Milking the Weather - September 2010 Edition
La Niña in focus
Daniel Watts, DPI Ellinbank and Zita Ritchie, DPI Warrnambool
So, it’s a La Niña year, but the question on everyone’s mind is, does that actually bring more rainfall? Read on to get the low down on La Niña.
What is La Niña?
In Australia (particularly eastern Australia) La Niña events are associated with an increased probability of wetter conditions. La Niña translates from Spanish, as ‘the girl child’ and relates to the extensive cooling of the sea surface temperatures (SST) in the Pacific Ocean along the equator. This is usually accompanied by warmer water over tropical Australia, Papua New-Guinea and Indonesia.
Above: A typical illustration of La Niña showing cooler than average SSTs in the central Pacific ocean with warmer SST off the northern region of Australia. Trade winds are strengthened. La Niña can be associated with increased rainfall for eastern Australia in winter/ spring.
The warmer water north of Australia is held in position by strengthened trade winds. As a result of the warmer water there is increased evaporation, cloud and humidity in the air, increasing the source of moisture to potentially feed down into Victoria.
La Niña can be measured using the Southern Oscillation Index (SOI), which is the daily air pressure difference between Tahiti and Darwin. A sustained positive value (over eight) is characteristic of La Niña. During La Niña the air pressure at Darwin is lower because of the rising warm air mass and the air pressure at Tahiti is higher. Fluctuations in the SOI occur as a result of changes in the trade winds and SSTs, which in turn affect the air pressure along the equator.
How does it affect Australia?
La Niña can be associated with higher than average winter, spring and early summer rainfall over much of Australia. La Niña events commonly last for around a year, however they can be shorter, or much longer. These events tend to begin in autumn and mature during winter, spring and early summer, then begin to decay in late summer. Events generally end in the autumn. In the past, the strongest La Niña events were established by winter’s end, with widespread above-average rain falling over Australia’s eastern half (e.g. 1988/89). If a La Niña develops late in the season (during spring) a typical rainfall response is less likely (an example was in 2007).
The map of Australia below shows the mean rainfall deciles for total winter/spring (June to November) rainfall for twelve La Niña years in Australia. South-eastern Australia, northern Victoria and northeast Tasmania have experienced decile 7 or decile 8 from the average of these twelve years (Bureau of Meteorology).
Above: Average rainfall distribution for the twelve strongest La Niña years for winter/spring (June to November) (from Bureau of Meteorology http://www.bom.gov.au/climate/enso/ninacomp.shtml)
Indian Ocean Dipole and La Niña
La Niña events can be associated with Indian Ocean Dipole negative events. A negative phase IOD increases the chance of rainfall for Victoria in winter/spring. IOD is related to small changes in sea surface temperature (SST) in the north east Indian Ocean (off the Sumatran coast).
Changes in SST in this area can influence the formation of cloud bands that stretch across Australia. We have entered into the potential start of IOD negative event. The last IOD negative event was in 1992.
