Doubling tropical cyclone risk to Hawai‘i possible

University of Hawaiʻi at Mānoa
Malte Stuecker, 808-956-9158
Assistant Professor, Oceanography & IPRC, School of Ocean and Earth Science and Technology
Marcie Grabowski, 808-956-3151
Outreach specialist, School of Ocean and Earth Science and Technology
Posted: Dec 22, 2020

Four tropical cyclones in the Pacific Ocean, Sept. 2015. Credit: NASA/NOAA GOES Project.
Four tropical cyclones in the Pacific Ocean, Sept. 2015. Credit: NASA/NOAA GOES Project.

Global warming will intensify landfalling tropical cyclones of a category three or higher in the Indian and Pacific Oceans, while suppressing the formation of weaker events. In Hawaiʻi, model simulations show a doubling of the risk of landfalling tropical cyclones, if CO2 concentrations double. That’s according to a study published in Science Advances and co-authored by Malte Stuecker, assistant professor of oceanography in the University of Hawaiʻi at Mānoa School of Ocean and Earth Science and Technology (SOEST).

Tropical cyclones, including typhoons and hurricanes, are the most fatal and costliest weather disasters on our planet. But how tropical cyclones will change in response to global warming has long remained a mystery.

To address this question, scientists for more than two decades have used the world’s largest supercomputers to run climate model simulations that show important aspects of these destructive storms. However, until recently the computing power has been insufficient to capture both atmospheric details and resolve the full interaction with the ocean on a global scale.

A team of scientists, led by researchers at South Korea’s Institute for Basic Science (IBS) Center for Climate Physics (ICCP) at Pusan National University, recently completed one of the most computing-intensive and detailed global warming simulations to date using the supercomputer Aleph at IBS. The global climate model records small-scale atmospheric and oceanic processes with unprecedented resolution. The model also more accurately simulates ocean temperature than the previous generation of climate models.

“This improvement was important for a realistic simulation of typhoons in the Indian and Pacific Ocean,” said Jung-Eun Chu, lead author of the study and a project leader at the ICCP.

Hawaiʻi ocean temperatures fuel tropical cyclones

While tropical cyclone numbers are simulated to decrease in the deep tropics, the situation is slightly different for Hawaiʻi, which is located at the edge of the tropics. Hawaiʻi is typically protected from tropical cyclones due strong northeasterly trade winds that flow in the opposite direction than the westerly winds aloft in the upper atmosphere. This so-called “vertical wind shear” normally tears tropical cyclones apart that approach the islands. In response to climate change, this wind shear is simulated to weaken and ocean temperatures are projected to warm near Hawaiʻi—fueling tropical cyclones.

“Our new model simulations show an approximate doubling of the risk of landfalling tropical cyclones in Hawaiʻi if CO2 concentrations are doubled” said Stuecker. “Importantly, the impacts of landfalling tropical cyclones will be much more severe due to higher rainfall and increasing sea level. Flood risk and storm surge will be much intensified in coastal areas.”

Fewer, more intense tropical cyclones in future

"A future reduction of rising motion in the tropical atmosphere will make it more difficult for tropical cyclones to develop, which explains the projected future suppression in tropical cyclone seeds and overall numbers in the Pacific and Indian Ocean,” explained Sun-Seon Lee from the ICCP, who conducted the simulations on Aleph. “Interestingly, the simulated pattern of future tropical cyclone changes is quite similar to the recent observed trends, which supports the notion that global warming is already altering global extreme weather.”

However, the way in which global warming will alter tropical cyclones is complex. Even though the total number of tropical cyclones is expected to decrease in future, developing events will have a higher chance to intensify beyond category three due to the higher humidity and energy levels in the atmosphere.

“This result confirms previous studies which used less-detailed global climate models,” said Axel Timmermann, co-author of the study and director of the ICCP. “By representing coastal processes more accurately than ever before in a global model, we now have a much higher confidence in these robust model projections, in particular for landfalling tropical cyclones.”