A persistent stationary front known as the Meiyu front is affecting East Asian countries since the rainy season began in May. The front is responsible for record-breaking rainfalls, mainly in Japan and China, and causing catastrophic floods and landslides that have been affecting millions of people.
The East Asian rainy season is caused by precipitation along an incessant stationary front called Meiyu, for nearly two months during the late spring and early summer between China and Japan, as well as Taiwan and Korea.
Also called the "plum rain," a Chinese term for the downpour in the fourth and fifth lunar month, it particularly refers to the historical belief that when plums turn yellow and fall at the south of the Yangtze in the fourth and fifth months, the moisture that evaporated from it becomes rain.
The rainy season usually lasts from May to June in Taiwan and Okinawa, Japan; June to July in Japan and Korea, and July to August in eastern China, especially the regions of Yangtze and Huai River.
According to BBC weather forecaster Chris Fawkes, the Meiyu front affecting East Asian countries, especially China and Japan, is gradually pushing northwards, bringing episodes of torrential rain that often leads to severe flooding and landslides. A line of clouds with thunderstorms running across central China eventually reaches parts of Japan and South Korea.
Recently, South Korea has also been hit by heavy rainfall as more than a month's worth of rain fell in the southern regions in a 24-hour period to July 13, leaving several people dead and causing damaging floods.
"It brings colossal falls of rain," Fawkes said. On July 4, Japan's Kyushu Island was lashed by severe flooding from record rains, including Kagoshima's highest rainfall ever of 109.5 mm (4.3 inches) in an hour on July 6.
In just three consecutive days, some parts of the region recorded more than 1 000 mm (39.4 inches) of rain.
"That has brought some catastrophic flooding, and according to authorities, the rain was unprecedented in intensity," Fawkes added.
The death toll from the devastating weather has reached 68 as of July 14. The rains left a major disaster on the island that tens of thousands of troops have been deployed to assist with relief operations.
The situation in China, on the other hand, was considered one of the worst rainfall seasons on record as almost 38 million people have already been affected by widespread flooding and landslides.
On July 13, up to 433 rivers have risen to dangerous levels, 33 of which have crossed historical highs. The damage to properties has overall amounted to more than 61 billion yuan (8.7 billion dollars).
The Meiyu front is powered by moisture through the depth of the whole atmosphere. Tropical air is moving in from the Pacific, also moving in from the Bay of Bengal across Southeast Asia.
"The moisture is one part of the Meiyu front. The other part is the winds higher up in the Earth's atmosphere," Fawkes explained.
As the winds flow across the Tibetan Plateau, a trough pushes the air to rise, resulting in the development of big thunderstorms. Sometimes it leads to the development of a massive storm system known as a mesoscale convective system.
A low-level jet stream occasionally forms. This is linked with the most severe rainstorms occur across East Asia that lead to catastrophic flooding. Once these areas of rain have been pushed away eastwards, the atmosphere returns as it was.
"It's the persistent nature of the front that leads to problems with severe flooding and landslides during the Meiyu front season."
Featured image credit: NASA/NOAA Suomi NPP/VIIRS, Acquired July 4, 2020.