Floods in the south and droughts in the north! Why is there such a big difference in weather between the north and the south?

Review expert: Ye Haiying, deputy director of the New Media Center of the Meteorological Publicity and Science Popularization Center of the China Meteorological Administration (China Meteorological News Agency)

Recently, there has been a sharp contrast between the weather in the north and south of my country: the north has been experiencing continuous high temperatures and the scorching sun is scorching the earth; while the south has been experiencing continuous rain and frequent floods. This extreme phenomenon of "floods in the south and droughts in the north" has attracted widespread attention. What exactly causes such abnormal weather? Let's talk about it today.

Image source: Weibo screenshot

Meteorological reasons for floods in the south and droughts in the north this year

According to data from the China Meteorological Administration, as of June 14, the maximum daily temperature at 228 national meteorological stations in my country exceeded 40 degrees Celsius. The overall intensity of the high temperature process was the fifth strongest among all high temperature weather processes in the country in the past five years.

Image source: Beijing Daily

" Periodic atmospheric circulation anomalies are the direct cause of regional high temperature weather. " Zheng Zhihai, chief forecaster of the National Climate Center, explained that the high temperature in eastern China is mainly affected by the subtropical high pressure in the western Pacific and the warm high pressure in the westerly belt. In areas controlled by warm high pressure, downward airflow prevails, the sky is clear and cloudless, and it is not easy for clouds to form and rain. At the same time, solar radiation is strong and near-ground heating is intense. Under the condition that the high pressure system is abnormally strong and stable, it is very easy to form persistent high temperature weather.

At the same time, in some arid areas of China, dry soil will accelerate local high temperatures, and high temperatures will intensify the loss of soil moisture. The local land-atmosphere interaction process will further increase the temperature, forming a vicious cycle between high temperature and drought.

What caused the continuous heavy rainfall in the south? Hu Xiao, chief meteorological analyst at China Weather Network, said that the subtropical high pressure was relatively strong this year, continuously delivering abundant water vapor to South China, and the cold air from the north continued to penetrate southward, confronting the warm and humid air flow, thus triggering continuous heavy rainfall and severe convective weather.

Image source: Xinhua News Agency

At the same time, with the development of low-vortex shear, the continuous transport of abundant water vapor, combined with favorable dynamic lifting conditions, can easily stimulate convection and form a "train effect" in South China, causing local rainfall to last for a long time and the impact effects to continue to accumulate.

In addition, the El Niño phenomenon is also one of the important factors affecting global temperature, and may add fuel to the fire of my country's summer climate anomalies.

Will El Nino cause floods in the south and droughts in the north? What is the El Nino phenomenon? El Nino is an abnormal phenomenon that occurs in tropical oceans . Its notable feature is a significant increase in sea water temperature in the eastern and central equatorial Pacific Ocean.

Image source: Middle school geography textbook

Under normal circumstances, the sea water temperature in the western Pacific is relatively high, the atmospheric upward motion is strong, and precipitation is abundant; while in the equatorial central and eastern Pacific, the sea water temperature is relatively low, the atmospheric motion is downward, and precipitation is very little.

When the El Nino phenomenon occurs, a large amount of warm water from the equatorial western Pacific flows to the equatorial eastern Pacific, causing the sea temperature in the equatorial western Pacific to drop, the atmospheric upward movement to weaken, and the precipitation to decrease, resulting in severe drought there. In the equatorial central and eastern Pacific, due to the increase in sea temperature and the strengthening of upward movement, precipitation increases significantly, and heavy rains become disasters. Changes in the large-scale atmospheric circulation in the tropics will inevitably affect and change the atmospheric circulation in the north-south meridian, thus leading to global atmospheric circulation and climate anomalies.

Image source: Middle school geography textbook

The El Niño phenomenon transmits abnormal signals of the atmosphere and ocean in tropical regions to other tropical areas and mid- and high-latitude regions through the influence of atmospheric circulation, and its impact is global.

Generally speaking, the impact of El Nino on China's climate is indeed floods in the south and droughts in the north. El Nino changes the atmospheric circulation, causing the monsoon rain belt to shift southward, resulting in more precipitation in the south, which is prone to floods, and less precipitation in the north, which is prone to droughts; El Nino also strengthens the intensity of the subtropical high, making it easier for humid air in the south to enter southern and eastern China, further intensifying precipitation in the south.

El Nino ended in May

According to monitoring by the National Climate Center, in May 2024, the sea temperature in the equatorial central and eastern Pacific dropped significantly compared with the previous period, with the Niño 3.4 index at 0.28°C and the sliding average of the index from March to May at 0.78°C.

Monthly Niño 3.4 index and 3-month moving average index evolution (unit: °C) Note: The moving average of the sea temperature index is the average of the sea temperature index values ​​of this month, the previous month and the next month. Source: China Meteorological Administration

According to the national standard "Method for Distinguishing El Niño/La Niña Events" (GB/T 33666-2017), the eastern El Niño event that began in May 2023 lasted until April 2024 and was confirmed to have ended in May ; the intensity of this El Niño event was moderate and lasted for 12 months.

At present, subsurface cold water in the equatorial central and eastern Pacific is developing rapidly, the surface sea temperature is significantly lower, and convection near the dateline is suppressed. Both thermal and dynamic conditions are conducive to the continuous decrease of surface sea temperature in the equatorial central and eastern Pacific. According to the recent evolution characteristics of the ocean atmosphere in the equatorial Pacific and the predictions of many climate dynamics and statistical models at home and abroad, it is expected that the sea temperature in the equatorial central and eastern Pacific will remain neutral in the summer this year, and may enter the La Niña state in late summer and early autumn.

Extreme weather is more frequent in the year after El Nino Although the current El Nino event has ended, its delayed impact is still ongoing. "Generally speaking, extreme weather will occur more frequently in the year after El Nino is formed. Compared with 2023 , 2024 may be hotter, with more extreme weather events," said Zhou Bing, chief climate service expert at the National Climate Center.

In the year following the El Nino event, the rise in sea temperatures in the Indian and Atlantic Oceans is conducive to the strengthening of the subtropical high pressure in the northwest Pacific Ocean. The interaction between the three tropical oceans will continue to affect my country's summer climate anomalies. Therefore, the decline of the El Nino event does not mean that the temperature will drop. On the contrary, the residual power of El Nino and the trend of climate warming may lead to more frequent high temperature weather in the summer of 2024.

Image source: Qianku.com

As the East Asian summer monsoon moves northward, my country has fully entered the main flood season, and the delayed response of the atmosphere to El Niño may become more apparent or amplified. Specifically, summer precipitation in southern my country tends to be higher, while large-scale high temperature and drought weather often occurs in northern China.

In the summer of the year following a strong El Nino event, floods are prone to occur in the Yangtze River Basin and the Jiangnan region of my country. In the second half of the 20th century, three major flood disasters in the Yangtze River Basin (1954, 1983 and 1998) occurred in the decay period of the El Nino event. For example, in the summer of 1998, severe floods occurred in the entire basin of the Yangtze River, Nenjiang River and Songhua River in my country. 29 provinces (autonomous regions and municipalities) in my country suffered from floods of varying degrees. Hundreds of thousands of PLA and armed police officers participated in the flood control and disaster relief that year. According to statistics, the number of people affected by the "1998 flood" reached 223 million.

Image source: Qianku.com

In addition, compared with the El Nino event, the impact of global warming is more persistent. Experts point out that global warming is the climate background for the occurrence of extreme high temperatures around the world. Therefore, while paying attention to phenomena such as El Nino, we should pay more attention to the fundamental issue of global warming, take positive actions to mitigate its impact, and jointly protect the earth on which we depend for survival.