It's been affecting your life all season, so here's a primer on the El Niño, La Niña phenomenon by Eammon Conaghan

Trade winds help determine WA weather patterns

PACIFIC TRADE winds have strengthened once again, auguring well for the export of WA grain. Not because they will provide swift transport, but because, in the global tapestry of weather activity, the vigour of the trade winds helps determine WA weather patterns. These patterns look to be kinder in 2003, perhaps restoring WA's grain production to more expected levels.

Presenting at the 2003 Crop Updates in Perth, Department of Agriculture clima-tologist David Stephens noted that 10 of the 12 commonly used statistical and coupled atmosphere/ocean models predict a breakdown of the El Niño weather system that has brought clear skies and limited showers over the past year.

The trade winds' return to normal along the equatorial Pacific and the cooling of sea surface temperatures (SST) in the eastern equatorial Pacific are changes that suggest the delicately balanced machinery of weather is turning through its cycle and preparing to relieve WA of its prolonged dry spell.

"The eastern equatorial Pacific is dominant in determining weather patterns because it occupies such a substantial patch of the global surface," Dr Stephens said. "Those ocean temperatures have been high, meaning the complementary waters off Australia's northern coast have been cooler. When the coastal water temperatures are cool, the air above them sinks, creating dry high-pressure systems. That is the broad scale El Niño pattern."

El Niño's alter ego, La Niña, occurs when the pattern is reversed and northern Australian waters are much warmer than the eastern equatorial Pacific. For local growers, La Niña is more readily associated with average or above-average rainfall.

El Niño and La Niña ride a giant equatorial seesaw as SST and consequent air pressures rise and fall between eastern and western ends. The seesaw usually changes from one extreme to the other in autumn (i.e. now) and sometimes too late for major farming lecisions.

To identify emerging trends and predict which way the seesaw is going, scientists refer to the Southern Oscillation Index (SOI), which is a comparison of the air pressures in Darwin and Tahiti.

Alternatives to the SOI

Dr Stephens has developed alternative predictive tools such as the 'El Niño Prediction Index' (EPI) and the 'Mean SOI' which have more lead time. The EPI averages air pressure anomalies over Alice Springs and Mildura between July and September, while the Mean SOI averages the traditional SOI with pressure oscillations between Indonesia and the eastern Pacific and between south-east Australia and Rapa Island, in the South Pacific.

"Late last year the EPI read +0.43, suggesting there is a higher chance of SST cooling in the eastern equatorial Pacific in the next 12 months," he said.

"Also, the running three-month Mean SOI bottomed out in September and has been rising since. Of the seven recorded transitions from El Niño to La Niña, the three-month running mean reached its lowest value before December."

Both indices hint at the end of the 2002 El Niño. The early dip in the three-month Mean SOI would suggest that El Niño will not be replaced by neutral weather patterns, but by a La Niña bringing average or above-average rainfall in 2003.

"When transferring from El Niño to a neutral system, the low point has always occurred after November," Dr Stephens noted.

"Also, when we examine analogous years from WA history, five emerge as very similar, all but one of which was followed by a year of average to above-average rainfall. The most recent of those years was 1995, where we experienced above-average rainfall but with dry stretches along the south coast.

"In 2003 we can expect average to above-average rainfall with an 80 per cent confidence, although a 20 per cent chance of below-average rainfall remains."

While this forecast could promise strong yields in WA's traditional dry regions, it sounds a cautionary note about eradicating weeds and bringing disease under control, especially stripe rust. Waterlogging-prone country also comes back into the risk analysis.