Read in Japanese

Threat of megastormsBomb cyclones increase;
more typhoons bear down on Tokyo

The number of "bomb cyclones," also known as winter storms, in the North Pacific Ocean has risen 20% compared with 30 years ago. These storms are characterized by high winds, heavy rain and snowfalls that can cause houses to collapse under their weight and strand cars on roads.

Typhoons in summer and autumn are also coming closer to the Japanese archipelago, approaching Tokyo 60% more frequently than 20 years ago. The threat of "megastorms" with disaster-level impacts looms over cities and is expected to continue increasing due to rising sea surface temperatures caused by global warming.


Bomb cyclones cause damage over a wide area

Photo of large vehicle stuck on Route 4

A large vehicle is stranded in Iwate prefecture in northeastern Japan, in December 2020 (Kyodo)

In December 2020, over 2,000 passenger cars and trucks got stuck on the Kanetsu Expressway, which connects Tokyo and Niigata prefecture, and on Route 4, which runs from Tokyo through the northern Tohoku region. Deliveries ground to a halt due to the storm.

Bomb cyclones which developed rapidly in the North Pacific, were a factor in the winter storms that caused the disaster.

The low-pressure system, which had a central pressure of 996.9 hectopascals at 9 p.m. on Dec. 12, dropped about 28 hectopascals to 969.2 hectopascals in 24 hours, at 9 p.m. on Dec. 13. The low stayed off the Kuril Islands, north of Hokkaido, for several days and strengthening an atmospheric pressure pattern peculiar to winter in Japan. That is, a high-pressure area located to the west and a low- pressure area to the east.

The 48- and 72-hour snowfall totals recorded at an observation station near the Kanetsu Expressway were 199 cm and 219 cm respectively, the heaviest ever recorded in Japan.

Winter storms cause about as much economic damage as Typhoons
Insurance payouts for wind and flood damage
  • Damage caused by bomb cyclone

In billions of yen. Compiled from data from the General Insurance Association of Japan, as of the end of March 2022.

Damage caused by bomb cyclones is similar to that caused by typhoons and heavy rain in summer and fall.

The bomb cyclone that crossed the Japanese archipelago in February 2014 had widespread effects. According to the Cabinet Office, up to 114 centimeters of snow fell in Kofu, Yamanashi prefecture, west of Tokyo. 679 houses were heavily damaged or completely destroyed, nationwide. According to the General Insurance Association of Japan, insurance claims paid for snow damage caused by this cyclone amounted to 322.4 billion yen ($2.3 billion at current exchange rates). This is close to the 387.4 billion yen paid out in the wake of Typhoon Songda in 2004 and exceeds the 195.6 billion yen paid out in response to torrential rains in western Japan in 2018. This snow damage in 2014 is the sixth-highest amount ever paid for wind and flood disasters.

Bomb cyclones becoming more frequent
Analysis of data over the past 60 years

Nikkei, together with Kyoto University and the University of Tokyo, examined the number of times an low-pressure systems that developed rapidly on the Pacific side of eastern Japan. Generally, a low-pressure system that drops 24 hectopascals or more in 24 hours is called a bomb cyclone. From data on sea level pressure published by the Japan Meteorological Agency every six hours, we examined the number of times a low-pressure system met the criteria for a bomb cyclone. For the analysis, we divided the 60-year period from 1961 to 2020 into 30-year halves, looking only at data from December to the following February each year.

Number of times rapidly developing low-pressure system rose 20% compared with 30 years ago
Chart showing increase of bomb cyclones

Based on an analysis by Akira Kuwano-Yoshida, an associate professor with Kyoto University, and Satoru Okajima, a research associate with the University of Tokyo, of the "JRA-55" the Japanese 55-year Reanalysis, a data set on sea level pressure released by the Japan Meteorological Agency. The period for each year is from December to February of the following year.

In the 30 years between 1961 and 1990, we detected low-pressure systems that met the criteria for a bomb cyclone 988 times, or an average of 32.9 times annually. In the 30 years between 1991-2020, we detected such systems 1,199 times, or 40 times a year, an increase of 21.4% compared with the first 30-year period.

The analysis was conducted in the North Pacific Ocean between 140-180 degrees east longitude and 35-50 degrees north latitude, where low-pressure systems are likely to affect Japan. A well-developed low-pressure system over the sea east of Japan intensifies the pressure pattern peculiar to winter.

The University of Tokyo’s Okajima warned, "The frequency of extreme phenomena, such as strong winds, high waves and heavy snowfalls along the mountains may increase in Japan."

What's the difference between a bomb cyclone and a typhoon?

A "megastorm" is collective term for very powerful storms that can cause catastrophic damage, such as hurricane. It is not a weather term, but megastorms also include bomb cyclones and typhoons.

Bomb cyclones have a different development mechanism than the typhoons that strike the Japanese archipelago in summer and autumn.

Typhoon = tropical cyclone

A typhoon is a tropical cyclone whose average speed near the center is over 17 meters per second. Cumulonimbus clouds, which develop over the warm seas far to the south of Japan, create a vortex, causing a tropical cyclone to develop. Because tropical cyclones are made up entirely of relatively warm air, updrafts tend to occur in vortexes, causing the cyclones to strengthen.

Bomb cyclone = extratropical cyclone

On the other hand, a bomb cyclone develops from a extratropical cyclone, which is created when cold and warm air collide around 30-40 degrees north latitude. Cold air from the north slips under warm air from the south, creating cumulonimbus clouds. The updrafts in an extratropical cyclone is not as strong as those in tropical cyclones, but when a trough near 5 kilometers in altitude comes into contact with a low-pressure system near the ground, the updraft strengthens and rapidly develops into a bomb cyclone.

Atmospheric pressure drops significantly, bringing huge damage

Typhoons have their strongest effects near the center of the storm. They typically approach Japan with a speed of 20-30 kph, as they develop in the Pacific Ocean, so it is rather easy for the Japan Meteorological Agency and other institutions when they will hit. After a typhoon passes, blue skies often prevail.

On the other hand, bomb cyclones develop rapidly and the atmospheric pressure drops significantly, potentially bringing huge damage. Nikkei, based on the Japan Meteorological Agency data, created an animation of a pressure pattern when a bomb cyclone hit northern Japan in February 2021. Let's take a look at how the central pressure dropped.

The central pressure of the low-pressure system over the Sea of Japan at 9 a.m. on Feb. 15 was 1000.7 hectopascals.

The low-pressure system moved to the Sea of Okhotsk, off the Shiretoko Peninsula, at 9 a.m. on Feb. 16, and the central pressure of the system was 952.9 hectopascals. The pressure dropped by 47.8 hectopascals in 24 hours.

The low-pressure system moved off the coast of Cape Soya in northern Hokkaido at 9 a.m. on Feb. 17. A violent storm blew through Hokkaido by this day. The maximum wind speeds hit records for February at 40% of around 170 wind observation stations in Hokkaido. In Kushiro, in eastern Hokkaido, and five other stations, windspeeds broke all-time records.

Bomb cyclones tend to make a long-lasting impact. When bomb cyclones are over the sea east and northeast of Japan, pressure patterns peculiar to winter strengthen, causing heavy snow to likely fall in parts of the country facing the Sea of Japan. The bomb cyclone in February 2021 stayed off the Kuril Islands, resulting in a blizzard in northern Japan.

Typhoon Nanmadol in September hits Kyushu with the fourth-lowest central pressure

The typhoon forced the Japanese archipelago into high alert again in 2022. When Nanmadol -- known as Typhoon No. 14 in Japan -- landed in the southern city of Kagoshima on Kyushu island on Sept. 18, the central pressure was 935 hectopascals. This was the fourth-lowest reading since JMA started keeping records in 1951. If the central pressure is low when a typhoon lands, strong winds are likely and the weather tends to be harsh. The U.S. Joint Typhoon Warning Center classified Nanmadol as a super typhoon before striking Japan.

Damage caused by Nanmadol

The town of Kunitomi in Miyazaki prefecture in southern Kyushu was flooded by heavy rain from Typhoon Nanmadol on Sept. 19. (Kyodo)

The town of Yakushima in Kagoshima prefecture in southern Kyushu recorded maximum wind speeds of 50.9 meters per second -- the fastest on record. It also rained heavily in neighboring Miyazaki. Five people died and 154 were injured nationwide as a result.

Typhoon Talas, or No. 15 in Japan, approached the Tokai region of Honshu island. Around 76,000 households in Shizuoka prefecture had water supply disrupted due to heavy rain and landslides caused by the storm.

The frequency of typhoons near Tokyo has increased 60%
Very strong tropical cyclones are expected in the future

Using JMA data, we counted typhoons within the southern half of a 300-kilometer radius of the Tokyo Metropolitan and Osaka city halls.

More typhoons are approaching cities. Using JMA data of typhoon paths, we counted the typhoons that approached Tokyo and Osaka in 1980-1999 and in 2000-2019. Typhoons approached Tokyo 30 times in the earlier period and 48 times in the latter -- an increase of 60% over the years. Similarly, typhoons approached Osaka 37 times and 52 times, respectively -- an increase of 41%. According to JMA, the number of typhoons that developed during each period did not change much, but their paths have shifted to the north.

According to the Sixth Assessment Report (AR6) by the United Nations Intergovernmental Panel on Climate Change (IPCC) published in 2021, very strong tropical cyclones are expected to develop more frequently, and peak wind speed of the strongest tropical cyclones will increase across the world, both due to global warming.

Hurricane causes "1-in-1,000-year" rain fall in the U.S.

Cars and debris from washed away homes line a canal in Fort Myers Beach, Florida on Oct. 5, one week after the passage of Hurricane Ian. (AP)

The U.S. has been suffering from megastorms almost every year. Hurricane Ian hit North America in late September to early October and caused more than 100 fatalities. Placida, Florida, north of where Ian made landfall, received more than 381 mm of rain over 12 hours on Sept. 28. CNN reported that the storm produced rainfall that exceeded a "1-in-1,000-year flood event," citing data from the U.S. National Oceanic and Atmospheric Administration.

According to NOAA, the cost of damages from weather and climate disasters in the U.S. between 1980 and 2021 exceeded $2 trillion.

Sea surface temperatures keep rising, contributing to bomb cyclones

Akira Kuwano-Yoshida, associate professor at Kyoto University, said: "One of the factors behind the recent increase in bomb cyclones is that sea surface temperatures near the East China Sea have risen and the area has become more humid." An analysis based on JMA data shows that sea surface temperatures in the southern part of the East China Sea in winter (December-February) were 0.7 C higher in the 1991-2020 period than 1961-90.

Higher sea surface temperatures increase the amount of water that evaporates into the air. Water vapor releases heat when it transforms into water droplets or clouds, strengthening updrafts and contributing to bomb cyclones from low-pressure sytems.

Sea surface temperatures in the East China Sea and the number of bomb cyclones

Based on JMA data. The period for each year is from December to February of the following year.

IPCC notes in the AR6: "It is unequivocal that human influence has warmed the atmosphere, ocean and land." IPCC adds that "Many changes in the climate system become larger in direct relation to increasing global warming." This means that the frequency of marine heatwaves, indicated by periods of unusually high ocean temperatures, is expected to increase.

Sea surface temperature is also related to typhoons. As it stands, the threat of megastorms is unavoidable. This makes it imperative for the JMA to refine existing technologies and develop new observation networks so that it can issue weather warnings earlier. The recipients of warnings and other information regarding weather disasters also required to take them seriously and to take appropriate evacuation actions.

Research and analysis methods:

We examined and visualized data provided by Associate Professor Akira Kuwano-Yoshida of Kyoto University and Research Associate Satoru Okajima of the University of Tokyo, who used the Japanese 55-year Reanalysis, or JRA-55 as their source. Kuwano-Yoshida and Okajima co-authored a research paper using JRA-55 with support from Japan's Ministry of Education, Culture, Sports, Science and Technology (MEXT).

Bomb cyclones are generally defined as low-pressure systems at 60 degrees longitude when its central pressure decreases by at least 24 hectopascals in 24 hours. But we modified the central pressure to include the latitude of each extra-tropical cyclone, as Kuwano-Yoshida and Okajima did in their research paper. We also counted significant low-pressure systems that were 28.8 hectopascals or more lower than the preceding 24 hours, taking into account the pressure near the center. Because the data presented here is in six-hour intervals, a single low-pressure system may be counted multiple times in the period between its onset and extinction.

We examined the paths of typhoons in JMA’s "Best Track Data," and organized it into six-hour intervals to count the number of typhoons that approached Tokyo and Osaka.