the South Equatorial Counter Current (surface current), which may be stronger during the El Niņo years. The increase of strength of the trade winds has been verified by Rasmussen and Carpenter (1982) but only on the west side of the dateline.

             At the end of the year (-1) preceding El Niņo, the sea level pressure (see figure below) in the Central and Southeastern Tropical Pacific starts to decrease and the easterly trade winds weaken (see figure below) to the west of the date line (180° of longitude), where sea surface temperatures become higher than normal. 

 

slpadec96.gif (24366 octets) wadec96.gif (18844 octets)
SLP anomalies in the tropical Pacific in December 1996

Source : IRI

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Wind anomalies at 850 hPa in the tropical Pacific in December 1996

Source : IRI

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             It can be noticed that, this situation can occur without leading to an El Niņo event. During the El Niņo years, because the trade winds weaken, the thermocline tends to flatten out, 1982). At the same time, the sea level flattens, dropping in the western equatorial Pacific and rising in the eastern equatorial Pacific. There is a transport of warm water from the western to the eastern Tropical Pacific. As the water reaches the coast it has to move either northward or southward. The equatorial upwelling weakens and bring only few or no nutrients and cold waters to the surface. east of the dateline, the trade winds weaken.  

El Nino situation

Source : NOAA

          In the early months of the El Niņo year (0), there is an amplification of the warm phase of the seasonal cycle in the eastern Tropical Pacific leading to a maximum of sea surface temperature and rainfall (see figure below) in the east at the time of their seasonal maximum. 

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Precipitation anomalies in July 1995

Source : IRI

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Precipitation anomalies in July 1997

Source : IRI

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            During an El Niņo year these conditions will persist for several months while the seasonal northward migration of the ITCZ is inhibited. In addition, the usual westward propagation of the cooling phase of the seasonal cycle does not occur. The cold tongue weakens or disappears. The warmer than average waters remain in the eastern region of the Tropical Pacific basin, enhancing sea surface temperature anomalies relative to the climatology. The sea surface temperature anomalies propagate westward at the speed of 50 to 100 cm/sec. The propagation coincides with the intensification of the westerly wind anomalies along the equator.

            The moist air above the warm surfaces heats up and becomes buoyant enough to form deep clouds which produce heavy rain along the equator. The collapse of the trade winds induces the eastward movement of the convergence zone (normally over the western equatorial part of the Pacific), the equatorward displacement of the ITCZ and the northward movement of the South Pacific Convergence Zone. The area of major rain is then shifted from the west to the east part of Pacific around April of year (0). The rainfall decreases and sea level pressure increases over Indonesia. In the east, from about 165°E to the South American coast, heavy rainfall occurs and northerly wind anomalies indicates a southward displacement of the ITCZ. 

            Usually by July of the year (0) the maximum anomalies conditions are reached in the eastern equatorial part of the Pacific while the anomalous conditions continue to amplify in the western and Central Pacific until the end of the year (0). The westerly winds extend to 160°E by September of the year (0). The shift in wind patterns is a consequence of the shift in the major atmospheric pressure systems in the South Pacific, known as the Southern Oscillation. The shift is accompanied by a displacement of the low-pressure cell, normally lying over the northwest of Australia to the east resulting in higher pressure and warmer temperature over this part of Australia.

          A secondary maximum in the anomalous conditions in the eastern equatorial part of the Pacific can be reach by the beginning of the year (+1) (see figure below). 

sstajan98.gif (39300 octets) wajan98.gif (26850 octets)
SST anomalies in January 1998

Source : IRI

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Wind anomalies at 850 hPa in January 1998

Source : IRI

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            The anomalies reached their maximum in the eastern Pacific earlier than in the western Pacific. A precursor of the end of El Niņo is the appearance of cold waters in the eastern equatorial Pacific in the middle of year (+1). The cold waters extend westward, signaling the beginning of La Niņa. The average duration of El Niņo is about 18 months.    

sstajun98.gif (27102 octets)

SST anomalies in June 1998

Source : IRI

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