Unsolved Problems of Typhoon Forecasting

According to the Central Weather Bureau (CWB), over the last 50 years an average of 26.7 tropical cyclones have formed per year in the northwest Pacific. But this year there was a noticeable decline by about 30%. Nonetheless, there was no corresponding drop in the number to hit Taiwan, with five having already come ashore as of the early October, above the annual average of between three and four. Of these, Typhoon Krosa, which smashed into northern Taiwan, took nine lives and caused NT$750 million (about US$25 million) in damge to the agriculture, fisheries, and animal husbandry sectors.

The effect of La Nina

How is it that the total number of typhoons could fall, yet the number to hit Taiwan not drop? Yu Jia-yuh, head of Chinese Culture University's Atmospheric Sciences Department, avers that this is because this year there was a La Nina effect in the central Pacific: ocean water temperatures were low, which is not conducive to typhoon formation, so that this year most typhoons took shape in warmer waters east of the Philippines. Given the locations where they formed, the proportion to hit Taiwan consequently increased.

CWB Weather Forecast Center director Daniel Wu has some reservations about this explanation. He points out that the prerequisite for a La Nina effect is that sea temperature in the eastern Pacific remain at least half a degree Celsius below average for three successive months. "But this condition only began in August, when most of the typhoon season was already over, so that there has been no clear link between the number of storms and the La Nina effect. The fact that this year typhoons formed in locations closer to Taiwan may have more to do with increased tropical disturbances in the band of low pressure stretching from the South China Sea to the ocean east of the Philippines, which is a condition conducive to the development of tropical cyclones."

Tough calls to make

Besides the fact that there are different views about the reasons for the location of typhoon formation, in fact the meteorological community has many different perspectives on making long-term predictions about the number and strength of tropical cyclones.

For instance, with respect to the number of storms, some scholars maintain that the lower number of typhoons this year is connected to the La Nina effect. But according to CWB statistics, over the last half century there has been a cyclical pattern of ups and downs in the number of typhoons formed in the northwest Pacific, with each swing lasting about ten years. Since 1997 the number has been on the low side, but it was on the high side the previous ten years, whereas the El Nino and La Nina effects occur only once every two or three years. Explaining the number of tropical cyclones in the Pacific based on this factor alone is inadequate.

Another school of experts opines that given the impetus to gradually rising sea temperatures from global warming, there will inevitably be a long-term increase in the number and strength of typhoons in the Pacific and hurricanes in the Atlantic. In 2005, the Atlantic broke all records by giving birth to 28 tropical cyclones (the average is 15 per year). Moreover, the super-hurricane Katrina that hit in late August of that year brought devastating wind and rains, especially to New Orleans and the rest of the state of Louisiana, taking at least 1800 lives. These events seemed to support the theory. But in 2006 there were only nine hurricanes in the Atlantic, and as of late September of this year only another nine had formed. These numbers run counter to the theory that there is a positive correlation between global warming and the number of tropical cyclones.

Why are there such differences of opinion? National Taiwan University professor of atmospheric sciences Hsu Huang-hsung, who studies long-term changes in global climate, reminds us that humans have only been accurately measuring the climate scientifically and recording its changes for about 60 years. This is too short a time and there remain many unconfirmed assumptions. At the moment, weather forecasting can only be done with any reasonable accuracy for a period of two weeks in advance; error thereafter increases as the time period is extended.

"Short-term weather forecasting is like a doctor who is able to diagnose the flu based on long experience, but long-term forecasting is more like the disease that only comes along once in a blue moon: it is hard to make the right diagnosis," says Hsu.

Global warming effect

In terms of long-term predictions, the biggest area of agreement among climatologists today is that average global temperatures are definitely showing a tendency to increase.

Since 1970, global warming has caused sea temperatures to increase by an average of 0.6°C. The UN's Intergovernmental Panel on Climate Change reported earlier this year that the rate of warming over the past 25 years is 2.8 times that of the average of the past 100 years, indicating that global warming is becoming more intense.

Compared to global warming, moreover, the speed of warming in Taiwan has been even faster. In 1901, Taiwan's annual mean temperature was 21.95°C, but in 2006 it was 24.07°C. The CWB's records show that from 1901 to 2006 Taiwan's annual mean temperature rose by 1.4°C, about double the global rise. The increase in winter temperatures has been even more marked.

In terms of rainfall, there has been a decline in precipitation in northeastern Taiwan, but in southwestern Taiwan the trend has been toward more rain. At the same time, there has been an appreciable islandwide drop in both the number of days with rain and the total hours of rain. In other words, given the fact that total precipitation has not fallen much, there has been a decline in the number of days of light rain, with an increase in the number of days of heavy rain, dry weather, and no rain.

Though this may be the case, so far there does not appear to be any linkage between changes in annual rainfall in Taiwan and the trend of rising annual mean temperatures. Indeed, whereas extreme weather conditions-like heat waves, droughts, violent storms, or blizzards-have hit a number of places around the world in recent years, it is difficult to link these directly to global warming.

Stepping back for a moment, field research like that done by the "wind-chasers" and "rain-chasers" may help us to get a handle on typhoons and monsoons that can bring disastrous weather, so that better preventive measures can be taken. But with respect to the many remaining mysteries about long-term climate change, scientific data is still inadequate. Perhaps the only thing we can do for certain is to become a little more humble in the face of nature, and even as we struggle to resist natural calamities, we can also minimize as much as possible the disasters that we bring on ourselves.