How La Nina shrinks Sydney’s beaches...and they recover

If you have noticed beaches looking thinner and rockier of late, you’re observing a phenomenon that’s being felt across the Pacific Rim.

Sydney scientists have shown that beaches along Australia’s south-east coast slim down by up to 20 per cent during cooler La Nina climate cycles and beef up again when the warmer El Nino cycles begin.

On the other side of the Pacific, beaches in the Americas see the opposite effect, expanding during La Nina and shrinking in El Nino times.

“For the first time, we’ve been able to measure how things are changing on a massive scale,” said Dr Mitchell Harley, a senior lecturer from the Water Research Laboratory at the University of NSW.

“Previously we’ve only been able to get precise measurements from a handful of beaches on any timescale to look at the patterns of change brought about by El Nino and La Nina.”

El Nino and La Nina are climate cycles driven by water temperature in the Pacific. Australia has just emerged from an unusual triple-dip La Nina, which fuelled storms and extensive flooding in eastern states last year.

Using a treasury of images sourced from NASA’s Landsat and the European Space Agency’s Sentinel-2 satellites, Harley and colleague Dr Kilian Vos built a software tool that could scan decades of change over 8300 kilometres of coastline around the Pacific Ocean.

“From that incredible imagery, we’ve developed new techniques to detect the shoreline changes using algorithms that are powered by machine learning,” Harley said.

The team found a strong correlation between La Nina cycles and increased coastal erosion at many NSW beaches as storms and strong waves gnawed away at the coast. El Nino cycles tend to be warmer and calmer, allowing beaches to gradually replenish lost sand.

But there is sometimes a lag after a La Nina, when the climate is in a neutral phase and beach erosion is still extremely high.

“This highlights how El Nino and La Nina can trigger prolonged erosion phases on sandy coastlines,” Vos and Harley wrote in the paper, which was published in the journal Nature Geoscience on Friday.

Using 38 years of satellite data, the study showed that 48 per cent of the beaches on the east coast shrank during La Nina years, while 70 per cent bulked up with sand when El Nino was in force.

Data over this timeframe had previously only been collected continuously at a few sites such as Collaroy on the northern beaches, where scientists have been gathering beach width data for more than 40 years by more old-fashioned methods – such as a tape measure stretched between two posts.

The research showed that most beaches that became heavily eroded did tend to recover. “But at a certain stage there is a tipping point where beaches can’t respond as dynamically and they can erode away,” Harley said.

University of Sydney’s Professor Andrew Short, a coastal erosion expert who was not involved in the study, said the research would help anticipate how beaches would change shape and how sand would be carried up and down the coast.

“It’s been suspected for some time that there is this correlation, but to have the database that shows the impact of La Nina and El Nino over this sort of timescale is very useful,” Short said.

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