A Little More On La Nina…

By Dan Hutton on January 21, 2011 in

Photo: Marcus Greig

If you had a few weeks off over summer you may have gone back to work feeling a little ripped-off, weather-wise. This season the number of perfect beach days seemed well below average, whereas the number of grumpy European tourists complaining about the rain seemed anomalously high.

Many Aussies outside of Sydney had bigger problems, with widespread flooding affecting vast areas of New South Wales and Queensland. As well as loss of life and damage to infrastructure, the devastation of important food crops by floodwaters is likely to send the prices of fruits and vegetables rocketing for months to come. So, on the driest inhabited continent on the planet, what on Earth is with all this rain?

It turns out that we exist on the eastern coast of Australia at the whim of the Pacific Ocean, which is typically cooler in the east than the west, creating convection currents in the ocean and air that interact to provide us with the weather systems we rely on so directly for survival. Indeed, the tropical waters lapping the shores of Darwin are usually up to ten degrees warmer than those at similar latitudes on the South American coastline because there, the Humbolt current brings cooler waters up from Antarctica.

Our present precipitation predicament is due to lower than average temperatures in tropical Pacific South America and the resulting intensification of the trade winds across the Pacific, which in turn have pushed rain clouds from the central ocean basin all the way to our beaches, townships and farms. This phenomenon occurs quasi-periodically and, in meteorological circles, is termed the ‘La Niña’ (‘the little girl’) phase of the ‘El Niño/La Niña Southern Oscillation’ (ENSO) weather system. Interestingly, despite the devastating floods this ‘little girl’ has brought about, she is an important and relatively benign phase of ENSO, upon which much biodiversity in wetlands and other important ecosystems depend.

It’s La Niña’s little brother, El Niño, who causes us big problems when he visits, every three to eight years. El Niño phases of the ENSO system occur when the water off South America warms-up and the convection currents and trade winds weaken, allowing rain clouds to move westward, away from our thirsty shores. Thus, El Niño phases can have catastrophic effects on Australia’s climate by causing droughts and increasing our susceptibility to severe bushfires. Alarmingly, the intergovernmental panel for climate change (IPCC) has predicted that extreme weather events like El Niño may become more severe in the future.

So a lot more goes into a great day at the beach than you may have realised. It’s amazing how small changes in temperature on one side of the world can so dramatically affect the weather here. Meteorologists reckon that La Niña and El Niño events usually end in autumn so there’s hope for a warmer, drier summer in 2011/12!
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