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Fast-moving tropical cyclones are becoming more powerful

Researchers at Nagoya University in Japan have investigated how global warming affects the intensity of typhoons. The study revealed that compact, fast-moving storms are becoming more powerful as temperatures rise.

Tropical cyclones pose a significant threat to East Asia with their capacity for widespread disruption, damage, and loss of life.

“Intense tropical cyclones (TCs) often cause extreme destruction. Therefore, to prevent future disasters, it is essential to understand how warmer environmental conditions will affect intense TCs,” wrote the researchers.

Tropical cyclone intensity 

Tropical cyclone intensity is closely linked to the sea surface temperatures, which are on the rise due to global warming. The increase in sea surface temperature (SST) has a varied impact on typhoons, depending on their size and speed. 

As the size of a cyclone increases, the sea surface temperature decreases, limiting its intensity. However, under global warming, the sea surface temperature is higher. As a result, typhoons may last longer. 

“The rise in sea temperatures is concerning because a typical compact, fast-moving storm, for example Typhoon Faxai in 2019, caused severe damage to eastern Japan,” noted study lead author Sachie Kanada. “Our findings show the intensity of such typhoons can strengthen under global warming conditions.”  

Advanced simulations

The researchers delved into the dynamics of the relationship between global warming and tropical cyclone intensity using the advanced CReSS-NHOES simulator.

The simulator combines the cloud simulation model CReSS from Nagoya University with the oceanographic model NHOES from the Japan Agency for Marine-Earth Science and Technology. This provides a sophisticated tool to assess the interaction between the atmosphere and the ocean, and its effect on typhoon intensity.

The research was focused on four typhoons – Trami (2018), Faxai (2019), Hagibis (2019), and Haishen (2020) – each varying in size and demonstrating the grave impact typhoons can have through extensive damage and loss of life. 

Critical new insights 

By evaluating scenarios across different sea surface temperature increases (from preindustrial levels to rises of 2°C and 4°C), the experts observed significant variations in how typhoons strengthen with each degree Celsius increase in sea surface temperature. 

“We found that the degree to which typhoons strengthened per 1°C rise in SST varies significantly from typhoon to typhoon,” said Kanada. 

The researchers were surprised by the change in hPa, a unit of pressure used in meteorology to measure atmospheric pressure and which represents the strength and intensity of a storm. “A typhoon, such as Trami, strengthens by only 3.1 hPa, while Faxai strengthens by as much as 16.2 hPa with a 1°C rise in SST.” 

Ocean-atmosphere coupling effect

The results underscore the importance of considering the ocean-atmosphere coupling effect, which acts to buffer changes in storm intensity due to global warming. 

The study revealed that large, slow-moving typhoons tend to cool the sea surface temperature around their centers, thereby limiting their intensity. Compact, fast-moving typhoons, such as Faxai, avoid this cooling effect, maintaining a constant heat source at their centers that can fuel their intensification under warmer conditions.

Typhoon intensity forecasts 

Building on these insights, the researchers developed a new model using a simple parameter called the non-dimensional storm speed (S0) to differentiate between typhoons likely to intensify under global warming and those more resilient to its effects. 

This model, thanks to its high-resolution and the coupled regional atmosphere-ocean approach, could significantly enhance the accuracy of typhoon intensity projections under climate change scenarios, as well as improve current forecasting methods.

“Currently, climate change projection research on typhoon intensity is conducted using models with coarse horizontal resolution or atmosphere-only models, which have difficulty reproducing the intensity and structure of strong typhoons,” explained Kanada. 

“This research using a high-resolution coupled regional atmosphere-ocean model can reproduce the intensity and structure of strong typhoons and the response of the ocean with high accuracy, so is expected to contribute not only to the quantitative projection of typhoon intensity under a warming climate, but also to the improvement of the accuracy of current typhoon intensity forecasts.”

The study is published in the journal Geophysical Research Letters

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