Why have we had so much rain this year?

You’ve heard about La Nina on the news – but what exactly is it?

As parts of western NSW prepare for heavy rainfall and severe flooding, the east coast is once again bracing for more wet weather throughout spring.

Greater Sydney had its wettest July on record according to the Bureau of Meteorology, recording around double the average rainfall for winter. And the rain isn’t set to slow down anytime soon, according to Dr George Takacs, Honorary Senior Fellow in the UOW School of Physics. 

“This year is shaping up to be the wettest on record in much of southeastern Australia,” he says.  

There are three major drivers of rainfall changes: the El Nino Southern Oscillation (ENSO), Indian Ocean Dipole and the Southern Annular Mode. These three patterns have combined over recent years, causing extensive rain across the east of Australia.  

“When the ENSO is in a La Nina phase, we get higher than average rainfall in eastern Australia. When the Southern annular mode is in a positive phase, likewise,” says Dr Takacs. 

“The Indian Ocean Dipole is also in a negative phase which brings higher than average rainfall to southeastern Australia. It is extremely rare for all these three drivers to be simultaneously in a phase which brings higher rainfall.”  

It is the third year in a row of La Nina, the phase of ENSO which causes stronger winds blowing east to west across the Pacific Ocean.  

“What happens in a La Nina, is there is an overall transfer of energy from the atmosphere to the oceans, so global average atmospheric temperatures are lower than they would otherwise be. The opposite applies to El Nino years, during which there is a transfer of energy from the oceans back to the atmosphere, raising atmospheric temperatures,” says Dr Takacs. 

“La Nina leads to warmer than usual waters off the east coast of northern Australia. In turn this means more water evaporating into the atmosphere, which then gets directed upward over land and condenses as rainfall. 

“It is unusual to have La Nina three years in a row, as we have had recently. Normally the time between one La Nina to another is around three to eight years.” 

Man in plaid shirt looking up on black background Dr George Takacs, from the UOW School of Physics

The ongoing effects of climate change 

It isn’t just the coincidence of these weather patterns causing major disasters. Recent data has shown abnormally high temperatures in the Arctic and Antarctic. Dr Takacs says the rapid heating of the poles, faster than the rate of the equator, can contribute to the natural disasters we have seen in recent years.   

“What creates, but also disrupts, our large-scale weather systems is the difference in temperature between the equator and the poles. As that temperature difference decreases, these systems may become more persistent. So long dry spells and heatwaves will become more frequent, which is going to predispose us to bushfires, but also long wet spells become more persistent and they're going to predispose us to floods and landslides.”  

Not only will these disasters become more common, Dr Takacs says the geographic extent of such events continues to grow.  

“The areas that were affected in the [Black Summer] bushfires in Australia, that was unprecedented. Likewise in these floods – normally if it was flooding on the south coast in Moruya, it wouldn’t be flooding in places like Bellingen or the Lockyer Valley in Queensland at the same time. The worry is the emergency services have to respond to a large number of places, but there isn’t the capacity,” he says.  

“It is unknown whether we will continue to have wetter summers in the future. While global warming is reducing the temperature difference between equatorial and polar regions, how our climate system operates is entering unknown territory. We need to prepare now for greater variability.” 

Dr Takacs will be speaking alongside other climate experts in a special webinar for Global Climate Change Week. This event is free and available to the whole community, however registration is essential.