Australia has recorded its wettest and coolest summer in at least five years, but the current conditions are not finished yet. 

La Niña events occur when sea surface temperatures of the tropical Pacific, particularly the central and eastern Pacific, get unusually cold. At the same time, the other side of the basin - the Western Pacific which is closer to Australia - gets warmer than average. 

The warm water near Australia increases moisture to the atmosphere and enhances the chances for more rainfall over northern and eastern Australia.

“What we are seeing now is actually past the peak of La Niña event and it's slowly fading,” says Dr Andrea Taschetto, an Associate Professor and ARC Future Fellow at UNSW Science’s Climate Change Research Centre.

“We are expecting that La Niña will fade and go back to normal conditions by April/May this year. Without La Niña we expect to receive normal average rainfall in winter, not exaggerated as we've seen during the summer and autumn.”

La Niña is the cooler phase of a phenomenon called the El Niño-Southern Oscillation (ENSO). The warm phase of ENSO is named El Niño.

“La Niña and El Niño generally have a three to seven year cycle so we are not expecting to see another event like this develop at the end of the year,” Dr Taschetto says. 

“Although La Niña can sometimes persist for two years, seasonal forecasting agencies, such as the Bureau of Meteorology, are predicting neutral conditions for the rest of the year and next summer.

“The strong La Niña event of 2010/2011 resulted in massive floods in Queensland. The 2010 spring season was the wettest spring in Australia since the 1900s. 

“We simulated a similar event at the time to check how unusual the 2010/11 La Niña was, and we found the warming ocean surrounding the northern parts of Australia was extremely important for that event. 

“It accentuated the chances of having extreme flooding in northeast Australia. 

“Our simulation showed that northeast Australia was three times more likely to experience extreme rainfall during that La Niña than if there hasn’t been a warming of the ocean north of Australia.  It was global warming making an appearance on top of La Niña.”