The underlying logic behind the recent rainstorms and floods in the south is still El Nino

Recently, heavy rains and floods in the south have concerned many people. On June 24 and 25, the Central Meteorological Observatory issued a red rainstorm warning for two consecutive days, which is the highest level of rainstorm warning signal.

Since the beginning of spring this year, southern my country seems to have been at odds with heavy rains and floods. In April and May alone, it experienced 14 regional rainstorms, especially the heavy rains in South China during the pre-flood season in April, which caused the second largest flood in recorded history in the Beijiang River Basin in Guangdong, second only to the 2022 flood. Since the Dragon Boat Festival in South China began in late May, heavy rains have intensified, causing floods and geological disasters in Guangxi, Guangdong, Fujian and other places.

Today, the plum rain weather in the Yangtze River Basin has officially begun. Its intensity is stronger than usual and has already caused floods in some places.

Everyone can't help but ask, what is the underlying logic behind the heavy rains and floods that have hit southern my country this year? Are there more and more heavy rains and floods?

(On June 19, the Lijiang River experienced its largest flood since 1998. Image source: Xinhuanet)

1. Why is the rainstorm in the south so fierce?

Speaking of this year's floods, we have to mention the El Niño event that ended in May this year.

El Niño-Southern Oscillation (ENSO) is a large-scale ocean-atmosphere interaction phenomenon occurring in the tropical Pacific region. It actually combines the abnormally cold La Niña event and the abnormally warm El Niño event in the central and eastern equatorial Pacific, as well as the Southern Oscillation based on the atmospheric pressure difference fluctuations between Tahiti and Darwin stations on both sides of the South Pacific.

(Global sea temperature anomaly in February this year. At this time, the tropical Pacific is still in the El Niño state, and it can be seen that the equatorial Pacific region is warmer. Image source: see the watermark on the upper left)

The moderate El Niño event that began in May 2023 will peak in December 2023 and end in May 2024, with the fifth strongest intensity in history. The National Climate Center predicts that the equatorial central and eastern Pacific may enter a La Niña state in the summer of 2024, and the La Niña event that may form in the future is likely to be weak to moderate in intensity.

2. El Nino ended in May this year, but its impact on my country's flood season precipitation is more obvious

Although El Niño ended in May, since the region where El Niño occurred is in the equatorial Pacific Ocean, which is a certain distance away from my country, my country's weather and climate will be affected through the tropical zonal circulation and the interaction between the tropical and extra-tropical atmospheres, that is, the atmospheric bridge or atmospheric teleconnection mechanism.

What are atmospheric bridges and teleconnections? The atmospheric circulation responds to the anomaly of sea temperature, and the anomaly is affected by atmospheric circulation or standing waves to other sea areas around the world. This connects the anomalies of sea temperature in two different sea areas like a bridge, which is called an "atmospheric bridge". The atmospheric circulation anomaly in one area can cause the atmospheric circulation anomaly in another area far away. This association between atmospheric circulation changes and anomalies at a long distance is called "teleconnection", and teleconnections are usually connected through atmospheric bridges.

This teleconnection takes time, so the atmospheric response has a certain delay. As a result, the weather during my country's main flood season (June to August) is still affected by the El Niño in the equatorial Pacific.

According to climate statistics, my country is prone to flood disasters in the summer following El Niño, especially the plum rains in the Yangtze River Basin and the Jiangnan region, which have a significantly intensifying effect.

The years with severe plum rains and floods in history include 1954, 1969, 1983, 1998, 2016, and 2020. Among them, 1983, 1998, 2016, and 2020 are all related to El Niño.

3. How does the aftermath of El Niño affect the South?

The internal logic of the correspondence between El Niño and floods in southern my country is very complex.

Generally speaking, affected by El Niño, the subtropical high pressure in the western Pacific is relatively strong and located to the south, which is conducive to the transport of ocean water vapor along its northwest side to my country. The sea temperature in the north Indian Ocean and the northwest Pacific is relatively high, and the evaporation of sea water increases, providing water vapor conditions for the frequent occurrence of heavy precipitation processes.

Specifically, the central and eastern parts of my country are located in the East Asian monsoon region. For the East Asian monsoon region, the El Niño event can cause abnormal sea surface temperature in the equatorial western Pacific during the decay period, thereby causing the Walker circulation anomaly in the region to affect the subtropical high pressure activity in the subtropical northwest Pacific, and through the Pacific-Japan Pattern (PJ) teleconnection between the tropical northwest Pacific and East Asia, thus causing summer climate anomalies in the East Asian monsoon region.

(Five key sea areas that affect the interannual variability of the Western Pacific subtropical high in summer, from He Chao et al.)

The North Pacific Anomalous Anticyclone (WNPAC) is an important bridge connecting the abnormal warming of sea surface temperature in the central and eastern Pacific (El Niño phenomenon) and the climate in East Asia. Therefore, maintaining the activity of the anomalous anticyclone in this region has an obvious connection with the summer climate in the East Asian monsoon region.

During the decay period of the El Niño event, many feedback mechanisms of sea-air interaction can maintain the anticyclonic activity in the western Pacific, such as the Indian Ocean-Western Pacific capacitor effect: This refers to the phenomenon that the Indian Ocean sea temperature anomaly affects the weather and climate in East Asia through a specific physical process: the tropical Indian Ocean sea temperature is warmer, and by stimulating the atmospheric equatorial Kelvin waves, it induces Ekman divergence in the tropical northwest Pacific, leading to the maintenance and strengthening of the anomalous anticyclone in the northwest Pacific. Similarly, the tropical Atlantic also has a similar mechanism that affects the anticyclone in the northwest Pacific.

The abnormal anticyclone will be superimposed on the climatological Western Pacific Subtropical High, making it stronger. When the Western Pacific Subtropical High is strong in summer, its ridgeline is located to the south. The strong Western Pacific Subtropical High makes the frontal rain belt in the south stable and less moving. At the same time, the abnormal southerly wind on the west side of the anticyclonic circulation transports a large amount of water vapor to southern my country, causing more precipitation in the south. At the same time, in the year after the occurrence of El Niño, the longitude of the mid-latitude westerly belt is strong, which is conducive to the southward movement of cold air, which is easy to merge with warm and humid air currents to form a more intense frontal rain belt.

(Global annual average temperature trend chart, from NOAA)

In recent years, extreme weather such as heavy rain and high temperatures seem to be more and more frequent. One reason is that extreme weather is becoming more and more severe due to the climate change dominated by global warming, which has caused various meteorological records of mankind in just over a hundred years to be broken. In 2023, under the influence of El Niño, the global average temperature set a new record, and so far in 2024, there is a trend of breaking the 2023 record and continuing to set new highs.

The development of the Internet, the increasing availability of mobile camera equipment, and the explosion of self-media have made the dissemination of weather news more convenient and have also allowed us to learn more about the frequent extreme weather events in various places.

In fact, the difficulty of predicting rainstorms and floods is gradually decreasing. While traditional supercomputer numerical forecasts are constantly improving and updating, the use of artificial intelligence AI in climate and weather models has also made forecasts more accurate than before. But even so, we cannot avoid the occurrence of such disasters. First, the atmosphere is a chaotic system. Although we can make more accurate large-scale trend forecasts for rainstorms and other weather, the small-scale forecast error accurate to each county and city is still not small. Second, facing the planetary-scale atmospheric circulation system (subtropical high pressure), we humans do not have the ability to change it. We can only do our best to prevent disasters and minimize losses.

References :

He Chao, Zhou Tianjun, Wu Bo. 2015. Key sea areas and influencing mechanisms affecting the interannual variability of the subtropical high pressure in the northwest Pacific in summer[J]. Acta Meteorologica Sinica, 73(5): 940-951

Author: Deng Feng, Earth Science Content Creator

Reviewer: Zhou Bing, researcher at the National Climate Center, chief expert on meteorological services at the China Meteorological Administration

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.