Jennifer Fitchett and Stefan Grab from the University of the Witwatersrand, South Africa, analysed storm data for the south-west Indian Ocean over the past 161 years. Their results are in line with previous studies that found there has been no increase in the number of tropical cyclones.
Much of the perceived increase in storm frequency could be due to better observations, the researchers believe. From 1850 to 1940, storms were often observed from ships, which would try to avoid regions of bad weather, leading to a significant undercounting of storm numbers.
Since the 1940s, aerial reconnaissance followed by satellite imagery have improved observations – researchers can now detect every single tropical cyclone that forms, moves and makes landfall on a coastline, Fitchett said.
Two other factors could have triggered perceptions of increasing numbers of tropical cyclones, the team believes. The era of global Internet connectivity has made people more aware of each storm that occurs as and when it happens, rather than just those in their immediate vicinity, or those that are severe enough to warrant global news coverage. "So the increase in reporting suggests an increase in numbers to the lay-person," said Fitchett.
And as the world population grows, more people are occupying coastal regions, and coastal towns and cities are getting bigger. "Consequently, when tropical cyclones make landfall in an area, they are now more likely to affect a town or city, and hence are worth writing about," said Fitchett.
The study found no statistically significant trends in the frequency of tropical cyclone landfalls over Madagascar and Mozambique over the past 60 years.
"Recent trends indicate an increasing number of tropical cyclones tracking to the south of Madagascar, potentially associated with the southward shift of the 26 °C isotherm, combined with a decrease in the steering flow during La Niña years," the researchers wrote in the International Journal of Climatology.
The 26.5 °C isotherm has moved south at a rate of 0.6° latitude per decade since 1850. "At current rates we could see frequent serious damage in South Africa by 2050," said Fitchett. "Our findings of a southward trend in storm trajectories will hopefully allow policymakers in South Africa and Mozambique to ensure that infrastructure is sufficiently sound to withstand the storms before they become a very regular occurrence. It allows for a certain amount of forewarning, which they previously did not have."

Related links