El Nino and the Monsoon

How to predict a monsoon

Monsoons are critical to India whose economy is agriculture driven.

Predicting the failure of the monsoon has been a tricky issue, but new researchers suggests that the key to better forecasts depends on understanding of a warming cyclic phenomenon of the Pacific Ocean, called El Niño.

Droughts have always been accompanied by an El Niño, but monsoons do not fail in every El Niño year.

In 1997, a predicted drought didn't occur but in 2002 and 2004, unexpected and severe droughts surprised an unprepared country.

Previous forecasts used an average of sea-surface temperatures to measure the strength of El Niño events and predicted the power of the monsoon.

Scientists led by meteorologist Martin Hoerling of the National Oceanic and Atmospheric Administration in Boulder, Colorado, discovered that the key is determining exactly where in the equatorial Pacific the sea-surface warming is strongest.

They used rainfall records dating back to 1871.

Three climate models were matched.

The team found that when the warm waters were closer to the coast of South America in eastern Pacific, the effect of the monsoon was weaker or nonexistent.

In the eastern Pacific waters, where Humboldt current greatly cools the water, more warming has to occur to increase temperatures over a limit before there is an effect on India's rains.

When an El Niño strongly warmed the central Pacific, drought resulted in India.

"The reason, says Hoerling, is that as the central Pacific warms, the atmosphere above it heats up and rises. This induces a large, dry air mass to sink over India, depriving it of nourishing rains."

"The trend could be mitigated as the eastern Pacific warms," Hoerling says.

In the western Pacific, where sea-surface temperatures are among the warmest in the world, less additional warming can increase the amount of rainfall during the monsoon.

Western El Niños can't have a serious influence on the monsoon.

"It's strong work, but I think its applicability is suspect," says climate scientist Peter Webster of the Georgia Institute of Technology in Atlanta.

"One problem is that forecasting precisely where the warmest waters will be during an El Niño is well beyond current capabilities," he says.

And the next El Niño will occur in April.

"Even if the forecast were perfect, you wouldn't get much lead time" to prepare for a drought, says Webster. "Still, it's a step in the right direction."

Rising ocean temperatures due to global warming may reinvigorate the monsoons, which have past through a decline over the past three decades.


By    2 Oct 2006, 07:15 GMT