Winds played a major role in Cyclone Fani's sudden intensification

Fani is not the first cyclone to leave governments and scientists perplexed. Last October, two very severe cyclones—Titli and Luban—had also behaved unconventionally on either side of the Indian mainland. Referring to them as the “rarest of rare” occurrences, the India Meteorological Department (IMD) said the movements of both these storms were unique.

While Titli made an unexpected shift in its direction (recurvature) towards the northeast after hitting the coast of Odisha on October 11, Luban made multiple recurvatures for nine days as it travelled through the south-eastern Arabian Sea on the Gulf coast before making landfall in Yemen on October 14.

Besides changing tracks, Luban saw rapid intensification and weakening over the sea while Titli saw rapid intensi fication, which means its wind speed increased by over 55 kilometres per hour within 24 hours. IMD could not predict it accurately even two days before the cyclone was to hit the coast and missed the landfall point of Titli by 27 km.

A month later, Gaja, a severe cyclone, hit Tamil Nadu with wind speeds of 100-110 km/hr. IMD’s prediction was 80-90 km/hr, which is the speed of a cyclone (63-88 km/hr). It also made landfall much later than predicted and remained a severe cyclone despite IMD saying that it would deintensify into a cyclone just before making the landfall.

The cyclone that left IMD completely flummoxed was cyclone Ockhi in November 2017. It took a convoluted route, and at the same time, witnessed rapid intensi fication in wind speeds — the two factors that make cyclone prediction difficult. On November 28, Ockhi developed near the southeastern coast of Sri Lanka.

The next day it started moving northwestwards before striking Lakshwadeep on Novem ber 30. Then the cyclone stopped its north-ward movement and turned towards India’s west coast.

According to IMD, the last time such a movement was recorded was in 1912, when a cyclone originated near Sri Lanka and ended up in Maharashtra. Ockhi had also transformed from a depression to a very severe cyclone in less than 40 hours, as against the normal period of 72 hours.

KJ Ramesh, director general of meteorology at IMD, at a conference organised by Delhi-based non-profit Centre for Science and Environment in 2018, said the failure to predict Ockhi prompted the agency to start using doppler weather radars and coupled ocean-atmosphere models that study local environmental conditions close to the cyclone.

Cyclones are becoming increasingly unpredictable worldwide. In recent years, the US has witnessed multiple cyclones that gained rapid intensification. In August 2017, hurricane Harvey strengthened from Category 2 (wind speed 154-177 km/hr) to Category 5 (252 km/hr or higher) in a little more than 24 hours, just before making landfall. A month later, hurricane Maria battered the US coast. Its intensification was even more dramatic.

It jumped from Category 1 (119-153 km/hr) to Category 5 in 24 hours. Even during hurricane Lane in August 2018, scientists could not predict its intensity when it hit Hawaii. A May 2018 study in the journal Geophysical Research Letters found that the magnitude of rapid intensification events increased from 1986 to 2015 in the central and eastern tropical Atlantic Ocean. In October 2018, two hurricanes — Willa in Mexico and Micheal in the US — witnessed rapid intensification.

Cyclones are massive and complex systems of wind circulation that are fed by moisture which is sucked up due to increasing temperatures over the sea surface. Ocean temperatures are on the rise due to global warming. This has fundamentally altered the character of cyclones.

Since 1970, sea surface temperatures worldwide have gone up by an average of 0.1°Celsius per decade as oceans act like heat sinks. Warm air holds more water vapour and this provides additional fuel for cyclones.

This was seen in Hurricane Katrina which intensified significantly when it hit deep pools of warm water in the Gulf of Mexico. The rising ocean temperature also results in seawater expansion, which, along with the melting of snow, has raised sea level by 0.2 metres since 1900. Higher sea levels mean cyclones start at a height, risking coastal and low-lying areas.

Apart from warming oceans, cyclones are also influenced by air pressure and local wind patterns. A cyclone forms around a low-pressure area that pulls air from the ground, expands and cools it down. When this process keeps repeating, the cyclone collects moisture and grows.

Apart from this, there is variation in air pressure throughout the interior of a cyclone which creates a large air pressure gradient force responsible for strong winds in the eye-wall of the storm, the deadliest part of the cyclone. The air pressure at different parts of a cyclone is measured using satellite imagery, weather balloons and radar-carrying aircraft and is usually factored in during forecasts.

Local wind movements, on the other hand, have always been difficult to record due to unpredictability in wind speed and direction in the lowest 10 km of the atmosphere which is called wind shear.

If vertical wind shear, which includes the winds moving towards the upper layers of the atmosphere and affects the top part of the cyclone, is high, the cyclone becomes unstable and dissipates quickly. When it is low, the cyclone gets access to more moisture and becomes intense.

The second factor is horizontal wind shear which constitutes the winds closer to the sea surface and either feeds the cyclone or corrodes it. When the direction of these winds is in sync with those of the cyclone, it can intensify the cyclone.

It can also push the cyclone towards warmer waters where moisture is easily available. But if the horizontal wind shear decreases then it allows the cyclone to grow. Though the theoretical aspects of both vertical and horizontal wind shear are well understood, actual obser vational data on these for a live cyclone are notoriously difficult to measure.

A complete understanding of the minutest details of winds around a cyclone forms the missing link in accurate prediction.

Unwieldy storms Cyclones are now rapidly gaining strength and unexpectedly altering their course Cyclone Titli / 2018 A severe cyclonic storm on October 10 in the Bay of Bengal. A day later, it shifted its direction towards the northeast after hitting the coast of Odisha. Cyclone Gaja / 2018 Originated from a low-pressure area on November 8 over the Gulf of Thailand. Intensified to 130 km/hr by November 16. It also made landfall much later than predicted and remained a severe cyclone contradicting IMD forecasts. Cyclone Phailin / 2013 Originated over the Bay of Bengal on October 8. Intensified into a very severe cyclone in just 24 hours before hitting land near Gopalpur, Odisha on October 12. Cyclone Hudhud / 2014 Maintained intensity even six hours after making landfall near Vishakhapatnam with sustained wind speeds of 170-180 km/h on October 12. Cyclone Ockhi / 2017 Intensified from a deep depression to a cyclonic storm in less than six hours on November 30. It becomes one of the worst cyclones to hit India in 50 years. Hurricane Willa / 2018 Formed in the Pacific Ocean on October 20. In a single day, it gained a speed of 129 km/hr, hit western Mexico on October 24, and caused damages worth US $536.8 million. Hurricane Michael / 2018 Rose from Category 1 storm on October 8 to Category 5 when it finally hit Florida on October 10. It was one of the strongest hurricanes to make landfall in the US. Hurricane Florence / 2018 Jumped from Category 2 to Category 4 a few hours before it slammed into North Carolina, USA, on September 14. Hurricane Maria / 2017 Underwent an “explosive intensification” on September 17, a day before it hit Puerto Rico, a Carribbean island. There it gained 130 km/hr in just 24 hours. At its peak, the hurricane had a wind speed of some 280 km/hr.

(This article will be published in Down To Earth's print edition dated May 16-31, 2019)

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