Cyclones and climate change: The case of Cyclone Batsirai
Tropical Cyclone Batsirai has left Madagascar after killing dozens, displacing tens of thousands and devastating the island’s agriculture, already suffering from drought hard hitting a vulnerable population, Unicef said, adding that many of the victims are probably children, who make up more than 50% of Madagascar’s population.
Batsirai hit eastern Madagascar with a Category 3 strength on the evening of Saturday 5 february 2022, bringing heavy rains and winds of 165 km/h. The city of Mananjary in the east of the country “has been completely destroyed”, one of its residents said. This new storm occurred in the wake of Tropical Storm Ana, which recently killed 55 people. Given that we already live in a world of 1.2°C global warming, such extreme events immediately raise the question of whether and to what extent climate change has altered the likelihood and intensity of these devastating storms. Here we briefly summarize the current scientific knowledge on such links.
Tropical cyclones formation
Tropical cyclones develop over ocean basins from large-scale tropical circulation disturbances and waves generated by atmospheric dynamics. Through enhanced convection these disturbances can self-organize into tropical storms under conditions of weak vertical wind-shear. When fuelled by high sea surface temperatures (SST), these storms can quickly escalate into cyclones with sustained winds exceeding 120 km/h. Strong cyclones can only develop when SSTs reach very high values such as 27-30°C. The complexity of mechanisms describing tropical storm development leave projections of cyclones under climate change poorly understood. The Western Indian Ocean, Madagascar and the South-Eastern coast of Africa; as well as islands like the Réunion or the Mauritius islands are regions prone to cyclones formed in the Indian Ocean.
Cyclones in Europe
In Western Europe tropical cyclones can hardly reach the Atlantic coasts with a tropical structure (warm core), while they do hit Portugal, France, Ireland and the United Kingdom in the form of post-tropical depression, i.e. hurricanes which have changed their dynamical characteristics because of their northward shift. Only a few hurricanes kept their tropical characteristics while reaching Europe: hurricane Chloe 1967 which made landfall in France with Category 2 winds, Hurricane Vince in 2005, which struck southwestern Spain as a tropical depression; and Subtropical Storm Alpha in 2020. The Mediterranean Basin hosts other kinds of tropical-like cyclones, the so-called “medicanes”, which features a smaller radius and a different dynamics than the tropical cyclones.