The El-Niño Southern Oscillation (ENSO)
(Updated to 31st July 2008)

John L. Daly

What is the El-Niño Southern Oscillation?

  The El-Niño Southern Oscillation is the result of a cyclic warming and cooling of the surface ocean of the central and eastern Pacific. This region of the ocean is normally colder than it's equatorial location would suggest, mainly due to the influence of northeasterly trade winds, a cold ocean current flowing up the coast of Chile, and to the upwelling of cold deep water off the coast of Peru.

  At times, the influence of these cold water sources wane, causing the surface of the eastern and central Pacific to warm up under the tropical sun - this is an EL-NIÑO event. This results in heavy rainfall in South America, but severe droughts in eastern Australia. The more intense the El-Niño, the more intense and extensive the Australian droughts.

  At other times, the injection of cold water becomes more intense than usual, causing the surface of the eastern Pacific to cool - this is a LA-NIÑA event. This results in droughts in south America and heavy rainfall, even floods, in eastern Australia. In this way, Australia experiences it's characteristic cycle of droughts and floods - all caused by the El-Niño/La-Niña cycle described above.

  For more detailed information on El-Niño see the Queensland DNR `Long Paddock' website, and the NOAA El-Niño Theme Page.

  The latest Global Sea Surface Temperature from the University of Wisconsin-Madison (all rights reserved). Particularly note the ocean temperatures in the eastern Pacific near Ecuador, Peru and the Galapagos Is., the source of the El-Niño Southern Oscillation.

Why are "El-Niño" and "La-Niña" so named?

"El-Niño" is named after a Peruvian Christmas festival where the warming of the waters off Peru is said to occur near the birthday of "The Boy" (El Niño), or the Christ child. Meteorologists thus named the phenomenon the "El-Niño Southern Oscillation", or ENSO for short. The reverse phenomenon, the cooling of the eastern Pacific waters, was at first called "Anti-El-Niño", until it was realised that this literally meant the Anti-Christ ! To avoid this unfortunate connotation, it was renamed "La-Niña" (or "The Girl").

What is the Southern Oscillation Index?

  It has been found that the cyclic warming and cooling of the eastern and central Pacific leaves it's distinctive fingerprint on sea level pressure. In particular, when the pressure measured at Darwin is compared with that measured at Tahiti, the difference between the two can be used to generate an "index" number. When there is a positive number, we have a La-Niña (or ocean cooling), but when the number is negative we have an El-Niño (or ocean warming).

Here is the monthly Southern Oscillation Index from January 2005 through July 2008


Here are El-Niño/Southern Oscillation data for the 30 days up through 31st July 2008

(These three graphs will be updated regularly)

For a longer-term perspective, here is the Southern Oscillation Index (monthly) since 1950

And the Southern Oscillation Index since 1876 (Annual) -

What does the SOI Index tell us?

  In the case of Australia, a negative SOI means there is an El-Niño under way, and therefore drought conditions can be expected in eastern Australia. The more negative the number, the further south does the drought extend. For example, the severe 1982-83 El-Niño was thankfully short-lived, but it's intensity was such (as shown by the deep plunge in the graph) that even regions as far south as Tasmania were severely affected by drought.

  Conversely, the period in 1988 when the index went positive indicated a major La-Niña event, which in Australia was accompanied by severe flooding in Queensland and New South Wales. In 1996, we had another La-Niña event, albeit a weaker one than in 1988. However, there was extensive rainfall throughout eastern Australia in 1996 as a result of this. The 1998/2001 La Niña ran true to form in that rainfall was greater than normal throughout most of Australia.

  The period from 1991 to 1995 was a weaker El-Niño than the 1982/83 event, but it was much more long-lasting. The effect of this was that southern states like Victoria and Tasmania were not seriously affected by drought, due to the weakness of the event, but Queensland and inland New South Wales suffered a long crippling drought for the full four years of the El-Niño. The 1996 La-Niña provided welcome relief.  El-Niño returned again in mid-1997, beginning with it a warming of the surface oceans off Peru and Ecuador, followed in mid-1998 by the return of La Niña.

  In other words, it is both the intensity of an event and it's duration which determines who gets floods and who gets drought, and for how long.

  ENSO research is a matter of vital national interest to Australia and deserves the major portion of any climatic research. Few, if any, other countries have such a vested interest in ENSO as Australia does and other countries would be unlikely to commit the necessary resources for successful prediction. It is a strong argument for Australia not to waste it's limited research resources on Greenhouse, (an issue which is already the subject of saturation research by other developed countries, and which is likely to prove to be a non-problem anyway), but to focus on those climatic phenomena which impact directly upon our national interests.

John L. Daly