Southern Oscillation
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Annual values of the Southern Oscillation Index. A negative value is associated with warm, El Niño conditions in the eastern Pacific. The values for the earliest years underestimate the year-to-year variability. |
The Southern Oscillation is a pronounced disturbance of the atmospheric circulation over lower latitudes of the Pacific sector associated with a re-distribution of mass during the course of the El Niño warming and La Niña cooling of the equatorial Pacific.
The Southern Oscillation Index is based on the standardized pressure difference between Tahiti and Darwin, stations near the centres of the two poles of the oscillation of mass.
Discussion
The El Niño Southern Oscillation phenomenon is the major cause of year-to-year variations in climate over lower latitudes and one of the most significant causes of global climate change on this timescale.
The phenomenon is associated with disruption to tropical climates in many regions: for example, drought occurs in the areas of Indonesia and Australasia and parts of Africa during El Niño events alongside flooding in parts of South America, and weakening of the Indian monsoon.
Advances have been made in understanding the mechanisms underlying this quasi-regular cycle, which has a variable period of three to seven years. And this understanding is gradually leading to a predictive capability.
Data availability
The Southern Oscillation Index and related pressure data for Darwin and Tahiti can be downloaded for the period 1866-1997. Other indices are available.
On the Web
The Tiempo Climate Cyberlibrary has compiled a listing of key Web sites covering various aspects of the Southern Oscillation and the El Niño phenomenon.
Key references
Allan, R.J., Nicholls, N., Jones, P.D., and Butterworth, I.J. A further extension of the Tahiti-Darwin SOI, early ENSO events and Darwin pressure.Journal of Climate 4:743-749, 1991.
Ropelewski, C.F. and Jones, P.D. An extension of the Tahiti-Darwin Southern Oscillation Index. Monthly Weather Review 115:2161-2165, 1987.
Acknowledgements
This version of the Southern Oscillation Index is maintained by Phil Jones, Climatic Research Unit, University of East Anglia.