30th Anniversary of Typhoon Mireille

Thirty years ago, in September of 1991, Typhoon Mireille whipped across Japan with winds in excess of 160 km/h (Figure 1). According to the General Insurance Association of Japan (GIAJ), Mireille caused damage in 41 of Japan’s 47 prefectures, destroying more than 170,000 houses, and resulted in total insured losses of JPY 573 billion (in 1991 currency).¹ At the time, this was the largest insured loss claim ever paid for a typhoon-related loss in Japan. AIR estimates that if Mireille were to recur today, with today's exposures, it would result in insured losses of JPY 1,046.49 billion (USD 9.81 billion), with the majority of these losses—JPY 967.77 billion (USD 8.92 billion)—caused by wind.

Mireille’s unusually enhanced winds contributed to insured losses so significant they would stand as a GIAJ record for nearly three decades, before being surpassed only recently by Typhoon Jebi in 2018 (JPY 1,067.8 billion) and Typhoon Hagibis in 2019 (JPY 582.6 billion). In the next sections, we consider how a recurrence of Mireille could have far-reaching impacts on Japan's modern exposures and demonstrate how the magnitude of a Mireille, modified by future climate change, might top the charts in another 30 years.

A Usual Track with Unusual Winds

Mireille's left-side track was typical of other typhoons that have caused major damage in Japan, such as Bart in 1999 and Songda in 2004 (Figure 2). When approached from the west side, the island nation is subjected to a typhoon's strongest winds, typically found on the right-front quadrant of a storm. Mireille, however, also developed strong winds behind it and to the left side—where strong winds don't usually form—which contributed to higher losses.

Five days prior to landfall, on September 23, Mireille had just strengthened, with maximum sustained winds of 212 km/h (132 mph), when it interacted with Typhoon Nat in a rare Fujiwara effect, causing Mireille’s forward speed to slow briefly and its direction to shift. Mireille passed 175 km west of Okinawa while producing winds of 90 km/h (56 mph) or greater for 27 hours across that island. At Kadena AB, wind gusts were recorded as high as 147 km/h (91 mph).

Mireille then accelerated northeast and made landfall in Nagasaki Prefecture on September 27 with a central pressure of 940 mb, and an estimated intensity of 176 km/h (109 mph). Mireille produced record wind gusts at 26 locations, with a peak gust of 220 km/h (136 mph). Mireille underwent an extratropical transition as it traversed northeast through the Sea of Japan before making another landfall in Hokkaido.

Influences on Extratropical Transition

Mireille's winds were strengthened asymmetrically as it underwent extratropical transition. Due to unseasonably cold air to the northwest of the storm, a strong upper-level westerly jet streak developed, which drove Mireille's surface winds toward Japan. An area of cold high pressure then moved in from the northwest, and the increasing temperature gradient where it met warm moist air to the southwest further amplified Mireille's west-side winds. Finally, an upper-level cold front developed on the western edge of the typhoon, creating a pressure dip that further enhanced the winds in the storm's left rear quadrant.

The resultant strong westerly winds behind the storm’s center were equally able to wreak havoc on onshore exposures. In fact, nearly two-thirds of the wind-observing stations monitoring Mireille reported the strongest winds coming from the west onto Japan, rather than from the east in the usual right-front quadrant. Extratropical transition also broadened the destructive reach of Mireille's enhanced wind field as the storm moved northeast.

The economic impacts of extratropical transition in Japan can be dramatic, as buildings and other exposures in the middle and high latitudes and well inland may be less prepared for storm-force winds over large areas.

Record-Breaking Damage and Loss: Japan's Costliest Typhoon at the Time

The majority of the damage and destruction was caused by the wide-ranging and enhanced winds, impacting 41 of Japan's 47 prefectures. In addition to the more than 170,000 buildings that were damaged or destroyed, there was extensive damage to the apple crop and trees. The storm's fast forward motion meant precipitation did not have much time to accumulate, but even so, 200 mm (nearly 8 inches) of rain fell across portions of Kyushu and Shikoku islands and as much as 350 mm (nearly 14 inches) of rain fell in Tokushima Prefecture. Storm surges as high as 3 meters inundated the western coasts of the Inland Sea of Japan. Flooding from coastal surge and precipitation inundated nearly 23,000 houses, including parts of Hiroshima City.² Nearly 6 million people lost power, with electric company officials at the time calling it the nation’s worst typhoon-related blackout on record.³ More than 50 people were killed, and 700 more were injured. Air, road, rail, and ferry transportation was hampered for days. More than 900 ships were damaged.⁴ JPY 18.5 billion in claims were paid in marine lines and JPY 26.9 billon in automotive, according to GIAJ.

What if Mireille Were to Recur Today?

If Typhoon Mireille were to recur today, we would still expect the majority of the damage and destruction—over 90%—to be caused by winds. Precipitation-induced flooding and storm surge would incur around 8% of the total insured property loss, when taking into account modern flood mitigation measures. Based on our estimation, Mireille would cause in total JPY 1,046.49 billion (USD 9.82 billion) in insured property loss across residential, commercial/industrial, auto, and mutual lines of business, placing it on par with Jebi's losses in 2018. Insured commercial/industrial losses alone would reach JPY 432.88 billion (USD 4.06 billion). Areas hardest hit by wind damage would include ports and industrial parks in Kyushu, such as port Kitakyushu and automotive manufacturing facilities, which have expanded substantially since the early 1990s.⁵

Capturing Japan's Exposures Today

Japan's modern building codes are stringent and reflect the country's exposure to wind and flood from typhoons and other storms, as well as to earthquake hazards. Modern wood-frame houses, or zairai, typically exhibit good lateral resistance to wind loads, making them less subject to collapse, but even minor wind damage to roof coverings and windows can allow wind-driven rain to enter a building's interior, where the water can damage contents. Larger multi-family apartment buildings and major commercial and industrial structures in Japan are generally engineered and made of reinforced concrete or steel—wind damage to these structures is usually confined to nonstructural components such as mechanical equipment or roofing. Such structures may also be more resilient to flooding due to the inclusion of flood mitigation features. A significant portion of Japan's industrial stock today is non-engineered light metal construction, which is particularly vulnerable to wind damage from a storm like Mireille.