Perfect storm: What is the Fujiwhara Effect?

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?

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.

Unpredictable Path

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.

No collated data

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.

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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