Why frequent heavy rains in the middle and lower reaches of the Yangtze River this summer
Economic Daily·China Economic Net reporter Guo Jingyuan
At 0:00 on July 12, in the water level well of Xingzi Station, the iconic hydrological station of Poyang Lake, the lake water passed a red mark-"the flood level of 22.52M in 1998", which marked the water level of the largest freshwater lake in China since the hydrological record broke through. Historical extreme value-Since June this year, southern China has witnessed continuous heavy rainfall, flooding disasters in many places, which has aroused social concern. Why are there heavy rains this year? Will there be more extreme weather events in the future? This reporter interviewed relevant experts to answer questions for everyone.
From June 1st to July 16th, there were 7 heavy rainfalls in the middle and lower reaches of the Yangtze River in China. The average precipitation in the Yangtze River Basin was 418.1 mm, which was the first since 1961. The Yangtze River Basin and Poyang Lake and Dongting Lake There was a severe flood situation. Why do the south continue to have heavy rain this year? Will there be more extreme weather events in the future? Economic Daily reporter interviewed relevant meteorological experts on this.
The impact of the subtropical high is stronger than in previous years
According to the statistics of the National Climate Center, since the flood season, as of July 16, there have been 16 large-scale heavy rainfall processes in southern my country. Among them, the middle and lower reaches of the Yangtze River, northern Hunan, northwestern Jiangxi, eastern Hubei, southern Anhui, and central Zhejiang, as well as Zhejiang, Anhui, and Jiangxi, have local cumulative precipitation exceeding 1,000 mm-daily rainfall in many places has exceeded historical extremes. The cumulative precipitation in several provinces exceeds half of the annual precipitation.
"Comprehensively considering the scope, duration, and rainfall, it was found that from June 27 to July 12, the comprehensive intensity of the regional heavy rain weather process in southern China was the second strongest since 1961 and ranked first in 1998." Associate Researcher, National Climate Center Zhai Jianqing said that although the regional rainstorm process in the south this year has a long duration and a wide range of influence, its location is more northerly and its overall intensity is slightly lower. Overall, it is still not comparable to the 1998 heavy rainstorm event.
However, since June, the frequent heavy rainfall process in southern my country not only has many heavy rainfall processes, but also one round after another, the intermittent period is very short. Why has the recent southern heavy rain plunged into a "wheel war"? Ma Xueke, chief forecaster of the Central Meteorological Observatory, explained that this is mainly related to the atmospheric circulation situation since June. The Northwest Pacific Subtropical High (Subtropical High), which has a huge impact on my country, is a stable and less active warm deep weather system. The southwestern airflow on the northwest side of the periphery is just an important channel to transport water vapor to the rainstorm area. Specifically, since June, the subtropical high has been stronger than in the same period of previous years. The southwestern airflow from the periphery transports abundant water vapor from the Bay of Bengal or the southern sea area of China to the south of China; cold air activities in the north are also more frequent, causing cold and warm air in the southern region. The continuous intersection leads to frequent and continuous occurrence of heavy rainfall.
What is the situation of rain in the south in the future, and will the pressure on flood control be reduced? Since July, my country's main rain belt has maintained from the east of the southwest to the middle and lower reaches of the Yangtze River. From July 17th to 21st, the eastern section of the main rain belt lifted to the north, and the western section remained in the eastern part of the Sichuan Basin and Hubei. Ma Xuejian reminded: "This is still a great coincidence with the previous heavy rainfall zone, and it is necessary to pay attention to the effect of superimposed precipitation. Even if the eastern section of the rain belt rises north in late July, the flood control pressure in the Yangtze River Basin will not be reduced. Precipitation will still pose a threat to rivers, lakes and lakes where water levels are high."
"Violent Plum" Superposition Factors
In recent years, extreme weather and climate events have been increasing. Is the abnormal climate behind this continuous heavy rainfall? Wang Yongguang, chief forecaster of the National Climate Center, introduced that it was Mei Yu, a "monster", that triggered the heavy rainfall. Specifically, the early arrival of the plum and the strong Meiyu front are the reasons for the abnormally high rainfall during the plum flood season in the middle and lower reaches of the Yangtze River. This year, Meiyu in the Jiangnan region was 7 days earlier than in previous years. Meiyu's "main battlefield", the middle and lower reaches of the Yangtze River, entered Meimei on June 9. The time was similar to that of previous years. The onset of the South China Sea summer monsoon was earlier this year, and the location of the ridge line of the Western Pacific Subtropical High in the mid-June is north, which together led to abnormally high rainfall during the Mei flood season in my country.
In the good weather, the rainy season usually starts on June 8th, and on July 18th, the average rainy season is 343 mm. Sooner or later is closely related to the amount of rain. According to statistics, the three years with the most rain in China were 1954 (789 mm), 2016 (662 mm), and 1998 (596 mm); the three years with the strongest plum rain were 1954 (intensity index 2.7), In 1996 (1.7) and 1998 (1.3), the legendary "violent plum" appeared. Among them, in 1954, over 76% of the Yangtze River Basin experienced heavy rain, with a maximum daily rainfall of 262 millimeters, and a century-old rare flood that occurred throughout the basin. On August 18 of the same year, the highest water level at the Hankou Station on the Yangtze River reached 29.73 meters, 1.45 meters higher than the highest water level in 1931.
This year my country has also suffered from "violent plum", and the amount of plum rain has increased significantly compared with previous years. So why do rainstorms "favor" the middle and lower reaches of the Yangtze River? "Mainly due to the relative stability of the rainy weather system this year. The sub-high has been maintained steadily in the past period, the edge of the warm and humid airflow is strong, and the low vortex shear system in the middle and lower reaches of the Yangtze River is active, resulting in a long period of rainy weather in the middle and lower reaches of the Yangtze River." Chen Tao, chief forecaster of the Central Meteorological Observatory, said.
And this year, this "monster" also obtained "energy" from other places, which caused Mei Yufeng to be strong. A weak El Niño event occurred in the autumn of 2019, and the temperature of the North Indian Ocean was abnormally warm, resulting in a significantly stronger secondary high. At the same time, the meridional circulation at middle and high latitudes develops and the cold vortex is active. The cold air exploded on the way to the middle and lower reaches of the Yangtze River. As a result, the cold and warm air converged in the middle and lower reaches of the Yangtze River, which caused the Meiyu front to be stronger and the precipitation in the middle and lower reaches of the Yangtze River to be significantly higher.
Ability to respond to extreme disasters needs to be strengthened
The South has made every effort to prevent floods and disasters, and the North cannot afford to relax. Affected by the monsoon climate and topography, my country's rain belt moves from south to north every year. Usually from late July to early August, the rain belt will be lifted from the north of the Yangtze River Basin to north China and northeast, and the north will enter the main flood season.
Meteorological forecasts show that in late July, the rain belt in the east of China moved northward to the area between Huanghuai, North China, and Northeast China. Cumulative rainfall was 30 to 70% more than normal and more than doubled in some areas; the main rainfall period was 7 From 22nd to 23rd, and around July 26th. Zhang Fanghua, chief forecaster of the Central Meteorological Observatory, said: "In late July, rainfall in the northern region will increase significantly, entering the critical period of the northern flood season. It is necessary to pay attention to the risk of secondary disasters caused by heavy rainfall, especially local heavy rainfall."
Every time extreme weather and climate events such as heavy rains, high temperatures, hurricanes, etc. occur, it may be a warning signal sent by nature to mankind. In the future, will such extreme events happen more and more frequently? The Climate Change Assessment Report issued by the United Nations Intergovernmental Panel on Climate Change (IPCC) pointed out that there will be more high-temperature and heavy rain events in some parts of the world in the 21st century, and the degree of drought will increase, threatening food, water and energy security in various countries.
"Although it is currently difficult to attribute a single weather and climate event directly to global warming, under the background of global warming, some extreme weather and climate events are indeed increasing." Huang Lei, a researcher at the Climate Change Office of the National Climate Center In fact, under the background of global warming, since 1951, China's average temperature and extreme temperature have shown a significant upward trend, and some extreme weather and climate events show the characteristics of stronger intensity, more frequent occurrence, and longer duration. .
The prediction results of the climate model also indicate that if the anthropogenic greenhouse gas emissions are not controlled, the frequency, intensity and duration of some extreme events in the world will increase significantly. By the end of the 21st century, the probability of high temperature heat wave events in land areas will be the current 5 to 10 times, the frequency of extreme heavy precipitation events will also increase in most parts of the world.
Huang Lei said that it is extremely urgent to further enhance China’s ability to respond to extreme disasters. It is necessary to address extreme disasters as the core content of adapting to climate change, strengthen extreme disaster risk prevention measures, strengthen extreme weather and climate event monitoring and early warning and meteorological disaster risk management, and develop key areas , Assessment of the impact of climate change in key industries, strengthen ecological and environmental meteorological services, and improve the government-led, departmental linkage, and social participation disaster prevention and reduction mechanism.