Welcome weather: It remains to be seen what effect La Niña will have on Australia's drought.

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ADELAIDE: El Niño is shifting to the anticipated La Niña weather pattern, according to the U.S. National Oceanic and Atmospheric Administration (NOAA) and Australia's Bureau of Meteorology – but it may not break Australia's drought this year.

According to meteorological records, broad La Niña shifts are typically well established by the end of winter in the Southern Hemisphere. However, the current trend indicates only a partial shift towards La Niña, making this year somewhat unusual.

Now that spring has started in the Southern Hemisphere, a late developing La Niña is less likely to yield a typical increase in rainfall, cautions the Bureau of Meteorology (BOM).

Late and weak

"Due to the relative lateness and weakness of the developing anomalies, the usual above average rainfall response across eastern Australia to La Niña cannot be relied upon," said climatologist Lynette Bettio at BOM's National Climate Centre in Melbourne. A La Niña normally increases the chances of rain in eastern and southern Australia, compared to El Niño years, which are generally drier than normal.

"While we can't officially call it a La Niña yet, we expect that this pattern will continue to develop during the next three months, meeting the ... definition for a La Niña event later this year," said Mike Halpert a meteorologist at NOAA's Climate Prediction Centre in Maryland.

La Niña is part of the El Niño/La Niña climate cycle, also called the El Niño Southern Oscillation (ENSO), which occurs about every four to seven years.

During a La Niña period there are cooler surface temperatures in the eastern equatorial Pacific Ocean, off the coast of South America, leading to strengthening of the easterly currents and trade winds. The winds can bring warm moist air that rises up over eastern Australia, generating rain and hopefully respite from drought.

Earlier in the year, the BOM predicted a La Niña event was likely in 2007, based on climate models and oceanic data from previous La Niña events (see, Australia's drought unlikely to break in 2007, Cosmos Online).

However, since late July the indicators of an approaching La Niña have been strengthening. The surface temperatures in the eastern equatorial Pacific have now passed the La Niña thresholds. Sea surface temperatures of 0.5°C or more below the average must persist for three months before a La Niña event is officially declared.

"With the exception of the Southern Oscillation Index, most ENSO indicators have reached a point that, should they persist at these levels until the end of the year – 2007 will be referred to as a La Niña," said the BOM in a report released this afternoon. "All the dynamic computer models predict La Niña conditions for the rest of 2007 and declining to neutral thereafter."

Mixed forecast

While a La Niña might be imminent, the bureau is not forecasting a wetter than average spring in all parts of Australia. The oceans to the northwest have not warmed, as generally occurs during a La Niña. This is inhibiting northwest cloudbands, a major source of winter and spring rain for central and southeastern Australia during La Niña years.

There has been below-normal rainfall over much the country since the start of July, according to the BOM. The odds favour a wetter than average spring in southwestern Western Australia, but southeast South Australia, western and southern Victoria, northern Tasmania, and northern Queensland are likely to have a drier than normal spring.

On the other side of the Pacific, NOAA says the La Niña will mean drier than normal conditions in the already drought-stricken U.S. southwest.