Not all waves are created equal

Why do some big wave events swamp Kiribati’s Tarawa lagoon shorelines while others pound the windward coast? A new UBC-led study disentangles the culprits and shows why future flooding risk will not be uniform across the atoll.

After analyzing 40 years of offshore wave varieties hitting the Tarawa atoll from 1979 to 2018, including 41 extreme events, it was found that when elevated water levels and the right wave direction line up, inundation risk jumps. This is particularly true during El Niño, when regional sea level sits higher.

“Tarawa offers us a broader lesson for low lying atolls across the Pacific,” said Julia Mayer, PhD student in UBC’s Institute for the Oceans and Fisheries, and first author on the study. “Not all extremes arrive in the same way. As sea levels rise, extreme wave events, especially those linked to El Niño conditions, are expected to become more destructive for the people of Tarawa.”

Instead of treating big waves as one hazard, the team used hierarchical clustering to group extreme days by full frequency and direction fingerprints. This linked each cluster to different atmospheric triggers and specific coastlines at risk. They identified three event types that drive extremes in Tarawa: first, El Niño related westerly wind bursts that whip up locally generated lagoon waves; second, long period swells from the south and southwest produced by Southern Hemisphere storm; and finally, swells from the north and northeast tied to Northern Hemisphere storms.

“Our results upended the common assumption that open ocean shores are always most vulnerable. Some of the most damaging events were the result of wind reversals during El Niño,” said Mayer. “The atoll’s partially open lagoon generated energetic waves that pushed water toward low lying, populated shorelines. Places that usually enjoy lower wave energy can become hotspots of risk.”

“Tarawa is a thriving place, the capital of the country, and one of the more densely populated atolls in the world,” said Dr. Simon Donner, senior author on the paper and professor in the IOF, as well as UBC’s Institute for Resources, Environment and Sustainability, and Department of Geography. “The rising population has led to more and more people living in marginal lands that at vulnerable to waves and flooding”.

“Sea-level rise is an existential threat to places like Tarawa. The islands may not ‘disappear’, as is often reported, but they will become increasingly challenging places to live. A better understanding of what drives high wave events can help inform adaptation planning and keep communities safe,” Donner said.

For Tarawa, and many atolls, the big question is not only how big the waves are. “It is which waves, when they arrive, and how high the ocean sits beneath them,” Mayer concluded.

“Clustering of Historical Extreme Wave Events to Assess Climate Variability in Tarawa Atoll, Republic of Kiribati” was published in Natural Hazards.