What is causing Japan’s “abnormal” weather as the rainy season draws to a close?

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(Mainichi)

TOKYO — The Japan Meteorological Agency (JMA) has declared the rainy season apparently over in Kyushu, Shikoku, Chugoku and Kinki areas in southwestern and western Japan, as well as in the Hokuriku region. on the coast of the Sea of ​​Japan on June 28 — the first time it was due to happen in June since sightings began in 1951. The announcement came a day after the Kanto-Koshin region in the eastern Japan, the Tokai region in central Japan, and the southern Kyushu region in southwestern Japan saw the end of the season.

Due to the effects of global warming, Japan has seen more extreme heat days of 35 degrees Celsius or higher in recent years. Also this year, a hot summer is expected and there are concerns about pressure on electricity supply due to increased use of air conditioning as well as water shortages. The Mainichi Shimbun spoke to experts about the causes of the “abnormal” weather.

— Concerns about showers after rainy season ends as weather front moves south

The record early end to the rainy season comes as a powerful Pacific high pressure sweeps across the Japanese archipelago, pushing the rain front north. According to the JMA, westerly winds near Japan shifted northward, causing the Pacific High to expand northward.

Hisashi Nakamura, a professor of climate dynamics at the University of Tokyo, said among the westerly winds, the meanders of the subtropical jet stream that flows at around 40 to 45 degrees north latitude played an important role in weather changes. This phenomenon, which begins near Europe and repeatedly spreads over Eurasia, occurs in summer and frequently brings abnormal weather conditions to Japan, including extreme heat. Since it involves the atmospheres of Europe and Asia, it is sometimes referred to as the “Silk Road Teleconnection” model.

This year marks the second time that the rainy season ends in the Kanto-Koshin region in June, the last time in 2018. In that year, after the end of the rainy season in the Kanto-Koshin region, western Japan was hit by torrential rains. rain.

The weather agency says this year is different from 2018 in that the Pacific high pressure system has extended westward. Nakamura pointed out that the Silk Road teleconnection scheme also happened in 2018.

“Even after the end of the rainy season in the Kanto-Koshin region, due to the weakening and southward movement of the Pacific High, there are fears that the rain front will move south If that happens, we have to be on our guard for heavy rain,” he said.

Abnormally hot weather for June was seen across Japan before the end of the rainy season. These temperatures are believed to have been triggered by the La Nina phenomenon, in which the sea surface temperature in the eastern Pacific Ocean near Peru and other countries remains at least 0.5°C below the normal for years. When La Nina occurs, summers in Japan tend to be warmer.

The La Nina phenomenon has been going on since last fall and is helping to cause the subtropical jet stream to creep northward near Japan. Takafumi Umeda, JMA Severe Weather Information Center Manager, commented: “For next week, meandering westerly winds and longer term La Nina” could be factors. extreme heat.

Some also believe that surface temperatures in the Indian Ocean play a role in the extreme heat. Takeshi Doi, chief climate dynamics researcher at the Japan Agency for Marine and Earth Science and Technology, said that a negative phase of the Indian Ocean Dipole, in which the surface temperature of the Indian Ocean is above normal in the east and below normal in the west, occurred from this month.

This phenomenon occurred at the same time as La Nina in 2016, a year marked by extreme heat. Regarding the impact of this on Japan’s weather, Doi commented, “The La Nina phenomenon has a greater effect than the Indian Ocean Dipole phenomenon. However, the JMA believes that the negative phase of the Indian Ocean dipole may play a role in the Hiroaki Ueda, a professor at the University of Tsukuba versed in climate systems, commented: “The combination of La Nina phenomena and the Indian Ocean negative dipole could lead to a extreme heat and torrential rain, which is extremely dangerous.”

This year, the early end of the rainy season has been deemed “abnormal”, but with global warming, temperatures and rainfall are also changing and our “common sense” regarding the rainy season may no longer apply. Professor Nakamura commented: “Global warming has progressed compared to 30 or 40 years ago, and both atmospheric and sea surface temperatures are increasing. Even with similar pressure patterns, it has become easier for large amounts of rain to fall.”

Kazuhisa Tsuboki, professor of meteorology at Nagoya University, pointed out: “It is difficult to determine the relationship with climate change, but this time too, the rain front is in the north and the age of ‘not rainy season in Hokkaido’ is coming to an end.”

(Japanese original by Yurika Tarumi, Tomohiro Ikeda and Kouki Matsumoto, Department of Science and Environmental News)

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