Contribution Understanding the impact of typhoons in Korea
Robert Muir-Wood is the Chief Research Officer at Moody's RMS, which is Moody's subsidiary offering climate risk modelling. Courtesy of Moody's RMS
By Robert Muir-wood
The western Pacific ― to the north of the equator, is a breeding ground for the largest concentration of tropical cyclones on the planet. The Philippines is the country that is most exposed to these devastating storms and is hit by twenty cyclones on average each year.
After the Philippines, tropical cyclones then initially move northwest to aggravate Taiwan, coastal southeast China, and the Japanese Ryukyu chain of islands. Recurving towards the north and northeast, cyclones then bring their strong winds and intense rainfall to the main islands of Japan and the Korean peninsula.
Tropical cyclones are rotating storms, with a chimney of rising air around the storm's central eyewall. The lower-level winds spiral in towards the storm center, while at a 15-kilometer elevation, the winds spiral outwards. As defined from the radius of maximum surface winds, the circulation can occur over a wide range of sizes from five kilometers to 200 kilometers. The faster the winds, the lower the central pressure vortex measured at the heart of the storm.
Formed over evaporating warm ocean water, a tropical cyclone's energy is fuelled by the heat liberated from the condensation of this warm moist air as it rises. The threshold sea surface temperature for formation is 27 degrees Celsius. Where a storm moves over cooler water or overland, the cyclone's circulation will weaken.
Typhoons and hurricanes
In the Western North Pacific, tropical cyclones are known as typhoons, but in the North Atlantic, identical tropical cyclones are termed hurricanes, and rated using the five-tier Saffir Simpson “category” scale. The scale ranges from Category 1 with one-minute sustained windspeeds above 119 kilometers per hour, to the extremely dangerous Category 5, where sustained windspeeds are above 252 kilometers per hour.
For Japan and Korea, meteorologists use three tiers when issuing warnings of storms above the 118 kilometers per hour sustained wind threshold. A Typhoon is a storm with sustained winds from 119 and up to 156 kilometers per hour (equivalent to Saffir Simpson Cat 1).
A Very Strong Typhoon is up to 193 kilometers per hour (a mid-range Saffir Simpson Cat 3) with a Violent Typhoon tier for higher wind speeds.
Using models to establish typhoon risk for Korea
So, how much should homeowners and businesses in Korea fear typhoons? Which locations are most at risk? What is the strongest typhoon that could affect the country? How much could a typhoon's damage cost, on average every fifty or one hundred years? And what about climate change; how is that going to alter the risk from typhoons?
The detailed meteorologic record of Korean typhoons is only complete for twenty years, and the historical record is for fifty years. This is by no means long enough to answer all these questions, which are vital for the functioning of insurance.
Instead, we need to build a catastrophe model, with, at its core, a synthetic history of 50,000 years of typhoons, carefully simulated to capture the detailed climatology. With the catastrophe model for Korea that Moody's RMS has built, we can hope to answer all these questions and provide a scientific foundation to risk pricing.
The Kuroshio Current and typhoon formation
The latitude of South Korea extends from 44 to 48 degrees,equivalent to the latitude of the U.S. mid-Atlantic states of North Carolina and Virginia. These regions share features of their tropical cyclone climatology. The mid-Atlantic states border a huge offshore warm-water current, the Gulf Stream, with its origins in the tropics, while south of Korea runs the Pacific equivalent: the warm Kuroshio Current.
Where typhoons linger over a warm water Kuroshio eddy they can be sustained at higher intensity, allowing an intense typhoon to make it through to landfall in Korea or western Japan. Without the Kuroshio Current, the summertime sea surface temperatures around Korea would not be warm enough to sustain a tropical cyclone circulation.
Typhoon activity in South Korea
Since 2000, six typhoons have made landfall in South Korea at a Category 1 intensity or higher, and all six arrived between Aug. 31 and Sept. 12 ― although the season runs from the end of July through to September.
Two Very Strong Typhoons ― Maysak and Haishen ― made landfall in South Korea in 2020 just a few days apart while a few days earlier there was Typhoon Bavi: a rare Cat 1 typhoon landfall in North Korea. We see a similar “clustering” of tropical cyclones both in Japan and the southeastern U.S.
The strongest typhoon landfall in South Korea in recent decades was Cat 3 Violent Typhoon Maemi in September 2003, which brought 193 kilometers per hour sustained winds to the region around Busan. Maemi broke the record for the most intense storm at landfall in South Korea, previously held by Typhoon Sarah in 1959.
For most bypassing cyclones or tropical storms which affect South Korea, on average two or three per year, high winds are no longer the primary hazard. Instead, the storms can bring intense rainfall from the typhoon's tropical-origin air mass, leading to flooding.
The total volume of rainfall is a function of the size and former intensity of the typhoon. The slower a storm advances, the higher the local rainfall totals. Daily typhoon rainfall totals reach 300 millimeters almost every year somewhere across the Korean Peninsula, while the most extreme rainfalls can reach 500 millimeters in 24 hours. However, not all high rainfall events in South Korea are associated with typhoons.
Fatalities from typhoons have been reduced because of better warnings, evacuations, and flood risk management. The death toll from Sarah (1959) was 669, while 117 were killed during the stronger Maemi (2003). As the storms weaken further north, typhoon risk is highest in the coastal south. The damage from Maemi, which made landfall near Busan, was costed at 5.5 trillion won, or $4.8 billion.
Korean climatologists have found an
increase in the frequency of Korean landfalling typhoons
since the mid-1990s. Climate change is also raising sea surface temperatures, potentially bringing stronger typhoons to landfall, while warmer air is increasing rainfall intensity.
The losses from typhoons are expected to rise, which is why sound insurance and reinsurance will be needed, underlain by catastrophe models for the country.
The writer is the Chief Research Officer at Moody's RMS, which is Moody's subsidiary offering climate risk modelling.