Distant swells and their impacts on atolls and tropical coastlines. The example of submersions produced by lagoon water filling and flushing currents in French Polynesia during 1996 and 2011 mega swells

Distant swells produced by high latitude storms are poorly studied despite their significant impact on almost all major ocean basins. This article studies the consequences of extreme southern swells on the islands and atolls of French Polynesia by focusing on lagoon water filling. This phenomenon is a product of great water influx in the lagoon coming from waves breaking on the reef. This analysis is based on archives in Tahiti, and on surveys and interviews conducted in the Makemo Atoll (Tuamotu Archipelago) to identify the impacts of giant swells during August 2011 and July 1996. These swells have strongly affected all the archipelagos of the Western Pacific and have been strongly felt on the entire Pacific coastline of the American continent. In French Polynesia they caused a rise in the lagoons that reached 2.75 m (above the lowest astronomical tide), causing a major submersion event that strongly impacted the life of the inhabitants by destroying many resources on which they depended. Comparing the swells of 1996 and 2011, this article shows that the filling of the lagoon is largely determined by the duration of the intense wave event and not only by the maximum intensity reached by the swell. Finally, this article raises the question of how this phenomenon is taken into account in town / atoll planning, and the vulnerability of the coastlines of the intertropical zone to distant swells in the context of sea level rise and changes of atmospheric circulation. •Swells from high latitudes can produce strong flooding on tropical coastlines.•Distant swells can produce a rise of lagoon levels up to 2.75 m.•The rise of lagoon water levels is slowed down by lagoon filling inertia.•Longer-lasting swells produce higher lagoon levels.