1 – A not-so-rare cyclone in the Indian Ocean
In the Indian Ocean and South Pacific, the term cyclone refers to a large system rotating around a center of low pressure. The phenomenon is called a hurricane in the North Atlantic and the Northeast Pacific, and a typhoon in the Northwest Pacific. The intensity of cyclones is defined by the Saffir-Simpson scale which ranks them from 1 to 5 depending on wind speed. To reach category 5, the most devastating, winds must exceed 250 km/h.
“In the south-west basin of the Indian Ocean, where Mayotte is located, we have a specific classification which goes up to the “very intense” cyclone, which roughly corresponds to category 5 of the Saffir scale. Simpson. Chido is considered “intense”, between Saffir-Simpson categories 3 and 4. We record three per year on average of this intensity,” indicates François Bonnardot, head of the forecast system at the Météo-France establishment in Reunion. A cyclone is “intense” for winds from 176 km/h average speed over ten minutes in the eyewall, the low pressure center.
2 – The most unfavorable trajectory
Mayotte's immense bad luck is due to the fact that Chido's trajectory made it transit right above the main island of the archipelago, Grande-Terre. “Most of the time, these systems evolve over the ocean. That the eye passes over an island is very unlikely. Chido would have gone 50 kilometers further north or further south, its effects on Mayotte would have been weak. It is really within a radius of a few dozen kilometers around the eye that the winds are devastating,” continues François Bonnardot.
“Usually, Mayotte is protected by the northern tip of Madagascar, to the east. Cyclones heading towards Africa commonly dissipate on the eastern coast of the big island. This is what could have happened for Chido. But its trajectory shifted towards the north in the last few days, it did not make landfall in Madagascar and it then came back down to Mayotte without becoming disorganized,” explains the scientist.
3 – The unclear influence of warming
Tropical cyclones are born over warm waters. With global warming, surface waters gain additional degrees, which can transfer additional energy to cyclones. Climate models actually predict an increase in their intensity by the end of the 21st century.e century, but not in their number. They are also expected to appear at higher latitudes, both in the Northern and Southern Hemispheres since the waters there will be warmer.
“However, it is complicated at this stage to connect Chido with global warming. The most intense systems are too few in number to identify a reliable trend over a few decades in the South-West Indian Ocean,” judges François Bonnardot.
