The wild ways of La Niña

Wild storms are lashing Australia’s east coast, with severe flooding in Queensland and New South Wales leaving a trail of destruction – one person is dead and 10 missing. The weather pattern is expected to continue well into next week, marking the culmination of two years of wet La Niña summers.

“The current extremely wet conditions in south-eastern Queensland and coastal New South Wales are caused by an easterly atmospheric trough moving over the region,” says Nina Ridder, a research associate at the UNSW Centre of Excellence for Climate Extremes, whose work focuses on weather and extreme precipitation.

“This trough is dragging moist air from the tropical ocean with it – from the Coral Sea, to be exact. This moisture is then released as rainfall over land.”

Ridder explains that two years of La Niña weather patterns have worsened these dangerous conditions, not only because of the higher-than-average rainfall this week, but because the ground is already saturated from two years of intense rainfall.

“Both La Niñas have contributed to significantly more rainfall than normal over eastern Australia,” she says. “As a result, the rain we are seeing now is falling on saturated catchments, which means the water cannot be removed fast enough and thus leads to extended flooding.”

La Niña is part of a broader natural weather cycle known as the El Niño–Southern Oscillation (ENSO). The ENSO cycle generally operates over timescales of between one and eight years, according to the Bureau of Meteorology. 

La Niña happens when equatorial trade winds become stronger, changing ocean surface currents and drawing cooler, deep water up from the depths. This cools parts of the eastern and central Pacific Ocean, but trade winds pile up warm surface water in the western Pacific and the north of Australia.

These factors produce the ideal conditions for rising air, cloud development and, consequently, heavy rainfall over eastern Australia.

Amid increasingly chaotic storm seasons, scientists are concerned that these patterns and their damage might be exacerbated by climate change as the planet warms.

“How much climate change contributes to the current event needs to be studied in more detail,” cautions Ridder. “Eastern troughs are a common occurrence in Australia and are known to contribute to rainfall over eastern Australia.

“However, the amount of rain that we are seeing at the moment is likely influenced by increased temperatures in the atmosphere and the ocean.”

Ridder explains that a warming atmosphere, coupled with a warmer ocean, creates ideal conditions for more rain.

“A warmer ocean releases more moisture into the atmosphere,” she says. And the warmer the atmosphere is, the more water it can hold. “As a rule of thumb, 7% more for each degree warming.

“As a result, the amount of rainfall an area receives during an extreme rainfall event is increased compared to what it would get if there was no change in the climate.” 

Agus Santoso, a senior researcher at the UNSW Climate Change Research Centre whose work focuses on the dynamics, impacts and future of El Niño and La Niña, and how climate change might interact with the ENSO cycle, believes climate change is likely to exacerbate these patterns considerably.

“As the climate warms, we can expect more extreme weather events,” says Santoso, who is one of the authors on a paper, published last month, that examined potential effects on the ENSO system under four different IPCC emissions scenarios. “Our studies found that extreme El Niño and La Niña events may occur more frequently as the climate warms further.”

We already know that the ENSO cycle has been more variable and volatile since the 1950s. 

“From observations, we know that over the past decades, extreme rainfall events have increased both in number and in their intensity over most of Australia,” says Ridder. “Studies into future conditions project this trend to continue with increasing temperatures, even if mean rainfall is projected decreasing.”

Ridder and Santoso both say climate-driven sea-level rise will also make flash floods more likely.

“Higher sea levels make it increasingly harder for runoff from the land or rivers to be released into the ocean, which mean that lower rainfall amounts are likely sufficient to cause extensive flooding,” Ridder explains. Santoso adds that sea-level rise also increases the risk of dangerous storm surges.

So, the future looks moist, but are there things we can do to prepare communities and urban centres for increasingly extreme conditions? 

CSIRO, Australia’s national science agency, has recognised the need to focus on preparing for these kinds of dramatic events. The agency notes that even as rising sea levels and climate change increase flooding and worsen storm surges, Australia’s eastern coastline is becoming ever more developed – around 85% of the population lives on the coast. 

Last year, the Federal Government released its National Climate Resilience and Adaptation Strategy, which sets out the nation’s plans for mitigating the ravaging effects of climate change, but it is sparse on actual details of mitigation strategies for extreme events such as flooding.

“We need to adjust building codes and urban planning, and determine flood-prone areas, all taking into account the compounding nature of these events,” says Ridder. 

Santoso agrees that we’ll need to change the way we build our cities to weather these extremes. “Infrastructure needs to be enhanced and maintained, including structures for preventing beach erosion, floods and water treatment,” he says.

Ridder says education and awareness are key. “We need to raise awareness of where those floodplains are and ensure that people understand the dangers of flooding and the correct or safe behaviour around flood waters,” Ridder says.

Santoso agrees.

“The community needs to be informed about extreme weather and climate events and what to expect in a climate that is changing.”