Rethinking Of Typhoon’s Randomness And The Impact On Economy
By Dr. Hiroshi Takagi 20 October, 2020
See why Dr Takagi from Tokyo Tech thinks typhoons have an unpredictable random nature & what we should do about it
My research field is on the risk of coastal hazards, with a focus on Asia. A strong research interest among many is to analyse the statistical variability of typhoons and the associated risks of storm surges and high waves.
Cyclone Nargis caused a huge storm surge that killed ~140,000 people in Myanmar; not a high risk country
I first became interested in this issue when I studied Cyclone Nargis, which made landfall in Myanmar in May 2008. This cyclone caused a huge storm surge that killed an estimated 140,000 people. It is well known that neighbouring Bangladesh is the most at risk of tropical cyclones in the world.
By contrast, prior to Nargis, Myanmar was not considered to be a high typhoon risk country. Although the typhoon itself had made landfall even before then, Nargis’ path turned out to be a very unusual route. It made landfall on the southern coast and hit its largest city, Yangon. None of the residents, including the elderly, had ever experienced a strong typhoon before.
The nature of typhoons is fundamentally of a rather unpredictable random nature
Since that study, I have come to believe that the nature of typhoons is fundamentally of a rather unpredictable random nature that does not fit clearly into any probability distribution. Many recent media reports have tried to link the frequent occurrence of strong typhoons to the problem of climate change at all costs. For example, the Japan Times quoted the comment of one of Ministries after Typhoon Faxai, which severely hit Tokyo Bay in 2019, said “Due to climate change, various changes, including wind speeds and rainfall that go beyond common expectations, are occurring”.
It is true that Typhoon Faxai was one of the strongest typhoons ever. The table below shows the rank of the high waves observed at Tokyo Port, demonstrating that the wave recorded during Faxai was the highest at least over the last five decades. However, it should be noted that there were other typhoons of the same level in the past. The other equivalent high waves over 3 m high were also caused by the past typhoon events, Irma in 1985 and Danas in 2001.
WS: winter storm
SS: spring storm
|TY Name||Wave height (m)|
Climate change under global warming may intensify future typhoons. However, I don’t believe that long-term climatic variability alone can adequately explain the fickle randomness of typhoons.
“I don’t believe that long-term climatic variability alone can adequately explain the fickle randomness of typhoons”
It is more important to consider factors such as the frequency of typhoons that hit a narrow bay with a concentration of economic assets on the worst possible course. Furthermore, from a disaster management perspective, I think it is more important to consider how much the risk of strong typhoons has potentially increased in areas, where have not experienced strong typhoons over the past few decades, due to population growth, economic development, urban development, etc.
We recently published a paper on this point. The target country is Vietnam, a typhoon-prone country with a long north-south coastline. Most of the major cities (e.g. Hanoi, Ho Chi Minh City, Da Nang, and Hoi An) are located close to the sea, so it would be possible to reveal a clear relationship between typhoons and damage because the typhoon’s energy doesn’t diminish much as it hits these cities.
As for human damage, the number of victims has been decreasing in recent decades due to improvements in Vietnam’s typhoon disaster prevention, but economic damage has been increasing over time. In 2017, the damage caused by the disaster reached a record high of USD1.5bn, mostly caused by typhoons.
We analysed this factor using statistical methods such as principal component analysis, which is a mathematical method for revealing characteristic statistical trends among many factors. The results show that there has been no significant trend in meteorological factors such as the frequency and strength (wind speed and central pressure) of typhoon landfalls for at least the past 40 years. On the other hand, there was a strong correlation between the overall strength of typhoons and economic damage (below figure).
Although the frequency of typhoons did not increase nationally in the decade 2008-2017 compared with the previous decades, the frequency of landfall was significantly higher in the northern region of Vietnam between 20 and 22 degrees north latitude, with Hanoi and Haiphong (below figure). In other words, the recent increase in economic damage may be mainly due to the fact that strong typhoons frequently happened to hit place by chance where the economy is concentrated.
By contrast, areas that have not experienced major typhoons in the past few decades may have experienced steady economic development during this calm period.
The risk of typhoons in ‘calm’ areas (usually with economic value) need to be re-evaluated
However, the potential risk of typhoons might be very high in such areas due to the growing economic value, the low level of preparedness and people’s lack of awareness of the disaster risks. The risk of typhoons in such areas needs to be re-evaluated.
Waves of 3 m are not uncommon in the open ocean, but they are quite rare in the inner bay, such as Tokyo Bay, occurring only once every 10 to 20 years. Most of these unusual high waves are caused by typhoons, as shown in the above table.
Conversely, unless the typhoon hits, the waves are usually very calm in the inner bay. If the period of calmness is sustained, the port developments and industrial activities are promoted, resulting in the regional population growth. However, at the same time, people’s awareness of disaster risk is reduced, and the next direct hit typhoon may cause unexpected damage to the coast with low defense design levels (below photo).
Therefore, it is necessary to consider that the period when the climate is mild is the period when the risk of future disasters increases, and disaster prevention measures should be taken.
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