Warming due to human-induced greenhouse gases is absorbed by the oceans, increasing their heat content; this helps strengthen cyclones
Vineet Kumar Singh’s designation was corrected on March 14, 2023.
Cyclone Freddy is now the longest-lived tropical cyclone on record. It has been active for 32 days over the south Indian Ocean, having made two landfalls and is likely getting ready to make another.
Freddy also holds the record for the maximum times a tropical cyclone has undergone rapid intensification at six. The second record hints at the role of global warming, specifically the heat in the oceans, in its record breaking streak.
Cyclone Freddy has had a major impact on people in Madagascar and Mozambique due to the impact of its swift winds and also the accompanying rainfall. It has killed 21 people and displaced thousands of people causing a crisis in both countries which are regularly hit by tropical cyclones. And it is not over yet.
“Cyclone Freddy accumulated 77 units of accumulated cyclone energy which is calculated as the square of wind speed added every six hours during its entire lifetime. Currently, it is ranked second in terms of accumulated cyclone energy after Cyclone Ioke in 2006 which had a total accumulated cyclone energy of 85.6 units,” Vineet Kumar Singh, research scientist at Typhoon Research Center, Jeju National University in South Korea, told Down To Earth. Accumulated cyclone energy gives the measure of the strength of a cyclone during its lifetime.
The cyclone which formed February 6, 2023 around the northern coast of Australia, has already traversed more than 9,600 km. It made its first landfall along the south-eastern coast of Madagascar February 21 where its winds and rainfall caused significant damage. It then crossed over into the Mozambique Channel, gained some energy and made a second landfall along the Mozambique coast February 24.
After this, the storm system spent several days over Mozambique and Zimbabwe causing rainfall. “Then, a high pressure area towards the west pushed it back towards the Mozambique Channel where it intensified again and affected south-western Madagascar, though it did not make a landfall there,” said Singh.
Freddy is now moving towards Mozambique again and would intensify again, according to the World Meteorological Organization (WMO). Such bouncing back and forth by tropical cyclones is common in the south Indian Ocean and the reasons are different every time, Singh said.
WMO’s regional centre in La Reunion has warned of heavy rains in southern Madagascar over March 8 and March 9 which could add up to 100 mm and 200 mm locally, according to WMO.
“Madagascar has received more than 300 mm in the last seven days, or around three times the monthly average,” said WMO in a press release March 7.
Freddy’s record for the longest-lived cyclone is still unofficial as the WMO has to analyse the cyclone’s track and intensities over the period to be able arrive at this conclusion.
Another record that Freddy has broken is of the highest number of times that a tropical cyclone has experienced rapid intensification. When the tropical cyclone undergoes rapid intensification it gains wind speeds of 56 kilometre per hour in 24 hours. Freddy has done this six times which is a record, according to Singh.
“Freddy kept finding pockets of warm sea water and low horizontal wind shear, which favour rapid intensification, throughout its lifetime,” said Singh. The sea water was warm not only on the surface but also in the sub surface.
Horizontal wind shear are the horizontal winds around a cyclone. When these winds have lower speeds they help a cyclone grow in strength and when they have higher speeds they make it dissipate.
Usually, the sub surface water is colder and when it gets pushed towards the surface due to mixing as a result of cyclonic winds, it creates conditions that don’t favour intensification of cyclones. “But in Freddy’s case, the winds pushed up warm sub surface waters which helped it rapidly intensify multiple times,” Singh noted.
Around 90 per cent of the warming due to human-induced greenhouse gases is absorbed by the oceans, increasing their heat content and have made them the warmest they have ever been, according to the latest data from WMO. There is really high ocean heat content along the south Indian Ocean right now, Singh said.
“Freddy is having a major socio-economic and humanitarian impact on affected communities. The death toll has been limited by accurate forecasts and early warnings and coordinated disaster risk reduction action on the ground — although even one casualty is one too many,” Johan Stander, services director, WMO, said in the press release.
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