Why Repeated Heavy Rains Then Extreme Heat in Japan?

Osaka - Sunday, July 18, 2010

Since the end of July there have been torrential rains almost nationwide (excluding Hokkaido) causing severe landslides and floods. Even though it had been in a rainy season it was quite unusual to have such repeated, concentrated heavy rains for such a long period of time. Heavy rains stopped for this weekend marking the end of the rainy season for this year but there still remains a risk of such torrential rains again. What is more, it is quite possible that hot summer days will follow the weather recovery.

1. What is the reason for concentrated and repeated heavy rains?

The fact that atmosphere has been apt to be unstable is the reason for the concentrated and repeated heavy rains, attributing to strong warm and humid air flowing in from high pressure in the Pacific Ocean, and to cold air occasionally coming in stratosphere, when rainy season front has stayed hear Japan much longer than previous years.

The rainy season front has now moved up north east and should gradually further move north and disappear, but thunder clouds are being developed in regions strongly affected by strong damp warm air from the south even far away from the front. Damp warm air comes in following the rim of the Pacific high pressure. When there is cold air stratosphere, upper air current will become more violent.

Professor Yamagata of University of Tokyo says that another background for torrential rains is El Nino phenomenon that occurred from last summer to this spring. According to his explanation, this phenomenon makes the sea temperature of Eastern Pacific tropical waters higher than usual. Together with the flow of atmosphere made high pressure near Philippines strong. This made more humid air from topics come into Japan.

2. What is expected to come immediately after the end of the rainy season?

With stronger Pacific high pressure covering Japan we should have more sunny days and be free from heavy rains in most part of Japan for a while. It is forecasted that in Eastern Japan the highest temperature of the day will be extremely high.

However, it is possible that there will be localized heavy rains when the temperature hits high in inland and mountains and upper air current develops thunder clouds. It is forecasted that in northern Japan there will be many days that is not so sunny.

3. What is expected to come in the future?

What is called La Nina phenomenon, which is the reverse of El Nino phenomenon, is expected to occur in the future. With this phenomenon, Japan is to be covered with high pressure and therefore more likely to be extreme heat.

According to JMA (Japan Meteorological Agency), there is not yet such a sign of active convection current near Philippines; however, professor Yamagata assumes that in western Japan it will be extremely hot summer days from the end of July to August. In fact, on July 17, the temperature rose as high as 35.2℃ in Fukui and above 30.℃ in many parts of Japan including Tokyo and Osaka, and it was officially announced that the rainy season for this year has ended.