Researchers from UNSW Sydney have shed new light on the behaviour of coastlines during different phases of the El-Niño-Southern Oscillation (ENSO) cycle.

ENSO is described as a naturally recurring climate pattern that causes predictable variations in ocean surface temperatures across the tropical Pacific Ocean. El Niño and La Niña are the warming and cooling phases respectively, and have a cascading effect on global weather, especially along the coasts. Coastal erosion can increase during these periods, shifting sand away from beaches and threatening habitats along coastlines.

For over 40 years, satellites orbiting the Earth have captured images of the world's coastlines at regular intervals. Using millions of these satellite photos, the UNSW researchers studied changes along coastlines across the Pacific Ocean. Their findings – published in Nature Geoscience – now reveal the unique behaviour of coastlines during different phases of the ENSO cycle.

"Scientists have known beaches respond to ENSO cycles for decades, but we've only been able to paint a small picture of this from a few sporadic beach monitoring sites," says Dr Mitchell Harley, co-author of the study from UNSW's Water Research Laboratory at the School of Civil & Environmental Engineering.

Tapping into satellite imagery

The researchers developed a new open-source tool called CoastSat, which combines image processing and machine learning algorithms to extract information from every pixel of thousands of sandy beaches along the Pacific Rim.

"The tool automatically maps the position of the interface between the sand and water. Doing this over multiple images and beaches, and correcting for the tide, we can discover changes in the coastline over a large temporal and spatial scale and how this correlates with ENSO cycles," explains Dr Kilian Vos, lead author of the study, also from UNSW’s Water Research Laboratory.

The tool, CoastSat helped the researchers process 38 years of Landsat satellite photos to find correlations in beach width and ENSO cycles across more than 8000km of sandy coastline. Interestingly, they found that beaches on the southeast coast of Australia narrow during the La Niña phase and widen during El Niño periods. However, beaches on the other side of the Pacific experienced erosion during El Niño and recovered during La Niña.

Forecasting coastal erosion risk

By understanding the impact of ENSO cycles on different coasts along the Pacific Ocean, stakeholders can anticipate coastal erosion risks, and plan and implement appropriate protective measures.

According to Dr Vos, beaches provide a buffer between ocean waves and the high-value infrastructure around them. In some conditions, they can completely disappear overnight and take years to come back. "It's essential we continue to monitor them through studies like this, which help inform how we can manage our beaches.”

The CoastSnap citizen science project led by Dr Harley encourages community members to share their shoreline photos, providing new insights into how beaches respond to changing conditions.

"Using all these incredible data sources from space and the community, we can continue to increase our understanding of how our coastlines are changing now and in the future," Dr Harley added.

Image: Beaches on the southeast coast of Australia narrow during prolonged La Niña, while they widen during El Niño periods. Credit: Water Research Laboratory, UNSW