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El Niño and the Southern Oscillation Index (updated)

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RESEARCH NOTE Number 9, 29 September 1997 ISSN 1328-8016 El Niño and the Southern Oscillation Index (updated) 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.

Information and



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.

Rainfall at Emerald Queensland and Average SOI January 1989 to August 1997












1989 1990 1991 1992 1993 1994 1995 1996 1997











Percentage of average annual rainfall (%)

SOI: Left Axis

Rainfall Percentage: Right Axis

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.

Southern Oscillation Index 1989 - 1997



























Southern Oscillation Index (SOI)

Greg Baker Statistics Group Information and Research Services Phone: (02) 62772488 Fax: (02) 62772454

Views expressed in this Research Note are those of the author and do not necessarily reflect those of the Information and Research Services and are not to be attributed to the Department of the Parliamentary Library. Research Notes provide concise analytical briefings on issues of interest to Senators and

Members. As such they may not canvass all of the key issues. Advice on legislation or legal policy issues contained in this paper is provided for use in parliamentary debate and for related parliamentary purposes. This paper is not professional legal opinion.

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