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Ofu's southern fringing reef has long intrigued coral researchers due to the presence of super-heated pools on the reef flat, where corals exhibit remarkable resilience in the face of elevated sea temperatures. However, the new study examines a previously unrecognized phenomenon: the occurrence of exceptionally large low-frequency waves traversing the reef flat. 

During a large southerly swell event, researchers observed low-frequency waves with mean heights reaching 0.7 meters and periods spanning 2 to 4 minutes. These waves, characterized by periods exceeding 100 seconds, are estimated to have contributed up to 50% of the total water levels during the swell event, underscoring their significant role in coastal dynamics on the reef. 

One particular observation stands out amidst the data—a low-frequency wave, likely amplified by reef resonance, had a trough-to-peak vertical height of 1.5 meters, potentially representing the largest ever directly documented on a reef flat. 

While extreme waves like these can be hazardous in terms of coastal flooding and erosion, they also hold relevance to coral health and resilience. Low-frequency waves are important drivers of mass transport on reefs, which influences coral health through sediment and larvae transport, as well as exposure to nutrients and contaminants. 

The interplay between these waves and coral ecosystems remains an area needing further study, with implications for improving our understanding of reef ecology and developing effective adaptation strategies in the face of climate change. 

The study, Extreme low-frequency waves on the Ofu, American Samoa, reef flat, appears in Coral Reefs: Journal of the International Coral Reef Society.

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