Warming oceans can raise probability of storms: Experts

Clouds and wind spin and grow, fed by ocean's heat, water vapour

Within a span of about a month, two tropical cyclones pummelled Japan and an island chain in the Caribbean, dealing widespread destruction to lives and property.

Last weekend, Typhoon Hagibis killed more than 70 people in Japan, overwhelmed river banks and damaged bullet trains.

It followed Hurricane Dorian which slammed into the Bahamas last month, leaving at least 50 dead and tens of thousands homeless.

While it may be difficult to link individual storms to climate change, Dr Xie Shang-Ping, a climate scientist at the Scripps Institution of Oceanography, said warming oceans can increase the probability of their occurrence.

"It's no coincidence that new records have recently been set in tropical cyclone intensity," said Dr Xie. "I think we are seeing the climate change effect. The warmer the ocean gets, the stronger tropical cyclones will become."

He cited Typhoon Haiyan, which devastated the Philippines in 2013, and Hurricane Patricia, which blitzed across Mexico and the US south-west in 2015. Typhoon Haiyan was the most intense tropical cyclone at landfall in the world, while Hurricane Patricia was the strongest tropical cyclone in the North-eastern Pacific, he said.

In comparison, Dorian was the most intense tropical cyclone at landfall in the North Atlantic.

 
 

Hagibis too had approached the Category 5 storm category - the highest level - but otherwise, it was not that extreme, Dr Xie said, adding that it was the heavy rains that caused huge damage in Japan.

Hurricanes and typhoons are different names for the same type of storm. The storms that rage across the western Pacific Ocean are typhoons, while those that blitz the Atlantic and eastern Pacific are hurricanes. They are all known as tropical cyclones.

These form when warm ocean water evaporates - forming an air parcel that is both warm and moist - away from the Equator, where Earth's rotational force can be felt.

Water vapour raises the energy of the atmosphere near the water surface, said Dr Xie. "The rising air cools, causing water vapour inside to condense. The latent heat released during the condensation warms the air."

Due to the Coriolis force, which is the force that can be felt from the Earth's rotation, clouds and wind spin and grow as they are fed by the ocean's heat and water vapour.

Said Dr Xie: "Tropical cyclones are often viewed as heat engines that can convert heat energy released from the condensation of warm, moist ocean air, into the kinetic energy of the storms."

 
 

With human activity pumping more emissions into the atmosphere and trapping heat on Earth, ocean heat content will also rise, said Singapore Management University's Associate Professor Winston Chow, a lead author of an upcoming report by the Intergovernmental Panel on Climate Change (IPCC). This could provide additional "fuel" for tropical cyclones and increase their intensity.

Last month, the IPCC said if global temperatures rise by 2 deg C, the average intensity of cyclones and the associated average rainfall rates are projected to also increase.

But weather scientist Koh Tieh Yong from the Singapore University of Social Sciences said the IPCC had only cited limited evidence linking rising global temperatures with an increased number of Category 4 or 5 tropical cyclones each year. More research is needed, he said.

A version of this article appeared in the print edition of The Straits Times on October 17, 2019, with the headline 'Warming oceans can raise probability of storms: Experts'. Print Edition | Subscribe