The Northwest Pacific (NWPAC) Basin’s vast expanse of warm water explains its propensity to spawn intense storms. Nowhere else in the world sees more typhoons and other tropical cyclones. On average, 29 tropical storms form each year—20 of which reach typhoon status, equivalent to a strong Category 1 on the Saffir-Simpson hurricane wind scale.
The location of South Korea, however, is somewhat sheltered compared to other coastal countries in Asia. The total number of storms reaching the country is far less than the number affecting China, Japan, or the Philippines, but at least one tropical cyclone makes landfall every year on average, and at least one additional tropical cyclone comes close enough to the coast to cause loss.
The first few months of 2020 were unusually quiet in the Northwest Pacific Basin. The first tropical cyclone did not develop until May 8, no tropical cyclones at all formed in July, and as of August 31 only nine had developed. But South Korea, already reeling from the longest period of monsoon rain it has experienced in many years and a soaking from the remnants of Typhoon Hagupit, has just been impacted by three typhoons in rapid succession.
After turning northwest and reaching peak intensity on August 26 near Jeju (South Korea’s largest island) in the Korea Straight, Typhoon Bavi closely bypassed the South Korean mainland. It made landfall in North Korea 50 km southwest of Pyongyang early the following morning, according to the Korea Meteorological Administration (KMA), and became an extratropical cyclone soon afterward.
North Korea reportedly experienced widespread flooding and structural damage, but little damage was noted in Japan and South Korea, mostly flooding, fallen trees, and minor damage to residential and commercial buildings. About 1,600 homes briefly lost power in South Korea, many of them on Jeju, where 399.5 mm (15.73 in) of rainfall was recorded near Mount Halla and one life was lost. On Jeju and in the mainland City of Busan (the second largest city in South Korea and the fifth-busiest port in the world) hundreds of domestic and international flights were canceled; rail services were also suspended.
The second tropical cyclone, Typhoon Maysak, made landfall just southwest of Busan at about 2 a.m. local time on September 3 (17:00 UTC on September 2). It was the equivalent of a Category 2 hurricane at landfall, with 1-minute maximum sustained wind speeds of 169 km/h (105 mph). In just a couple of days from September 1, some 1,000 mm (39 inches) of rainfall was reportedly recorded on Jeju. Maysak set a low-pressure record for South Korea at 950 hPa and is only the sixth storm of Category 2 equivalent or greater to have struck South Korea since 1951, when reliable records began. Five of those six storms made landfall close to Busan.
Storm surge and flooding occurred along the eastern coast and The Korea Herald reported at least 17 homes destroyed and damage to a further 850. More than 270,000 homes lost power, most of them in Busan or on Jeju. Again, hundreds of flights were canceled in the region and rail services were disrupted. More than 2,000 people were evacuated ahead of Maysak’s arrival, but the storm nevertheless claimed at least two lives in South Korea.
The third tropical cyclone, Typhoon Haishen, became the first super typhoon of the 2020 Pacific typhoon season when it underwent a rapid intensification on September 3 as it bypassed Kyushu, Japan’s third largest island. Kyushu experienced major hurricane-force winds of 195 km/h (121 mph), the equivalent of a Category 3 hurricane. But as a result of an eyewall replacement cycle, Haishen lost intensity and when it made landfall in South Korea, its 1-minute sustained wind speeds were 143 km/h (89 mph), the equivalent of a strong Category 1 hurricane.
Haishen made landfall near Ulsan, South Korea around 9 a.m. local time (00:00 UTC) on September 7, bringing strong winds and yet more heavy rains of up to 70 mm (2.7 inches) per hour in some locations. Power outages were reported, and more than 100 homes were destroyed or flooded. Hundreds of flights were canceled, and some bridges and sections of railroad were shut down. By Monday afternoon, local time, however, trains between Busan and nearby cities were running and flights had resumed.
A Climatological Perspective
As recently as August 13, 2020, NOAA’s Climate Prediction Center had forecast a ~60% chance of La Niña development during the northern hemisphere fall of 2020 and continuing through the winter of 2020-21 . This is based on recent observations of below average sea-surface temperatures in the equatorial Pacific and forecast model predictions split between neutral or La Niña development in the near-future.
What could this mean for tropical cyclones in the NWPAC Basin? As we head into a potential La Niña season, tropical cyclones may continue to favor a westerly track. As has already been seen in 2020, five of the ten tropical cyclones have impacted southern China, the Philippines, and Vietnam. Zhan et al. 2017 states that La Niña developing years allow for more landfalls in the countries surrounding the South China Sea . By contrast, El Niño developing years allow for tropical cyclones to recurve toward the north and make landfall over Japan and the Korean Peninsula. While our analysis of the JMA historical record has found similar patterns, due to its north-central location within the NWPAC Basin, it is not unlikely for South Korea or northeastern China to be the impacted by storms that track into the East China Sea. Such tracks are represented strongly during La Niña preceding seasons.
Because the equatorial Pacific was experiencing El Niño conditions as recent as early 2020, conditions within the basin are undergoing a transition and an explanation for this year’s tracks has yet to be understood.
Peak Season, Highest Exposure
One typhoon in this region at this time of year is not unusual and the Korean peninsula is often bypassed, experiencing peripheral effects from storms passing through the Sea of Japan. Unlike the North Atlantic, which experiences hurricanes mostly within a recognized season running from June 1 to November 30, the Northwest Pacific Basin can generate tropical cyclones at any time of year. Most, however, form between May and October, and the most active period for typhoons in South Korea is June through October, with a peak in August and September.
It is rare for the country to experience a storm stronger than a Category 2 and the last storm of Category 3 or greater to reach South Korea was Typhoon Sarah, which made landfall at that intensity a few miles west of Busan in 1959. Generally the country is not exposed to extremely severe winds, and as a result, the country faces a more serious threat from flooding than from wind. As we have seen in the last few days, the areas of the country with the highest exposure are in the south and southeast, such as Busan and Jeju.