Cyclone modelling in the Western Indian Ocean – an application to the Comoros Islands

The Union of the Comoros, an archipelago located in the Western Indian Ocean, is subject to tropical depressions and sometimes to cyclones. In April 2019, cyclone Kenneth passed to the north of Grande Comore. It was particularly intense and caused casualties and significant infrastructural damage, requiring that such extreme and rare events are taken into account in future coastal management and engineering projects. For a more accurate definition of extreme offshore conditions in the area in view of designing a boat landing stage in Ourovéni (Grande Comore Island), a regional cyclonic modelling tool is developed using the open TELEMAC-MASCARET solver suite. Based on the Holland surge-wind model, it enables the determination of extreme wind, surge/water level and wave conditions during cyclonic events. The model is calibrated using satellite altimetry data and used to reproduce a sample of historical cyclones that have affected the Comoros archipelago since the 1980s. Based on this sample, the 100-year return values for offshore significant wave height and total water level are extrapolated and found in agreement with available studies. This tool ultimately allows a more realistic representation of forcing parameters for downscale modelling in an area where few cyclonic data are available.