Taiwan research team analyzes causes of off-season super typhoons

Taipei, Nov. 7 (CNA) The occurrence of super typhoons in winter is more likely when the solar cycle is amplified by atmospheric and ocean interactions, according to a research team with National Taiwan Normal University (NTNU) in Taipei.

At a presentation Tuesday, Wu Chau-ron (吳朝榮), a professor with the NTNU Department of Earth Science, said his team found in a recent study that solar activity could induce typhoons or even super typhoons in the off-season, despite the lower sea surface temperature.

The study, which was jointly conducted by Wu's team and a team led by Yu Jin-yi (余進義), professor of Earth System Science at the University of California, Irvine, collected tracking data for 402 off-season super typhoons in the western North Pacific from 1945 to 2018.

Wu said that based on their findings, the occurrence of super typhoons outside the normal typhoon season can result in devastating loss of life and property damage.

An example of this type of super typhoon is Typhoon Haiyan in 2013, Wu added.

The study looked into the three enhancing conditions that cause off-season typhoons, the first being the solar cycle, according to Wu.

Every 11 years, the sun's magnetic poles change their polarity, and the Earth gains more momentum, heating up the tropopause, which causes the Hadley Cell and northeast trade winds to weaken, Wu said.

Secondly, he went on, atmospheric condition such as global warming strengthen the connection between subtropical and tropical zones, which increases the relationship between the solar cycle and the formation of typhoons.

Thirdly, the ocean-atmosphere interaction heats up the Northeastern Pacific and that warmth reaches the central Pacific, which reduces the Walker circulation and results in deep ocean circulations moving east.

Such interaction also weakens the vertical wind shear in a way that facilitates the formation of off-season typhoons, according to Wu.

As typhoons are formed further eastwards in the off-season and there is a lack of steer flow they move more slowly, which means more energy and water are absorbed from the ocean, increasing the probability a super typhoon will form, Wu said.

Wu added that as the Earth faces an active solar period over the next two years, the likelihood of super typhoons occurring is greater and people should remain vigilant.

The study was published in the "npj Climate and Atmospheric Science" journal in late October.