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Australia's big wet: La Niña explained

The Bureau of Meteorology, the CSIRO, and the World Meteorological Organisation have each released a review of Australian and world climate in 2011. According to these three sources, 2011 in Australia was generally cooler than recent years...but warm for a La Niña year. It was also a wet year for Australia...but then, that’s normal for a La Niña year. So what is La Niña? How do we understand climate change and variability in the context of a La Niña event?

La Niña in simple terms
Globally and regionally, high or low pressure systems, cold or hot fronts and strong winds coincide and interact to influence weather and climate. One domino in this chain reaction of events is the Southern Oscillation. If the Southern Oscillation is ‘swinging’ strongly one way or the other, an El Niño or La Niña event results. In between, there are neutral periods.

The Southern Oscillation plays out in the South Pacific Ocean, the space between Darwin and the Peruvian coast (going through Tahiti). In neutral periods, the ocean surface temperature around Darwin is about 28˚C, and near Peru it’s around 20˚C. In very simplistic terms, ignoring all other factors (such as trade winds and zonal air circulation patterns) an El Niño or La Niña event occurs when the ocean surface temperature difference between the northeast Australian coast and South American coast is disrupted. This happens for a host of reasons (occurring both above and below the ocean surface), some of which are not fully understood.

An El Niño event occurs when the Australian waters become relatively cooler and the South American waters relatively warmer. High atmospheric pressure tends to result over cold water, just as the pressure is generally low over warm water. As a result of these pressure differentials, during El Niño conditions the Australian climate is generally sunny,dry and warmer than usual, especially between June and November. Of course, the effects are not uniform across the continent, but overall there are drier conditions and fewer clouds, leading to an increased likelihood of bushfires. However, because of reduced cloud cover, at night the air cools more quickly, meaning that warmer-than-average days end in cooler-than-average nights.

A La Niña event is the opposite of an El Niño. Warmer waters off the coast of northeastern Australia favour lower atmospheric pressure. The greater evaporation from the warmer water brings more clouds and rain, and generally cooler conditions. This means increased chance of flooding, especially between October and March.

El Niño and La Niña periods can last anywhere from a few months to a couple of years, and repeat at irregular intervals. The graph below from the US National Oceanic and Atmospheric Administration shows the occurrences of El Niño and La Niña periods since 1950.

The year 2011 began partway through one of the strongest La Niña events on record, which lasted until April/May. A second La Niña period began shortly afterwards, in August/September 2011.

Australian temperatures and rainfall in 2011
La Niña doesn't just affect Australia. It has major ramifications over a large area and, as a result, global average temperatures in 2011 were lower than in 2010. However, they were the warmest temperatures ever recorded under La Niña conditions. While much of Australia was colder than usual, Europe, North Asia and the Americas were generally warm, although there were none of the record-like heatwaves experienced in 2010. For Australia, even though 2011 was the coolest year since 2001, the 2002–11 average temperature was the equal warmest 10-year period on record. But temperatures were not uniform across the country. While northern Australia was cooler than average, the South was actually warmer than average over the year. This was punctuated by three heatwaves: the January/February hot spell across the southern mainland, the early August heatwave across southeastern Australia and the period in late December which affected northwestern Australia.

During the January/February heatwave, Sydney had its longest hot spell in 152 years, with 7 consecutive days above 30˚C and 5 nights above 24˚C. This spell ended on the 6th of February 2011, when Sydney recorded its highest ever daily minimum temperature of 27.6˚C. Perth experienced some testing times in March with 22 consecutive days above 30˚C. The March daily average for 2011 in Perth was the highest on record at 31.9˚C. Despite these record-breaking occurrences, on average 2011 was generally a cold year Downunder, and for Australia that usually means more rain.

Mean rainfall for Australia in 2011 was 705 mm, and this makes 2011 the second wettest year since 1900. The combined rainfall from 2010 and 2011 was 1408 mm, which is the highest recorded two-year total, just one millimetre more than the next highest which was for 1973–74.

A number of records fell in this unusual year. March 2011 was the wettest March on record for Australia as a whole, and also for the Northern Territory and Queensland. January 2011 was the wettest January on record for Victoria, and February was the wettest February for South Australia. Even the West didn't miss out; overall, 2011 was also the wettest year on record for Western Australia.

And, of course, 2012 has started with more flooding in many parts of the country as the second La Niña showed its effects. But the situation seems to be stabilising, and a return to neutral conditions within the next few months is forecast.