What's behind Japan's record-breaking rainfall?

Japan has witnessed record-breaking weather in the last ten days, with destructive and sometimes deadly consequences. Some places have been lashed with more than three times the average rainfall for the entire month. NHK World meteorologist Mori Sayaka explains why the rain has been so extreme this year.

The island of Kyushu, located in the southwest of Japan and sandwiched between warm ocean fronts, is the wettest place in the archipelago. And its mountainous terrain makes it the most disaster-prone area. Around 60% of the landslides that occur in the country are in Kyushu.

Heavy rain is common there, but the downpours in early July broke records. They also cost the lives of at least 66 people.

The Japan Meteorological Agency issued an emergency warning – the highest possible level of alert – for much of the island. It was the first time the warning had been issued for Kumamoto and Kagoshima prefectures since the alert system was introduced in 2013.

How much rain has fallen?

From July 3rd to 9th, the city of Kanoya in Kagoshima Prefecture received more than a meter of rain. That's three times the average for all of July. Even more astonishing is that most of it fell within just half a day. Kanoya and the city of Minamata in Kumamoto Prefecture saw 415 millimeters in just 12 hours, while Ashikita, also in Kumamoto Prefecture, recorded 326 millimeters in six hours. All of these far exceeded previous records.

This extreme rain caused many rivers, including the Kuma River, one of the three most rapid in Japan, to overflow. The rain quickly ran downstream, breached levees and caused deadly flooding.

The bars in the graphic above indicate the amount of rainfall.

What made the rain so heavy this year?

In June and July, humid maritime air and dry continental air collide, creating a seasonal rain front in and around Japan. The front usually moves northward as the season progresses. However, a Pacific high-pressure system normally responsible for pushing the front along has not done so this month. That left it stalled in the same area for more than a week. Officials at the Japan Meteorological Agency say they cannot recall the last time this occurred.

In addition, a plume of unusually strong water vapor called a "wet tongue" surged into Kyushu and strengthened the stalled rain front.

And much narrower bands of rain called "trainings" developed over Kyushu. A training is a series of thunderclouds that forms over the same area for an extended period of time. The thunderclouds are often short-lived, lasting only about 30 minutes, but inside the training the clouds are continuously forming, and that means extended periods of rain in one location.

More rain on the way

The main islands of Japan, including Kyushu, are still in the rainy season. The wet period typically lasts through mid-July in Kyushu, and until late-July in central Japan. We often see dangerous amounts of rain near the end of the rainy season, so caution will still be needed for the next few weeks.

Torrential rain leaves dozens dead in Japan (0:58)

Global warming brings more rain

Over the past 100 years, global temperatures have risen by 0.73℃, while the temperature in Japan has gone up by 1.13℃. The rapid rise has increased the amount of moisture in the air, resulting in an increase in rainfall. Studies suggest the frequency of heavy rain at a rate of at least 50mm per hour has increased by 40% over the last decade. Some scientists even believe the length of the rainy season could increase.

On top of all that, Japan is physically vulnerable to natural disasters. Some 70% of the land is mountainous and around 1,000 landslides occur every year. Around 10% of the land is flood-prone and half of the population lives in these areas. With rising temperatures, an increase in rain is unavoidable. Each of us should learn about the possible hazards where we live and prepare a clear disaster plan.