EL NIÑO AND THE SOUTHERN OSCILLATION
Compiled by D. O'Sullivan Co-ordinator Risk Management and
Drought, QDPI October 1994
The words Southern Oscillation and El Niño are now
commonly used when we speak about weather. But what do they mean and how do
they affect our weather?
Southern Oscillation
The Southern Oscillation is a see-saw in air pressure between
the central Pacific and Indian oceans. A high air pressure in the Indian ocean
will generally correspond with a low air pressure in the Pacific region and
vice versa.
The Southern Oscillation Index (SOI) is simply the index that measures the differences
in air pressure between Tahiti and Darwin. The index scale ranges from about
+30 to -30.
El Niño and La Niña
When the SOI is strongly positive (greater than +5) trade
winds blow strongly across the Pacific Ocean picking up moisture. In this situation,
much of eastern Australia has a 75% chance of receiving greater than average
rainfall.
With a strongly negative SOI (more negative than -5) trade winds are weakened
or reversed and there is a 75% probability of below average rainfall in eastern
Australia.
The name El Niño comes from the Spanish word for `boychild'. When the
waters in the eastern equatorial Pacific warm slightly, about every three to
seven years, an El Niño situation develops which causes flooding and
disruption to normal lifestyle along the South American coast. At the same time
a large proportion, but not all, of eastern and northern Australia experiences
drier than average weather.
A converse, the La Niña or girl child is sometimes used to describe the
opposite of the El Nino, where heavy rain may be experienced over eastern and
northern Australia.
General Weather Patterns
Our weather systems result from integration of the following
major factors:
• Warm sea and land heat the air above them to create zones of low pressure,
whereas cold areas create zones of high pressure.
• Maximum heating from the sun occurs in an equatorial zone, which moves
northi and south as the inclined earth circles the sun each year.
• The air and oceans then circulate, and also interact, to redistribute
the heat and to even out the pressure differences. This circulation occurs in
three dimensions with the main movements occurring in a horizontal and/or vertical
direction. For example, heated air rises in equatorial regions, and is replaced
by cooler air flowing in horizontally from higher latitudes.
• The circulation of air and water is modified by the earth rotating towards
the east.
What causes El Niño and La Niña Weather?
Australia's weather patterns associated with El Niño
and La Niña weather is a result of air pressure changes caused by varying
sea surface temperatures. A change of only 1 oC to 2 oC is all that is needed
to change our weather patterns.
Figure 1: El Niño and La Niña weather patterns.
(From: "Will it rain ?", 2nd Edition, QDPI )
High sea-surface temperatures in northern Australia cause
warm moist air to rise resulting in above-average rainfall for northern and
eastern Australia. This often coincides with a positive SOI. A relatively low
sea-surface temperature in northern Australian waters, as occurs during an El
Niño situation, causes high air pressure areas and below average rainfall
over much of eastern Australia. see Fig 1(b).
A typical El Niño episode
In a typical episode affecting eastern Australia, the SOI
becomes strongly negative from late autumn and winter, and the normal dry winter
conditions gradually become more severe as rain in spring is light. The dry
becomes drought as the main summer rains start late and are weak. In autumn,
the SOI often becomes positive and the drought breaks with heavy rain. We have
seen that a rapid change towards a positive value may be enough to herald the
arrival of rain.
Autumn is the usual time when the SOI and weather patterns change.
Climate and SOI
The records of air pressure at Darwin and Tahiti were used
to calculate the SOI back to 1851. When the SOI was compared with climatic records
for the last 100 years it was found that the Southern Oscillation had a strong
influence on cloud cover, temperature, humidity and evaporation, rainfall, number
of wet days and the frequency of cyclones. Using these records it has been possible
to determine a correlation between positive SOI and higher rainfall, and negative
SOI and lower rainfall.
Figure 2: Rainfall pattern in eastern Australia during a typical El Niño
episode.
(From: "Will it rain ?", 2nd Edition, QDPI)
Using the SOI
A strongly negative or positive SOI can indicate whether
the coming season is likely to be wetter or drier than normal. This assists
farmers, graziers and agribusiness managers to make better management decisions.
The summer-winter shift of the seasons as the equatorial trough and subtropical
ridge move south and north has the strongest control over our weather. However,
the Southern Oscillation is the most important influence on the year-to-year
variability.
In Queensland, the winter and spring SOI values have been the most reliable
indicators of the weather in the next season. The trend in the SOI during autumn
is also a useful indicator of winter and spring rain.
The SOI improves estimates of probabilities of rainfall, but it has never been
a totally reliable predictor.
Use of sea-surface temperatures and climate forecasts may add value to the SOI
as a useful predictor of our rainfall patterns.
History of El Niño and La Niña years
(up to 1994*)
A list of El Niño years and La Niña years
is given below. There is a good correlation between El Niño years and
the occurrence of droughts, and between La Niña years and floods.
El Niño years:
1896 , 1902 , 1905 , 1911 , 1913 , 1914 , 1918 , 1919,
1923 , 1925 , 1932 , 1940 , 1941 , 1946 , 1951 , 1953 , 1963 , 1965, 1969 ,
1972 , 1977 , 1982 , 1987 , 1991 , 1992 , 1993 , 1994.
La Niña years:
1892, 1893, 1906 , 1908 , 1909, 1910, 1916 , 1917 , 1921
, 1924 , 1938 , 1947, 1950 , 1955 , 1956 , 1964 , 1970, 1971 , 1973 , 1974 ,
1975 , 1988.
El Niño and La Niña years run from autumn
to autumn e.g. autumn 1982 to autumn 1983
Reference and further reading
This subject is covered in much more detail in the QDPI
saleable publication Q194015: `Will it rain? - The effects of the Southern Oscillation
and El Niño on Australia', edited by I.J. Partridge; it is available
from most QDPI offices.
Acknowledgments
Dr Roger Stone, QDPI, Toowoomba.
* The list of El Niño and La Niña years is taken from: Mean Sea
Level Pressure Indices of the El Niño-Southern Oscillation: Relevance
to stream discharge in south-eastern Australia, April 1996. By R.J. Allan, G.S.
Beard, A. Close, A.L. Herczeg, P.D. Jones, and H.J. Simpson. CSIRO Divisional
Report 96/1 ISSN 1033 5579.
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