El Nino and the Southern Oscillation Index

In a "normal" year, the northeast trade winds blow off the Pacific onto NE Australia, bringing with them warm, moist air. This is an important process for feeding moisture over Australia. The western Pacific is, in such years, warmer than the east, and drives the process. Air over warm water rises (taking moisture with it). For reasons that are not yet fully understood, sometimes the eastern Pacific warms up. A well studied circulation system establishes, and a large pool of warm forms along the west coast of South America and extends into the Pacific along the equator. There is currently such a pool of warm water in place - it is over 3 degrees C warmer than normal and growing.

The end result is rising air over the eastern Pacific. This pulls in air from over the western Pacific, reversing the air flow near NE Australia - and taking with it the moisture feed for Australia.When this reversal occurs it is called an El Nino (which refers to a link between the failure of the South American anchovy fishery and Christmas - El Nino is "the boy child").

El Ninos occur with different intensities, durations and months of onset - all of which make it hard to predict the results of their occurrence, let along predicting when they will occur. Recent El Nino activity has been:

Normally El Ninos are separated by normal years or "more than normal" years (dubbed La Ninas). In the 90's this has not been occurring, with a series of El Ninos stringing together what should normally have been minor droughts.

An El Nino is a process that has potentially severe symptoms for us. We measure its onset with the Southern Oscillation Index, the SOI. This compares air pressures at Darwin and Tahiti. At each site the number of Standard Deviations from the norm for the pressure anomaly are measured (an anomaly is the difference between a measurement and the average). The data from the two sites are them merged to form the index.

The SOI is strongly negative during an El Nino, around 0 on a normal year and strongly positive during a La Nina. The usual range for the SOI is between -20 & 20, with extremes out past -30 & 30.You may also hear of the blanket term ENSO - which stands for El Nino Southern Oscillation.

There are lots of sources of information about this phenomenon, in published or electronic form. Its worth the effort if you have an interest.

El Ninos are probably part of a global-scale process that is yet to be resolved. For example, cooling in the central Atlantic seems to be linked. Sea surface temperature anomalies off Australia's northwest coast are likely to be as important as El Ninos for the ACT. These drive the northwest cloudbands that are at times the main source of rain for us.

So, what does it all mean for the ACT firefighter?

The onset of drought in the ACT is linked to El Ninos at about the 70% level. Reasonable odds, maybe. But the results of an El Nino driven drought are major wildfires for which we need to be very well prepared. The failure of such a drought is a relatively 'easy' summer's firefighting.

  Prepared Not prepared
Drought occurs, big fires Lots of work
Lots of damage
$millions extra lost
Crew fatalities
No drought 'Easy' summer 'Easy' summer

As a result, the need to predict these events is critical so that we can increase our preparedness. If, as seems likely based on past BKDI records, we are entering a dry spell in the ACT, and, as seems possible, we are entering an intense El Nino event, then the implication is clear.

Rick McRae
Concpets & Risk Analysis Unit
July 1997