The world is seeing a rise in cyclonic formations and in cases of cyclones merging to form megacyclones, courtesy warming oceans
On September 1, 2022, super typhoon Hinnamnor, the strongest tropical cyclone of the year, was hurtling towards Taiwan from the western Pacific Ocean.
Another tropical storm called Gardo was moving towards Hinnamnor from its southeast.
As the two approached each other, they started a dance around the central line between them, showcasing a textbook example of what is known as the Fujiwhara Effect.
After the dance, which lasted over a day, Hinnamnor devoured Gardo and eventually made landfall in South Korea, drowning seven people on September 7.
In this case, Hinnamnor’s intensity decreased and Gardo vanished. But in an increasingly warming world, a dance and merger between two large enough tropical cyclones over any of the global oceans could lead to the formation of a mega cyclone, causing devastation along coast-lines.
“When two ocean storms form in the same region, their wind circulations start meeting each other at the mid and higher levels. This meeting of winds makes a bond between the two storms like a connecting limb through which they start influencing each other,” Ravi Shankar Pandey, a research scholar at Earth System Sciences of Academia Sinica and Department of Atmospheric Science at the National Central University in Taipei, Taiwan, said.
In recent years, several storms have come close to undergoing the Fujiwhara Effect.
Just a week after Hinnamnor engulfed Gardo, two hurricanes—Danielle and Earl—formed one after the other in the North Atlantic Ocean, sparking fears of the Fujiwhara Effect.
(Hurricane, cyclone or typhoon are names of huge swirling wind circulations that form over warm ocean waters and are named depending on where they form. They are fuelled by heat, wind and moisture and under the right conditions can gain significant strength and size. The big ones can have diameters of more than 1,000 km and wind speeds in excess of 250 km per hour.)
Around the same time, another hurricane—Kay—also brewed in the eastern Pacific Ocean. In 2020 hurricanes Marco and Laura had formed back to back in the small region of Gulf of Mexico and created a possibility of the Fujiwhara Effect. Is the frequency of the Fujiwhara Effect increasing?
JOINING FORCESThe Fujiwhara effect can turn two cyclones into one megacyclone
The Fujiwhara Effect is any interaction between tropical storms formed around the same time in the same ocean region with their centres or eyes at a distance of less than 1,400 km, with intensity that could vary between a depression (wind speed under 63 km per hour) and a super typhoon (wind speed over 209 km per hour).
The interaction could lead to changes in the track and intensity of either or both storms systems. In rare cases, the two systems could merge, especially when they are of similar size and intensity, to form a bigger storm.
There are five different ways in which Fujiwhara Effect can take place. The first is elastic interaction in which only the direction of motion of the storms changes and is the most common case. These are also the cases that are difficult to assess and need closer examination.
The second is partial straining out in which a part of the smaller storm is lost to the atmosphere.
The third is complete straining out in which the smaller storm is completely lost to the atmosphere. The straining out does not happen for storms of equal strengths.
The fourth type is partial merger in which the smaller storm merges into the bigger one and the fifth is complete merger which takes places between two storms of similar strength.
During a merger interaction between two tropical cyclones the wind circulations come together and form a sort of whirlpool of winds in the atmosphere.
Fujiwhara effect was identified by Sakuhei Fujiwhara, a Japanese meteorologist whose first paper recognising the Fujiwhara cases was published in 1921. The first known instance of the effect was in 1964 in the western Pacific Ocean when typhoons Marie and Kathy merged.
Pandey’s research shows that it is. He found that just between 2013 and 2017 there were 10 cases of the Fujiwhara Effect, mostly weak interactions, in northwest Pacific Ocean. Of these, seven involved super typhoons which are tropical cyclones with wind speeds in excess of 209 km per hour.
Pandey contends that as the oceans get warmer and there are more number of stronger cyclones the possibility of the Fujiwhara Effect would increase drastically, like it already has.
“There has been a 35 per cent increase in the strength of typhoons that have hit Taiwan between 1977 and 2016. This happened due to a 0.4 to 0.7°C rise in the sea surface temperature during these 40 years in the northwest Pacific. It shows how global warming is responsible for making cyclones stronger, and thus increasing the chances of the Fujiwhara Effect,” says Pandey.
This highlights the need for a closer examination of tropical cyclones and storms for studying Fujiwhara interactions.
The occurrence of the Fujiwhara Effect makes cyclones more unpredictable due to their rapid intensification, carrying of more rain and newer ways of moving over warming oceans. This is because each of the interactions between the two storm systems is unique and very difficult to capture with current climate models.
In the case of typhoons Parma and Melor in 2009, it became extremely difficult for forecasters to track the movement or intensity of either of the storms, especially Parma, and provide people with early warning because of Fujiwhara interaction between the two storms.
This resulted in large-scale devastation in the Philippines due to the weaker typhoon Parma gaining strength, changing track and remaining stagnant over the Luzon region because of its interaction with the stronger typhoon Melor.
The typhoon took multiple U-turns and made three landfalls over the Luzon region, according to a research paper published in the journal Weather and Climate Extremes in September 2020.
In April 2021, a similar event happened in the Indian Ocean, when cyclone Seroja interacted with cyclone Odette just off the coast of western Australia gaining strength and moving in uncharted ways, according to Verisk, a catastrophe financial modelling company based in Boston, US.
Seroja had already caused flooding and landslides in Indonesia before moving towards Australia. After the interaction Seroja maintained its intensity and caused damage to 70 per cent of buildings in the small resort town of Kalbarri in Australia.
Seroja became a unique storm because the northwestern coast of Australia is prone to cyclones while the southwestern part hit by the cyclone does not get much storm activity.
Assessment of Fujiwhara Effect cases over longer periods is a problem. Researchers across the world tend to select the Fujiwhara cases they find interesting to investigate, but there is no collated dataset.
The main problem is that “there is no worldwide accepted technique or recording agency dedicated to recognise and collect the cases of the Fujiwhara Effect as these are rare events and tough to assess,” says Pandey.
A Down To Earth analysis found only 10 cases of the Fujiwhara Effect, apart from those investigated by Pandey, from across all the oceans of the world either studied in research papers or mentioned by Meteorological organisations such as the US’ National Hurricane Centre of the National Oceanic and Atmospheric Administration.
Data dearthThere is no comprehensive data on the observed cases of the Fujiwhara Effect. Here's a snapshot of the phenomenon since its first recorded case
Pandey insists that current climate models being used for tracking tropical cyclones around the world should take the Fujiwhara Effect into account which would make them more efficient and help in identifying when and where the effect would take place. This will help scientists know if a mega storm is gathering momentum.
This was first published in the 1-15 October, 2022 edition of Down To Earth
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