What is the reason for the sudden change from drought to flood in Henan? Why are there so many floods this year?

Produced by: Science Popularization China

Author: Half Lazy (Institute of Atmospheric Physics, Chinese Academy of Sciences)

Producer: China Science Expo

Image source: China Newsweek

In June 2024, Henan was on the hot search several times because of the severe drought. The Flood Control and Drought Relief Department of the Henan Provincial Emergency Management Department stated that in May 2024, the rainfall in Henan Province was more than 70% less than normal, and more than 50% of the meteorological monitoring stations reached the level of moderate drought or above. On June 12, 2024, 16 cities in Henan monitored that the meteorological drought reached the level of severe drought or above, and lasted for 10 days.

Image source: National Climate Center

Henan: Difficult! One wave has not yet subsided, another wave rises

Under the extreme drought, there are anxious farmers and crops waiting to be fed. On the night of June 15 and June 20, 2024, some areas in Henan finally waited for significant rainfall, and the fourth-level emergency response for drought relief was lifted on June 22, 2024. Just when everyone thought that the drought had finally eased, they did not expect that one crisis would come after another, and a new crisis had been secretly brewing.

Image source: Henan Meteorological Bureau

From mid-to-late June to July 2024, as the main rain belt moved northward, Henan welcomed several rainstorms. From 8:00 on July 14 to 8:00 on July 18 alone, the average precipitation in the province reached 117.2 mm, and Dafengying in Sheqi County, which had the largest precipitation, reached 867.1 mm. The Nanyang Sheqi National Meteorological Observation Station recorded a rainfall of 418.6 mm from 20:00 on July 15 to 06:00 on July 16, 2024, greatly breaking the local daily rainfall record. On July 17, Henan was already in a first-level emergency response state for major meteorological disasters (rainstorms) . Just three years after the heavy rain in Zhengzhou, Henan in 2021, the pain is still there. Henan has once again become a hot topic because of extreme rainstorms.

Image source: Henan Meteorological Observatory

A village in Sheqi County, Henan Province after heavy rain

(Photo source: CCTV News)

Drought and flood: the problem is not only the scarcity of food but also the inequality of food distribution.

It took less than a month for Henan to go from severe drought to severe flood. This is the phenomenon of rapid transition from drought to flood.

Drought-flood abrupt transition refers to the situation where regional climate alternates between drought and flood in the same season, that is, the drastic change and extreme imbalance of precipitation in a short period of time, including drought-flood events and flood-drought events. Simply put, it is the uneven distribution of precipitation within a certain period of time, which mainly occurs in spring, summer and their transition period. Compared with the occurrence of drought or flood anomalies alone, although the probability of occurrence of such compound events is low, the threat posed to production, life, disaster prevention and mitigation is greater.

Frequency distribution of drought-flood transitions in China over the past 40 years: (a) drought-to-flood events and (b) flood-to-drought events

(Image source: Reference 1)

In 2011, the sudden change from drought to flood in the middle and lower reaches of the Yangtze River in China from May to June affected 33.947 million people and caused direct economic losses of 29.36 billion yuan, which was one of the top ten weather events of the year. In June and July 2023, precipitation in North China was less than normal, and droughts of varying degrees occurred in many places; in early August, extreme rainstorms occurred in most parts of North China, and 1.29 million people were affected in Beijing alone, 33 people died, and the affected area of ​​crops reached 15,000 hectares.

This rapid change not only affects the growth of crops. The continuous drought in the early stage will cause the soil to lose moisture and weaken its water absorption capacity. When faced with sudden heavy rain, rainwater will be more likely to gather on the ground. The resulting hysteresis effect of geological disasters may cause secondary disasters such as urban waterlogging, mountain torrents, and slope collapse and landslides along highways. Therefore, although the frequency of sudden drought-flood transition is low, it is often the most disastrous.

Why is July the tear of the North?

In fact, it is not only Henan. Drought-flood sudden changes occur frequently in my country, mostly concentrated in central, eastern and northeastern China, the Huaihe River Basin, the Yellow River Basin and the Yangtze River Basin.

Spatial distribution of drought-flood transition frequency from 1961 to 2021

(Image source: Reference 2)

When Henan was hit by heavy rains, neighboring Shandong was also "suffering". Since July 2024, the average precipitation in Shandong Province has been 392.9% higher than the same period in previous years. From July 6 to 9 alone, there were 167 stations in Shandong with precipitation exceeding 250 mm, with the maximum precipitation in Fangqian, Linyi reaching 607.6 mm, and single-day rainfall in many places breaking historical records.

Image source: China Weather

Due to the influence of monsoon climate and complex geographical environment, changes in meteorological conditions are an important driving factor. The coupling of atmospheric circulation, abnormal sea surface temperature and abnormal northward transport of warm and humid air currents will affect the occurrence of drought-flood abrupt transition events. For example, the drought-flood abrupt transition in the middle and lower reaches of the Yangtze River and the southwest region is mainly related to the high-altitude westerly winds in the middle and high latitudes, the position of the subtropical high pressure in the western Pacific in the middle and low latitudes, and the water vapor transport conditions in the low latitudes.

Main factors affecting the sudden change from drought to flood

(Image source: Reference 3)

Why do we always hear news about heavy rains in the north in July? This is because under the influence of the subtropical high pressure in the western Pacific (referred to as "subtropical high"), heavy rains in the north are most likely to occur in the critical flood prevention period of "the end of July and the beginning of August".

Image source: China Meteorological News

For 2024, the combined effects of global warming and the El Nino event have resulted in significantly higher temperatures in the northern region (except the Northeast) in the first half of the year. As the subtropical high intensifies and moves northward and remains in the Huanghuai region, the southwestern warm and humid air currents continue to be transported northward along the edge of the subtropical high, forming a high temperature and high humidity situation in Henan, where they meet the southward cold air from the north (yes, even in the hot summer, there is cold air moving south). The terrain uplift combined with the conflict between the cold and warm air of completely different temperaments can easily produce and maintain heavy rains, causing the rapid transition from drought to flood here.

Future: More and more drought-flood cycles

It is worth noting that against the backdrop of climate change, the number of sudden changes from drought to flood in my country is on the rise.

The scope and trend of drought-flood sudden-change events in China from 1961 to 2021

(Image source: Reference 2)

On the one hand, as the greenhouse effect increases, global temperatures rise, leading to increased surface evaporation, reduced moderate and light precipitation, and increased drought risk. Therefore, the total area of ​​regions experiencing increased frequency and severity of droughts in the future will further expand as warming intensifies.

The changes and attributions of extreme heat events since the mid-20th century are as follows: North America includes NWN (western North America - northern region), NEN (eastern North America - northern region), WNA (western North America), CNA (central North America), ENA (eastern North America); Central America includes NCA (northern Central America), SCA (southern Central America), CAR (Caribbean); South America includes NWS (western South America - northern region), NSA (northern South America), NES (eastern South America - northern region), SAM (South American monsoon region), SWS (western South America - southern region), SES (eastern South America - southern region), SSA (southern South America); Europe includes GIC (Greenland/Iceland), NEU (northern Europe), WCE (western and central Europe), EEU (eastern Europe), MED (Mediterranean); Africa includes MED (Mediterranean), SAH (Sahara), WAF ( The following are the main areas of focus: Western Africa (region), Central Africa (region), North Africa (region), East Africa (region), South Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa (region), West Africa

(Image source: Reference 4)

Water cycle diagram

(Image source: Reference 5)

On the other hand, the ability of the atmosphere to retain moisture will increase, and the water cycle will continue to strengthen. For every 1°C increase in global warming, the water holding capacity of the near-surface atmosphere will increase by about 7%. On a global scale, this will manifest as an increase in total precipitation and an increase in precipitation extremes, and an increase in the risk of floods.

Changes and attribution of heavy precipitation events since the mid-20th century

(Image source: Reference 4)

Furthermore, while the global humid areas (mainly including tropical, monsoon, mid- and high-latitude regions) will become wetter due to the increase in total precipitation, the temporal distribution of precipitation will become more uneven, and the fluctuations between dry and wet periods will be more dramatic.

Precipitation change patterns based on precipitation mean and variability changes, with the color being the ratio of precipitation variability change to mean change

(Image source: Reference 6)

This trend of increasing and intensifying regional drought events and extreme precipitation events will make it more likely that "severe floods will follow severe droughts" will occur in the future.

For my country, drought-flood abrupt changes will become more frequent in summer, and will increase significantly in most parts of China in spring. The North China Plain, as China's most densely populated area and the largest agricultural production area, is expected to see more and more drought-flood abrupt changes, and their intensity will increase.

Projected changes in the area-weighted average of (a) flood-to-drought events and (b) drought-to-waterlogging events in different seasons for China’s 10 climate zones. Values ​​greater than 0 indicate that the frequency of drought-to-waterlogging events is expected to increase, while values ​​less than 0 indicate that the frequency of DWAA events is expected to decrease.

(Image source: Reference 7)

Henan: A microcosm and a representative

Why do we have to single out Henan for discussion?

“The history of Henan is half of the history of China.” The suffering and tears of Henan are a microcosm of the millions of people in China who have been affected by extreme weather, especially farmers .

Henan is a major grain-producing province, contributing one-tenth of the country's grain, and is a well-deserved granary. The grain harvest directly determines the survival of farmers. However, located in the transition zone between the north and the south, it has always suffered from both floods and droughts, and has encountered too much suffering.

The extremely heavy rainfall in Henan Province in mid-to-late July 2021 alone affected 14.5316 million people in the province, killed 302 people, collapsed and damaged 784,265 houses, and caused direct economic losses of 114.269 billion yuan.

Behind the numbers are people who desperately fought drought in June by diverting water for irrigation despite the high temperatures, and fields that were helplessly flooded by heavy rains in July. The increase in extreme weather is a bloody reality . It is no longer a slogan, but has a definite impact on the survival of tens of millions of people .

Because not only for Henan, but also for any region affected by extreme weather, not only in rural areas, but also in large cities, resilience building must be taken seriously.

Heavy rain in Henan

(Photo source: China Newsweek)

Fortunately, my country has always adhered to the principle of putting people and life first. After every extreme disaster, effective rescue and risk avoidance measures are taken at the first time to help the affected people with basic living security and other disaster relief work.

Rescue site in Qingtai Village, Lidian Town, Sheqi County, Nanyang, Henan

(Photo source: China National Radio)

As my country's comprehensive disaster prevention and mitigation capabilities improve, even though extreme disasters are increasing, the number of people affected is showing a downward trend.

The number of people affected by disasters and the direct economic losses in China from 2002 to 2022

(Image source: Reference 7)

However, we still need to be vigilant. The direct economic losses caused by extreme disasters to my country and even the world have not been significantly reduced. Meteorological disasters will be long-term, sudden, catastrophic and complex for a long time, and accurate prediction of extreme disasters is still difficult. For individuals, we need to establish awareness of the normalization of extreme weather, enhance awareness of flood prevention and risk avoidance, and self-rescue and mutual rescue capabilities to adapt to the severe challenges brought by climate change.

References

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