El Nino opens the southern rainstorm model

Recently, heavy rain has hit many provinces and cities in southern China. I believe what you hear most in the report is "affected by El Ni? o", but do you know how El Ni? o affects the weather? How should we deal with the rainstorm to ensure our own safety to the maximum extent? In this issue, follow Xiaobian to learn science to answer your questions!

With the south wind blowing continuously from the South China Sea to the mainland, it marks the beginning of the rainy season in China. From the end of March this year, South China entered the first flood season, nearly 16 days earlier than before. According to the news of China Meteorological Bureau, there have been 17 times of heavy precipitation in the south since the flood season. The monitoring shows that the precipitation in southern China is obviously higher than that in previous years. The precipitation in the whole southern region is generally higher by 25%-100%, and even reaches 100%-200% in some areas.

Taking Guangzhou as an example, the cumulative precipitation in Guangzhou from March 6, 2016 to June 1 has been nearly 300 mm more than in previous years, and nearly 50% more than in normal years. As can be seen from the figure, there was a sudden increase in precipitation at the end of March (between March 16 and March 21), because South China, where Guangzhou is located, entered the pre-flood season, and the precipitation in Guangzhou was obviously higher after the start of the pre-flood season.

The so-called pre-flood season in South China refers to the rainy period in South China, which starts from the end of March and early April and ends around June. It usually marks the beginning of the summer rainy season in the whole China monsoon region. During this period, cold and warm air meet in opposition, and besides rainstorm, there are often strong convective weather phenomena such as thunderstorm, gale and hail.

What needs to be known is that the most important weather system affecting China is the western Pacific subtropical high, which is referred to as "western Pacific subtropical high" for short or directly called "subtropical high". The west side of the subtropical high will guide the water vapor northward, while the northern boundary of the subtropical high is often connected with the cold air southward, so the position of the subtropical high often determines the position of the rain belt. The movement time and advance and retreat speed of subtropical high determine the movement time and advance and retreat speed of rain belt.

During the first flood season in South China, the northern boundary of the subtropical high is located in the coastal area of South China. The subtropical high and the southwest jet in the westerly trough of the south branch jointly transport warm and humid air over South China. Before the cold air goes south, the warm and humid air accumulates in South China, which makes the air humidity gradually increase and the air gradually reach saturation, forming dew on the ground and walls. This is the "returning to the south sky" in South China.

The intersection of cold and warm air masses is called "front", and when the "front" lingers and moves less, it is called "quasi-static". When the cold air in the north continues to go south, it conflicts fiercely with the warm and humid airflow, forming a quasi-static front, which is called "the quasi-static front in South China".

At this time, because the subtropical high can’t go northward immediately because of the blocking of cold air, it stagnates in South China, and there will be a continuous precipitation weather process in South China, which is the first flood season in South China.

In the first flood season in South China, due to the frequent cold air activities and the water vapor can’t completely go north, the precipitation in North China is often less, and high temperature and dry weather often occur. This is the spring drought in North China. During this period, the weather situation of flooding in the south and drought in the north often occurs in China.

In late June, the southern part of China is usually in summer, and the subtropical high gradually becomes stronger, and the subtropical high will gradually move northward and stagnate in the middle and lower reaches of the Yangtze River. The quasi-static front on the ground will gradually move northward to the Jianghuai area, forming the so-called quasi-static front in Jianghuai. During this period, there will be a continuous precipitation weather process in the middle and lower reaches of the Yangtze River, which is the prestigious Jianghuai Meiyu, and the quasi-static front of Jianghuai is also called "Meiyu Front".

In fact, in addition to China, the continuous precipitation weather processes in the same period in South Korea and Japan are also called Meiyu, and they are called Meiyu together with Jianghuai Meiyu. Internationally, Meiyu sometimes directly refers to the summer and rainy season precipitation in East Asia, and that quasi-static front is directly called Meiyu front.

However, in some years, when it is in the pre-flood season in South China, the subtropical high will sometimes suddenly move northward to the Yangtze River basin, and the rain belt will also move northward to the Yangtze River basin, and the precipitation in South China will have a stagnant period. However, there will be a weather process similar to plum rain in the Jianghuai basin, which is called "early plum rain". For example, in late May of this year (2016), the Yangtze River Basin experienced an early rainy weather process. At that time, the Yangtze River Basin did not enter summer, but there was a continuous rainy weather process similar to Meiyu, which was actually the reason why the subtropical high suddenly moved northward. The biggest difference between early Meiyu and Meiyu is that after the early Meiyu, the subtropical high will retreat to South China again, and the rainy area will return to South China again; After the Meiyu process, the subtropical high will continue to move northward to North China, forming a rainy season in North China.

After the Meiyu, the subtropical high moved north and North China entered the rainy season. At this time, South China will be re-controlled by the tropical system going northward with the subtropical high, and there will be a wave of precipitation concentration period, which is called the late flood season in South China. During this period, North China and South China entered the flood season, while the Yangtze River basin was located in the subtropical high, and entered the summer drought period, so the precipitation situation in the whole country easily became a "waterlogging-drought-waterlogging" model.

Since last year, the public has generally heard the news of El Ni? o, and El Ni? o has also made 2015 the hottest year since the temperature record was recorded in 1880. The appearance of El Nino will make the sea water temperature in the eastern Pacific rise. However, due to the weakening of the easterly wind, it is difficult for the warm sea water in the eastern Pacific to spread westward to the northwest Pacific, while the sea water in the northwest Pacific will be relatively cold, which will lead to the weakening of convection and the weakening of the western Pacific subtropical high, which usually occurs in the year after El Ni? o. According to this El Ni? o, the next year of El Ni? o is 2016. The weakening of the subtropical high will lead to the lack of motivation to go north, which means that the time of staying in the south will become longer, and the time of continuous precipitation in the southern basin may also become longer, which is prone to flood disasters.

For example, 1998 was also the year after El Nino. At that time, there was a big flood in the Yangtze River basin, and the people in the Yangtze River basin experienced a difficult period of fighting floods. The intensity of El Nino this time was the same as or slightly weaker than that in 1998. With the continuous improvement and development of national flood control facilities, the people have a strong flood protection. Although it may not be the same as in 1998, it is also necessary to learn from history. In order to prevent the historical disaster from repeating itself, we need to be vigilant.

When heavy precipitation weather occurs, stay away from low-lying areas. People in mountainous areas need to guard against geological disasters such as landslides, do not wade reluctantly, stay away from rivers, embankments and other areas, and go to safe areas in time when danger is found. Similarly, it is also necessary to pay close attention to the official information released by meteorological departments and flood control departments, take a responsible attitude towards the safety of their own lives and property, cooperate with the disaster prevention measures of government departments, and follow the instructions to evacuate in time when they are in dangerous areas.

Note: In this paper, the precipitation anomaly map in the eastern region is drawn by the National Atmospheric and Oceanic Administration (NOAA), and the precipitation in Guangzhou region is drawn by the National Climate Prediction Center (CPC). Meteorological data required for mapping are provided by the observation stations participating in the global exchange of the World Meteorological Organization (WMO).

The article was produced by China, a popular science student, and was authorized to be published in the column "Learning Science with Xiaobian" of CCTV. Please indicate the source for reprinting.

Production: Mobei

Producer: computer network information center