La Niña may reappear. Will floods in my country increase in summer?

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The El Niño phenomenon has ended, and La Niña may reappear. Will floods in my country increase in summer?

According to the latest bulletin released by the World Meteorological Organization (WMO) on June 3, the El Niño event that boosted global temperature rise and extreme weather events from 2023 to 2024 has shown signs of ending, and La Niña may reappear later this year.

"El Niño is a basket, and all anomalies are put into it." This is the language often used by the industry to jokingly explain climate anomalies, such as the following news headlines: "Many places are so hot that people 'doubt life'. Is it all El Niño's fault?", "Many Latin American countries are experiencing high temperatures, droughts, and frequent fires. Are they all El Niño's fault?", "Brazil floods killed nearly 100 people. The World Meteorological Organization said it was related to El Niño", "El Niño may continue until next spring. Will this year be a super warm winter?", "El Niño is on the scene. How hot will this summer be?", "El Niño is forming in the tropical Pacific, which will lead to frequent extreme weather."

In contrast to El Nino, there is another climate phenomenon called La Nina . Every time the climate is abnormal, La Nina is also one of the "scapegoats", such as "La Nina event will form! Will China be colder this winter?", "Winter economy heats up! La Nina brings the coldest winter in 60 years?", "Affected by La Nina, China's cold winter this year, smog may be alleviated."

Many people cannot distinguish between La Nina and El Nino, and think that climate anomalies are either caused by La Nina or El Nino. In fact, these two phenomena are almost completely opposite . When El Nino occurs, the sea temperature in the tropical eastern Pacific is high, and when La Nina occurs, the sea temperature in the tropical eastern Pacific is low. From the name, El Nino means "little boy" in Spanish, and La Nina means "little girl", so the little boy has a fever and the little girl has a chill , which is in line with the personalities of the two.

Although the names of El Niño and La Niña both mean children, the spatial scales of the two are astonishingly large . The sea temperature anomalies they cause span the entire Pacific Ocean, with an east-west span of nearly 20,000 kilometers. If the Indian Ocean affected by it is included, the entire system is more than 20,000 kilometers. It is also the strongest interannual change signal in the global ocean-atmosphere system. When any event occurs, it will stir up global climate anomalies, which is particularly evident in tropical regions. It can be said that these are two "naughty kids" that cause climate fluctuations.

When La Nina occurs, the sea temperature in the tropical eastern Pacific is low, while the sea temperature in the tropical western Pacific is high. Generally, areas with low sea temperatures and their vicinity are prone to drought, so the southwestern United States and western South America are prone to drought and wildfire risks, while areas with high sea temperatures have more rainfall, so Southeast Asia and Australia may have a higher probability of floods. When El Nino occurs, the above distribution pattern is reversed.

Generally speaking, El Nino and La Nina appear alternately, forming a quasi-periodic phenomenon with a period of 2-7 years. Researchers refer to the two and the changes in the sea-air circulation that match them as the ENSO phenomenon . El Nino corresponds to the positive phase event of ENSO, and La Nina corresponds to the negative phase event of ENSO. El Nino and La Nina events generally develop from March to June and reach their strongest point from December to February at the end of the year. This is called phase locking, and then they begin to weaken in the spring. The whole process lasts for 9-12 months. Since ENSO events last for a long time and are slow-changing signals compared to flood events, the judgment of the progress of ENSO events can be used to predict climate anomalies in the next few months to several quarters .

Although ENSO has cycles in long-term statistics, its changes are more complex when it comes to each process, and it is a "quasi" periodic phenomenon. After an El Niño event ends, it is likely to turn into a La Niña event, but there have been individual cases where El Niño events have redeveloped. For example, the El Niño from 1957 to 1959 lasted for 24 months, and the El Niño from 2014 to 2016 lasted for 19 months. There are also many cases where La Niña lasts for more than a year, and it is more persistent. For example, the La Niña from 1973 to 1976 lasted for 36 months, the La Niña from 1998 to 2001 lasted for 32 months, and the La Niña low sea temperature from 2020 to 2023 lasted for 35 months.

El Nino and La Nina also have a huge impact on China's climate and are the "black hands" behind the abnormal climate in China. After the occurrence of El Nino and La Nina events, the impact on China mainly appears in the following year. The most serious floods in the Yangtze River Basin in China in the past 100 years occurred in 1931, 1954, 1998 and 2010, all of which occurred in the year after the El Nino event .

However, China is located in the mid-latitude region and is not only affected by the tropical ocean, but also by the mid- and high-latitude atmospheric circulation. At the same time, China faces the vast Pacific Ocean, backs the vast Eurasian continent, and is influenced by the world's "third pole" Qinghai-Tibet Plateau, forming a typical monsoon climate. Precipitation is concentrated in summer, and the rain belt has the characteristics of "three stagnations and two northward jumps" from south to north. This makes it easy for the influence of the tropical ocean to mix with the influence of other factors, making the formation and characteristics of climate anomalies more complicated . As far as ENSO events are concerned, although the most serious floods in the Yangtze River Basin are related to El Niño and there is a certain statistical relationship, the two are not one-to-one corresponding. This is the difficulty in seasonal prediction and is still a hot area of ​​research.

In addition, the intensity, structure and location of ENSO events vary greatly , and can be subdivided into ENSO events with different characteristics. The climate anomalies caused by them also vary in intensity, structure and location. The spatial scale of ENSO events is large, far beyond the territory of China and the main river basins in eastern China. For example, the Yangtze River Basin and the Huanghuai River Basin are only 300 to 400 kilometers apart. It is difficult and uncertain to predict disturbances of hundreds of kilometers caused by climate phenomena of tens of thousands of kilometers.

In addition, precipitation in China is mainly concentrated in the summer, which is prone to heavy rain . In extreme cases, the rainfall in one day can reach the rainfall in a normal year or even more, such as the "7.20" heavy rain in Zhengzhou in 2021 and the heavy rain in Beijing, Tianjin and Hebei in 2023. Such events are generally the result of the combined effects of multiple circulation factors such as water vapor transport, mid-latitude disturbances, high-level circulation anomalies, and typhoon activity. In many cases, each factor is indispensable, and its occurrence is somewhat unpredictable. To predict local heavy rains that only last for a few days based on the ENSO phenomenon that lasts for several months, there are also many problems that need to be solved, involving the interaction of multiple temporal and spatial scale phenomena, which is also a difficult problem in the research.

Against the backdrop of global warming, it is becoming increasingly difficult to predict China's flood season climate. As global temperatures rise, the water vapor content in the atmosphere increases, the energy of precipitation systems increases, and weather events become more extreme. At the same time, the atmospheric circulation is adjusted, the form and intensity of disturbances from mid-latitudes, and the paths of water vapor transport in low latitudes are all adjusting, which greatly increases the difficulty of prediction. Therefore, the El Niño "basket" may not be as useful as before. These are new challenges that require continued efforts from scientific research and business personnel.

This article is a work supported by the Science Popularization China Creation Cultivation Program

Author: Wei Ke, Institute of Atmospheric Physics, Chinese Academy of Sciences

Review | Dai Yunwei, Senior Engineer, China Meteorological Administration

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.