OPINION: Why Indian Ocean keeps spawning lethal cyclones
More than 1 000 people died. This makes it the most deadly tropical cyclone on the southern African subcontinent. Until Idai, tropical cyclone Eline, which struck in 2000, was the most devastating tropical cyclone to make landfall in Mozambique.
After Idai, Eline was the strongest - though not the deadliest - to have hit the south-eastern coast of Africa. The ranking was soon afterwards challenged by tropical cyclone Kenneth, a category 4, that made landfall over the border of Mozambique and Tanzania six weeks after Idai.
Kenneth took the region by surprise. The storm was the northernmost tropical cyclone to make landfall on Mozambique, and the first to make landfall on Tanzania. It occurred late in the season. Most occur from January to March. It was also unusual for the Mozambique Channel to experience two severe tropical cyclones that made landfall within one season.
The third major cyclone to emerge from the Indian Ocean came a few weeks after Kenneth, when cyclone Fani, a tropical cyclone on the border of Category 5 intensity wind speeds, hit the east coast of India. Category 5 tropical cyclones were first recorded in the North Indian Ocean from 1989 so the storm was unusually severe in the context of the longer historical records.
Their high intensity has been tied to the warm sea surface temperatures. Temperatures of 30°C are occurring more often and over longer periods as a result of gradual warming on a global scale. Warmer ocean temperatures allow stronger storms to form.
The conditions are exacerbated by global forcing mechanisms including El Niño and the Indian Ocean Dipole, which concentrate warm ocean waters in smaller geographic areas.
High-intensity storms have been a feature along the coast of the US throughout history. Their increased frequency in the Indian Ocean should be raising alarm bells because countries like the US are much better equipped to help people prepare, and to handle the fallout.
Intensity is classified according to the Saffir-Simpson scale. Categories are measured on the wind speed and the storm’s central pressure. Each category is accompanied by estimates of the probable severity of damage and storm-surge height. Tropical cyclones form an intensify due to seven primary climatological conditions, among them warm sea surface temperatures, high humidity levels and atmospheric instability. For a storm to intensify, the conditions have to be maximised while the storm remains over the ocean.
Tropical cyclones require a sea surface temperature of 26.5°C to form, while the highest intensity storms require much warmer sea surface temperatures of 28°C to 29°C.
The South Indian Ocean is warming rapidly. This means regions that experienced the temperatures of 26.5°C that facilitated tropical cyclone formation are experiencing temperatures of between 30°C and 32°C.
Regions further from the equator which didn’t have sufficiently warm water for tropical cyclone formation, with sea surface temperatures of from 24°C to 26°C, are more regularly experiencing the threshold temperature.
The warm sea surface temperatures are influenced by forcing mechanisms, among them El Niño Southern Oscillation, the Indian Ocean Dipole and the Southern Annular Mode. Scientists are seeing the strongest impact from the Madden-Julian Oscillation. This is a band of moisture in the tropical regions which moves eastward over 30 to 90 days. It is also affecting tropical cyclones in Australia.
Ranking storms on the Saffir Simpson classification can’t take the characteristics of the location of landfall into consideration. It doesn’t take the flooding potential into account. This is difficult to identify for a storm because it’s not only a function of how much rain is experienced and over what period but also the nature of the region of landfall.
Lower-lying, relatively flat areas are more prone to flooding than higher elevation regions. This is part of the reason Idai caused severe flooding. Some regions will have better suited stormwater infrastructure. When flooding does occur, some regions are better able to warn and evacuate people.
Another factor is population density. The higher it is, the more people who are at threat of losing their life, their homes and livelihoods. This also means more people who would need to be evacuated and would need shelter. This is why Idai and Eline resulted in far greater losses and fatalities than the stronger intensity Kenneth, and why the damage from Fani is projected to be devastating.
We need to start measuring storm destructiveness in addition to climatological metrics. The Conversation
Jennifer Fitchett is a senior lecturer in physical geography, Wits University.