China News Service, Beijing, September 26 (Reporter Chen Su) Since the summer of this year, the Yangtze River Basin, which should have been in the rainy season, has encountered extreme droughts that have not been seen for many years. According to statistics from the Meteorological Department, many places in the middle and lower reaches of the Yangtze River have been experiencing droughts for more than 70 days.
Why is the Yangtze River Basin dry and rainy this year?
Some studies have pointed out that under the background of global warming, the East Asian summer monsoon may move northward. Has the Chinese rain belt also moved northward?
According to statistics, since July, the cumulative rainfall in the Yangtze River Basin of China has been nearly 50% lower than the same period of the previous year, which is the lowest in the historical period since complete data were available in 1961.
Among them, the middle and lower reaches of the Yangtze River are 40% to 80% lower than the same period of the previous year.
As of September 25, the drought in Jiangxi has lasted for more than 70 days, and the precipitation has decreased by 70% within three months.
At 18:00 on the 25th, the Central Meteorological Observatory continued to issue a yellow warning of meteorological drought. So far, the drought warning has been issued for nearly 40 consecutive days.
Affected by high temperature and little rainfall, many rivers and lakes in the Yangtze River Basin have been severely dried up.
On September 6, Poyang Lake fell below the extremely dry water level of 8 meters, 85 days earlier than the earliest year in history (November 30, 2019).
According to media reports, the water area of Poyang Lake has shrunk by 80%, and a "sandstorm" is blowing on the lake bed.
Data map: Aerial photography of the ancient bridge "Qianyan Bridge" in the waters of Poyang Lake, Duchang County, Jiujiang City, Jiangxi Province.
Photo by Liu Zhankun
June to August is usually the main flood season in China, and each geographical area should enter the rainy season. This year, the precipitation in the Yangtze River Basin is abnormally low. What is the reason?
Chen Lijuan, chief forecaster of the National Climate Center, told a reporter from China News Agency that the direct cause of the continuous low precipitation in the Yangtze River Basin is the continuous abnormality of the atmospheric circulation in East Asia, especially the western Pacific subtropical high is obviously strong and extends westward, controlling the Sichuan Basin to Jianghan for a long time. , Jiangnan, Jianghuai and other regions.
In addition, the south pressure high pressure over the Qinghai-Tibet Plateau is abnormally strong and eastward. Under its control, sinking airflow prevails in the above-mentioned areas, causing the ground to warm up, and the sky is not easy to form clouds, making it easier for solar radiation to reach the ground, and frequent high temperatures. And stronger.
In addition to the subtropical high, the anomalous anticyclone circulation in the lower troposphere in the northern part of the Bay of Bengal has continued to maintain since midsummer, which is not conducive to the eastward transport of water vapor from the Bay of Bengal into China.
"The persistent anomaly of the Western Pacific subtropical high is also affected by ocean conditions. Since the spring of this year, the La Niña event has developed and strengthened again, and the SST in the tropical central and eastern Pacific has been low, inhibiting local convection; The interaction modulates the local meridional circulation and latitudinal circulation, and the combined effect leads to the development of the anticyclone in the Northwest Pacific, which is conducive to the strengthening of the western Pacific subtropical high and the large area, which makes the high temperature, heat wave and drought in southern China continue." Chen Lijuan said.
According to the forecast of the meteorological department, the rainfall in the Yangtze River Basin will continue to be less in mid-to-late September and October, the inflow of rivers will continue to be less, and the water levels of rivers and lakes will continue to decline. severe.
Chen Lijuan said that it is expected that in the late autumn (October-November), the precipitation in the upper reaches of the Yangtze River will be close to the same period of normal years to slightly more, but the precipitation in the middle and lower reaches of the Yangtze River will be 20 to 50% lower than the same period of normal years.
The drought in the middle and lower reaches of the Yangtze River will further develop, and there is a high possibility of continuous drought in summer and autumn.
In view of the drought in the middle and lower reaches of the Yangtze River, it is necessary to pay close attention to the impact of the continuous drought in summer and autumn on agricultural production, forest fire risk, and power supply.
At present, the Ministry of Water Resources has carried out two consecutive special actions of "Joint Dispatching of Reservoir Groups in the Yangtze River Basin for Drought Relief and Water Supply" to ensure the safety of drinking water for the people, focusing on ensuring the water demand for irrigation of autumn crops in the middle and lower reaches of the Yangtze River and the two lakes.
While the Yangtze River Basin is dry, northern China has experienced significantly more rainfall in recent years.
In 2021, the north will have the second-most precipitation on record, after 1964.
In July this year, there were 6 regional heavy rain events in China, 4 of which occurred in the north.
Xinjiang, which is arid and drier all the year round, has frequent regional extreme rainfall. According to the statistics of the Ministry of Water Resources of China, from late May to late September this year, the runoff of the upper Tarim River was 1.5 times higher than that of the same period of the previous year, which is the first time in the same period since the measured data in 1957. 1 person.
In stark contrast to the drought in the Yangtze River Basin, the "Yellow River is roaring" has continued to be staged. In the past month, the Yellow River Basin has received relatively sufficient rainfall. Among them, Gansu, Shanxi, Shaanxi and other regions have experienced 100 mm to 200 mm of rainfall.
The south is dry and the north is rainy. Has China's rain belt really moved north?
Chen Lijuan said that from the perspective of interannual variation, in the past three years, the rainfall in the North China rainy season in northern China has been more than the climatic value.
The direct cause of the high precipitation in the north is the midsummer stage, when the subtropical high in the western Pacific moves to the north, which is conducive to transporting water vapor from the northwestern Pacific to the north. The La Niña event continues during this summer. When the La Niña event occurs, the tropical Pacific sea surface temperature The distribution is warm in the west and cold in the east. The western Pacific warm pool is significantly warmer than normal, and the convection is active. Through the interaction of air and sea, it affects the local Hadley and Walker circulations, making the Pacific subtropical high stronger and northerly.
In addition, in the context of global warming, the "expansion" of the subtropical high and the northward shift of the northern boundary will also affect the intensity of the East Asian monsoon and the spatial distribution of precipitation in China.
"At the same time as the increase in precipitation in the north, the precipitation characteristics of the Yangtze River Basin show complexity. In June-July 2020, the Yangtze River Basin will have super-strong plum rains, and in August 2021, the Yangtze River Basin will have super-strong second-degree plum rains, and this summer, the Yangtze River Basin will appear Severe summer drought. It can be seen that under the background of global warming, the East Asian summer monsoon system shows complex characteristics, and the impact on China's rain belt is also complex. The future climate evolution will be affected by natural variability and human activities. Its changing trend still needs in-depth research." Chen Lijuan said.