El Niño and the Southern Oscillation Index (updated)
Greg Baker
Statistics Group
29 September 1997
Since the mid-1970s the climatic phenomenon known as El Niño and the
measure called the Southern Oscillation Index have been regularly referred
to in radio and television weather segments and in the rural press.
El Niño-pronounced el ninyo-was originally the name given to a
warm ocean current that flows intermittently in the Pacific Ocean off
the coast of Ecuador and Peru. Because it usually developed at around
Christmas time it was referred to by the Spanish word for the baby Jesus:
El Niño, the boy Child.
It is now recognised that this warm coastal current is one manifestation
of a global climatic event that particularly affects the 15 000 kilometres
of the equatorial Pacific ocean from the South American continent to Australia
and beyond into the Indian Ocean.
Normally, the western Pacific around Australia has warm surface water
with low air pressures; during these times eastern and northern Australia
receive variable rainfall. The eastern Pacific along the South American
coast has cooler water with little rainfall and high air pressures.
Periodically the centre of warm water around northern Australia spreads
east eventually warming the waters in the central Pacific and towards
the South American coast; it is this wider phenomenon that has taken on
the name El Niño. Although it is irregular, this change occurs
every three to seven years and typically lasts for 12 to 15 months. During
such an El Niño event, eastern and northern Australia receive less rainfall;
if it continues drought results.
Occasionally the sea surface temperatures are lower than usual towards
the South American continent and higher than usual in the western Pacific.
This is referred to as La Niña-pronounced la ninya-and it results
in higher than average rainfall in northern and eastern Australia.
The periodic reversal is called the Southern Oscillation and the cycle
of events affecting the western Pacific ocean is called collectively El Niño-Southern Oscillation (ENSO).
One way to measure the phenomenon of ENSO is an index called the Southern
Oscillation Index (SOI). The SOI is simply an index based on the difference
in air pressures between Tahiti and Darwin. It ranges from a high of +30
to a low of -30 and rainfall is correlated with this index. During an
El Niño event the SOI is strongly negative and there is an increased probability
of less than average rainfall across eastern and northern Australia. When
the SOI is strongly positive during a La Niña event there is a strong
probability of increased rainfall in eastern and northern Australia.
Southern Oscillation Index Jan 1989 to August 1997
1989 1990 1991 1992 1993 1994 1995 1996 1997
January 12.7 -1.9 4.2 -26 -8.9 -2.9 -5.8 7.7 3.5
February 8.5 -18.4 -0.2 -10.3 -8.7 0.3 -3.3 0.6 12.4
March 5.5 -8.2 -10.1 -22.8 -6.5 -9.6 2.8 5.2 -7.0
April 18.1 -0.7 -11.5 -16.5 -18.5 -20.1 -14.4 5.3 -14.4
May 15.1 13.6 -17.9 1.3 -8.0 -11.6 -8.6 1.7 -18.7
June 6.1 0.0 -5.5 -11.9 -14.3 -9.4 -2.2 10.5 -24.3
July 8.9 5.2 -1.5 -6.5 -9.8 -16.7 4.0 6.7 -8.9
August -5.6 -4.4 -6.8 1.7 -12.0 -15.7 1.2 5.3 -18.7
September 5.8 -7.3 -16.2 1.1 -6.8 -16.2 3.4 6.2
October 7.8 -1.2 -13.5 -18.0 -14.9 -13.5 -0.6 6.2
November -1.8 -5.0 -6.9 -6.9 1.5 -7.3 1.7 -0.8
December -6.3 -3.7 -18.3 -6.6 0.0 -13.1 -7.8 7.3
Graph 1
The table and Graph 1 show that the SOI is now strongly negative.
Graph 2
Graph 2 shows for the period 1989 to 1997 the relationship between average
SOI and the percentage of average annual rainfall for Emerald in Central
Queensland. Note that the 1997 figures are for the eight months to August
1997. These figures show clearly the relationship between the SOI and
rainfall in northern Australia. They are used here for illustrative purposes
only and are not used in this form for scientific predictions of rainfall.
It is also important to note that the climatic effects of El Niño are
not consistent and vary from place to place and with time of year.
The SOI is now strongly negative and sea surface temperatures are high
in the eastern Pacific. Meteorologists have predicted that areas of northern
and eastern Australia are likely to receive lower than normal rainfall
in the period to the end of this year.
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