By Peter Sousounis | July 27, 2020

Thanks to seasonal rains and glacial meltwaters entering the headwaters of the Yangtze River in the Tibetan plateau, China is no stranger to summer flooding, particularly in the south and central parts of the country. From April to August it is rainy season somewhere in China and significant precipitation can be experienced, but this year’s has been particularly heavy.

From southern China in early spring the rainy season typically moves through the Yangtze River basin in eastern China in May and June and continues into northern China and Korea July and August. In 2020, heavy precipitation began in eastern China in late May and early June and has been both intense and persistent since then, drenching some areas for days on end.

By June 4 flooding was already an issue in central China's Hunan Province, where landslides had occurred and homes had been damaged. The following day, flooding in Jiangxi Province impacting 10 cities was reported damaging 24,300 hectares of crops and collapsing 117 houses. Rainfall in the Yangtze River basin broke records set in 1961 according to the Vice Minister of Emergency Management.

Persistent and intense rainfall and flooding have continued to impact vast areas. Of the 32 provincial regions in China, 27 have been affected, with the worst-hit provinces being Jiangxi, Hubei, and Hunan in central China and Anhui, Zhejiang, and Jiangsu in the east. According to the Chinese Ministry of Water Resources, 443 rivers flooded, 33 of which broke historical records. More than 150,000 houses have been damaged and 27 million people have been affected, of whom 2.4 million were evacuated and at least 141 are reportedly dead or missing.

Basinwide Impacts

The Yangtze River is China’s largest river and the third longest in the world. It borders or crosses 10 provinces and drains more than 1.8 million km2 of land, called the Yangtze River basin. This basin is home to almost a third of China’s population; its agricultural economy produces enough rice to feed half the country, as well as many other crops, and its industry produces almost half of the country’s GDP.

For three quarters of its length the Yangtze passes through mountainous regions, but its middle and lower reaches cross extensive flood-prone lowlands on its way to the sea. Below the Three Gorges Dam the river enters a region of lakes and marshes in the provinces of Hunan and Hubei that have historically helped to absorb floodwaters.

The Three Gorges Dam was constructed to produce electricity, but it is also one of the many dams and reservoirs on China’s rivers used to regulate floodwaters. The water level behind the dam was lowered in May ahead of anticipated floods and rose in response to several rounds of flooding after June 2. From July 7 the dam eased pressure on the river’s main course downstream by reducing its outflow by almost half according to the Yangtze River water resources commission. The dam experienced a second peak flood on July 18, withholding 45% of the floodwater and opening its sluice gates to release the remainder; the reservoir’s water level nevertheless rose to a record high of 164.18 meters.

Despite such measures, water levels in the Yangtze River basin have widely exceeded the record levels reached in the devastating floods of 1998, and many lakes and rivers have reached or passed their warning marks. The Yangtze, at least 400 of its tributaries, and some lakes have burst their banks, resulting in unprecedented flooding. By July 14, for example, 2,531 km of embankments in the Jiangxi section of the Yangtze River in the Poyang Lake region had seen water exceeding warning levels and in Poyang County alone 14 levee breaches were noted. Flood defenses are being repaired or enhanced in the hope of preventing further breaches.

The state-run news agency Xinhua reported on July 15 that 583,900 hectares of crops in Jiangxi had been damaged, of which 123,600 hectares are destroyed. A landslide in Hubei Province blocked the Qingjiang River, a tributary of the Yangtze, on July 20 and formed a barrier lake. Wuhan, the capital of Hubei, located on the Yangtze above the Poyang Lake region and epicenter of the COVID-19 outbreak in China, has so far escaped serious flooding. The third flood of the year reached the Three Gorges Dam on July 26 and is expected to peak on July 28. But as relentless rainfall continues and more flooding is anticipated to the north in Shanxi, Henan, Shandong, Anhui and Jiangsu provinces the pressure has shifted to the Huai River Basin and there is growing concern for the major cities located toward the coast.

The Role of Climate and Climate Change

At the very beginning of this blog I noted that heavy rains across southeastern China are an annual occurrence in late spring to early summer. They are associated with the Mei Yu Front, which stretches from southeastern China east-northeastward to Japan during that time. It is also the rainy season in Japan and, to a significant degree, the heavy flooding that has been occurring in parts of Japan this year is related to that in China.

The front forms as warm unstable tropical air is transported into China on southwesterly winds on the back side of subtropical high pressure as part of the East Asia Monsoon. A weather front, by definition, is a boundary between air masses and what happens farther to the west and north matters. The higher elevation of the Tibetan Plateau generates a heat low that causes warmer but much drier air that meets the very moist air coming from the tropical western Pacific.

The front is actually a humidity front rather than a temperature front, but it does the same thing. Density differences from the two air masses on either side force rising motion and precipitation. The situation stays more or less that way, acting as a corridor for precipitation systems to move through the same latitude belt, until the Tibetan Low dissipates and allows the Mei Yu Front to do the same.

Changes in large-scale climate factors control the interannual variability of the front. Years when the tropical western Pacific are warming, as in the spring of 1998 when a strong El Niño was transitioning rapidly to a strong La Niña, can amplify the strength and precipitation of the front. As the climate warms, other effects come into play. It has long been known that for every degree Celsius the atmosphere warms, about 7% more water vapor will be in the atmosphere and available to fall as rain. A recent study found that precipitation associated with the Mei-Yu front has been increasing.

More may be at play than just the Clausius Clapeyron effect, though. Changes in the pole-to-equator temperature gradient may be changing how quickly large-scale weather patterns move. This could conceivably be prolonging the Mei-Yu season. And, although not presumably a factor this year, amplifying El Niño Southern Oscillation events like those witnessed over the last 50-some years—and which may continue because of climate change—may also cause periodically more intense Mei Yu seasons in the future.

Despite some plausible physical explanations for why Mei Yu precipitation will increase in the future, the latest available results from climate models show a wide range of results spanning outcomes in either direction. Lower-resolution models show a decrease, while higher-resolution models show an increase. Given that we are talking about a front that is a small-scale feature, in the cross-section at least, and the small-scale dynamics associated with it that I did not mention (e.g., low level jets), and given the recent historical increasing trend some studies have shown, a betting person may put their money on the high-resolution models and the smart money might go to improving flood defenses.

Read the AIR Current article “Climate Change: A Reckoning and a New Approach to Modeling Risk”

Categories: Flood , Climate Change

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