One of the strongest La Niña events on record has finally concluded, after playing a major role in the record breaking rainfall and flooding seen through eastern and northern Australia over the last 10 months, according to weatherzone.com.au.

By many measures this was one of the strongest La Niña events on record, surpassing those seen in the 1970’s and rivalling the 1917 event.

La Niña events are characterised by colder than usual sea surface temperatures through the central and eastern Pacific Ocean (near South America), along with warmer than usual waters near the Australian continent.

These sea surface temperature anomalies in turn drive stronger than usual southeast trade winds across the tropical Pacific Ocean, boosting moisture levels across eastern and northern Australia.

In addition, ascending air over the east and north leads to areas of low pressure and rainfall. The lower than usual pressures in Darwin are measured against the atmospheric pressure readings in Tahiti, producing a value for what is known as the Southern Oscillation Index (SOI). Consistently positive SOI values, combined with cold sea surface temperature anomalies across the central and eastern Pacific, indicate a La Niña phase.

“We have seen a return of near normal sea surface temperatures through the Pacific over the last month, while the SOI index has also returned to neutral territory in the last week. Therefore we can safely say that the current La Niña has concluded, following a once-in-a-lifetime event,” said Alex Zadnik, Senior Meteorologist at Weatherzone.

“While sea surface temperature data prior to 1950 is limited, using the SOI value over the last 10 months, we can say the current La Niña event was the second strongest on record,” continued Mr Zadnik.

The rainfall seen over the past year has been on a par with heavy rainfall events seen during the La Niña events of the mid 1970’s and the 1950’s, but based solely on atmospheric data, this event was stronger.

As a result of the persistent and heavy rainfall through the La Niña event, record flooding was experienced through central and southern Queensland, western and southern NSW and northern Victoria.

In some cases, 100 year-old rainfall records were smashed, partcularly through central Queensland. Taroom in central Queensland had more than 230% of its annual average rainfall in 2010. The accumulated total for the year was 1613mm, which is the highest in 125 years of records. In the Northern Territory, some suburbs of Darwin were swamped by more than three metres of rain through the wet season, while northwestern Victoria also had record falls. Mildura had more than double its annual average rain with 591mm, which was their highest annual total since records began in 1889. Broken Hill in western NSW had already picked up more than its annual average rainfall in the first three months of 2011.

Some flooding rain even reached as far east as South Australia and the Northern Territory. Adelaide had its wettest summer since 1973/74 and Alice Springs its wettest year since 1974.

Southern parts of Western Australia don’t usually see above average rainfall in La Niña times, as was the case in 2010, when much of the wheat belt had its driest year on record.

With the La Niña event now fading and the arrival of the dry season across northern Australia, the likelihood of another major flood event has drastically reduced.

“Looking ahead to next spring and summer, it’s not yet clear whether we will see La Niña conditions redevelop, or even return to the drier El Niño pattern. The scientific consensus is for neutral conditions into spring, although it is still a little too early to tell which way things will swing,” concluded Mr Zadnik.

– Weatherzone