Why is Yunnan, a “watery” province, experiencing a severe drought?

Author: Wang Su (Institute of Atmospheric Physics, Chinese Academy of Sciences)

The article comes from the Science Academy official account (ID: kexuedayuan)

——

The moon is out and it's bright/bright

Thinking of my brother in the mountains

Brother is like the moon walking in the sky/walking in the sky/Brother, brother, brother, brother

The stream at the foot of the mountain flows quietly

"Little River Flowing Water" is a Yunnan folk song. Its melodious tune immediately brings us to the wonderland of Yunnan, where water gurgles. Cangshan Mountain and Erhai Lake, Lijiang Ancient Town, Yuanyang Terraces... This south of the colorful clouds always seems to be filled with water vapor, slow, wet, and intoxicating.

Lugu Lake scenery (Photo source: veer photo library)

Indeed, as the province with the most plant species in the country, it has numerous rivers and lakes, and its forest area ranks third in the country.

However, recent news shows that due to continued high temperatures, Yunnan is experiencing the worst drought in nearly 10 years. As of April 15, 1.4779 million people have run out of drinking water, 100 rivers have dried up, and 4.6 million mu of crops have been affected by drought.

Seeing this, you may wonder: Why is there drought in Yunnan, which has such abundant water resources?

Yunnan: Sometimes wet, sometimes dry

Why do we say that Yunnan has abundant water resources?

If China is divided into different regions according to the 800mm annual precipitation, a dividing line can be obtained, which runs through the southeast edge of the Qinghai-Tibet Plateau and turns to the Qinling-Huaihe line. The south is the humid area and the north is the semi-humid area. The average annual precipitation in Yunnan Province has reached 1,258 mm, and the total water resources are 222.2 billion cubic meters, which naturally belongs to the humid area.

(Photo source: National Geographic China)

But for Yunnan, this kind of humidity is not available all year round.

Due to its special geographical location, my country presents significant monsoon climate characteristics. Monsoon, as the name implies, refers to seasonal wind. In winter, it is cold and dry due to the influence of cold and dry air currents from the continent; in summer, it is warm and rainy due to the influence of warm and humid air currents from the ocean. The southwest monsoon and southeast monsoon will bring the humid and hot "care" of the Indian Ocean and the Pacific Ocean respectively. Yunnan is located in the southwest of my country, closer to the tropical Indian Ocean and the Bay of Bengal, making the southwest monsoon water vapor transport the main water vapor channel for summer precipitation. It is the significant monsoon climate that determines its distinct dry and wet seasons. The precipitation in the rainy season (May-October) accounts for more than 80% of the annual precipitation, which is prone to floods; the dry season (November-April of the following year) accounts for only 20%, which is prone to droughts in winter and spring.

Schematic diagram of monsoons affecting my country (Photo source: China Meteorological News Agency)

Why can’t we keep you, rain in Yunnan?

At this point, you may want to ask: Since there is abundant rainfall in the rainy season, why not save it for use in the dry season?

This is because under normal circumstances, although Yunnan does not lack rain, it cannot retain it. Under Yunnan's unique karst landform, the province's karst area reaches 110,000 square kilometers. As the saying goes, "people go to high places, water flows to low places", most of the precipitation here quietly leaves through underground runoff, and it is simply unable to "extend life" during the dry season.

(Karst landform of Yunnan Stone Forest, image source: pixabay)

At the same time, due to its location on the Yunnan-Guizhou Plateau, the mountainous area accounts for 88.64% of the total area of ​​the province. Yunnan has a rugged terrain, with high in the north and low in the south. This special mountainous plateau terrain is very likely to form a "foehn effect".

In summer, the southwest monsoon brings warm and humid air from the Indian Ocean and the Bay of Meng and advances at full speed. When it encounters the "roadblock" mountains in western Yunnan that run approximately north-south, it is forced to rise and cool, and the water vapor condenses into rain and falls. After finally crossing the mountains, it has lost water along the way. Even if it succeeds in crossing the mountains and reaches the "Bazi area (local plain area, accounting for only 6% of the province's land area)" where 2/3 of the population is concentrated, the warm and humid air with a declining momentum has weakened its ability to "call the wind and rain" and has become dry and hot.

It is precisely because of the extremely uneven distribution of rainfall that although there is a lot of rainfall, very little of it can be used. Facing the eager eyes in the "Flower City", the water vapor can only sigh and look at the "mountains".

Most of the lakes that are good at storing water are located in low-altitude areas. It is a huge (and expensive) challenge to move the water resources here to higher altitudes.

Under the synergistic effect of the seasonal drought brought by the monsoon climate and the special topography, the water in Yunnan is like a teenager who eats less and exercises more to lose weight, and is rapidly "losing weight".

Precisely because of the inability to "come over" and "stay", "wet and dry" Yunnan has become one of the most drought-prone regions in China, with a saying that "nine out of ten years there are droughts".

One of the main culprits of the drought: El Nino

This year's drought in Yunnan is generally believed to be closely related to abnormal climate. When it comes to the causes of abnormal climate, we have to mention the 2019 El Niño.

What is El Niño? An El Niño event is considered to occur when the ocean temperatures in the tropical oceans of the central and eastern Pacific Ocean become abnormally and persistently warm.

We know that high temperatures and low rainfall are the direct causes of drought. Affected by El Nino, the warmer central and eastern Pacific Ocean has caused the subtropical high pressure to move southward, westward and stronger. The cold air from high latitudes in winter and spring is not as strong as the subtropical high pressure, resulting in generally higher temperatures in Yunnan. Since March 2019, the average temperature in the province has been 4-5℃ higher than usual throughout the winter. Southern Yunnan was the first to enter summer in February this year.

(Photo source: China Weather Weibo)

The transport of water vapor is one of the prerequisites for the formation of precipitation. When El Nino appears, the tropical northwest Pacific, which is the key bridge for its influence on the climate in East Asia, is controlled by abnormally high pressure. The resulting anticyclonic circulation prevents water vapor from the Bay of Bengal from reaching Yunnan, causing it to flow to southern China and the southeastern coastal areas. At the same time, the colder western Pacific will weaken the atmospheric activity in low latitudes, weaken the strength of cold air, weaken the southern branch trough, and weaken the southwest wind in front of the trough. Without the help of wind, the water vapor transport channel is naturally affected and no longer active, and the water vapor provided to Yunnan is naturally reduced.

Schematic diagram of the El Nino phenomenon (Source: Sohu.com)

At the same time, the abnormal circulation caused by the warming of the tropical Pacific and Indian Oceans has caused the eastern part of the plateau to be controlled by downward airflow. Such a configuration of high-level convergence and low-level divergence makes it difficult to produce precipitation, and also causes water vapor to move away from Yunnan like "scattered flowers from the sky".

Since February last year, the average cumulative precipitation in Yunnan province has been 915.5 mm, 18.8% less than the same period last year and the third lowest in the same period in history (data source: Yunnan Provincial Meteorological Bureau).

It is the long-term high temperature and significantly low precipitation that led to the outbreak of this year's severe drought. Of course, the author only discussed the qualitative results here. What kind of physical mechanism caused this year's drought? I believe there will be more detailed research in the future.

What other abnormal climate is responsible for the drought in Yunnan?

El Nino did play an important role in the drought in Yunnan. The extreme drought in 2009-2010 was caused by the Modoki-type El Nino (i.e. the tropical central Pacific warming type). The El Nino event in 2014-2016 also led to the relatively severe high temperature drought in early summer in Yunnan in 2015.

But this does not mean that El Niño is solely responsible for the drought in Yunnan.

The summer drought in 2006 was mainly caused by the strengthening of the western Pacific subtropical high and the reduction of snow cover on the Qinghai-Tibet Plateau in the previous winter. The autumn, winter and spring droughts in 2012-2013 were affected by the La Niña event. Based on previous studies, Associate Researcher Wang Lin of the Institute of Atmospheric Physics and his collaborators summarized all the factors that may affect drought in southwest my country. Circulation anomalies in the tropical western Pacific, the Bay of Bengal, the Indochina Peninsula and the eastern Qinghai-Tibet Plateau, water vapor transport flux divergence anomalies, tropical Indian Ocean sea surface temperature anomalies, and Arctic Oscillation (AO) anomalies are all included.

Schematic diagram of important candidate areas that may affect the formation of drought in southwestern China. The yellow line represents the border of China, while the red box represents southwestern China. TNWP stands for Tropical Northwest Pacific, ENSO, WPSH and AO stand for El Nino-Southern Oscillation, Western Pacific Subtropical High and Arctic Oscillation. (Image source: Reference 2)

Due to the complex interaction between land and sea, there is no simple "1+1=2" derivation model between them. Due to the existence of strong nonlinearity, there is still great uncertainty in the coupling of the ocean and related atmospheric circulation to the feedback of Yunnan drought. Therefore, although people have a clear understanding of the variability of Yunnan drought at different time scales, there is no conclusion on its possible influencing mechanism, and further research is still needed.

If linearity is compared to a straight line, nonlinearity is like this ball of thread. We know the beginning and the end, but it is difficult to tell what happened in the middle (Image source: dreamstime)

Apart from the climate, who else should be blamed?

It is not just the climate that causes the drought in Yunnan. The reduction of high-quality forests is also to blame. (Please click: What causes large-scale forest death, and you will find a true vicious cycle)

The unique soil and water conservation function of vegetation cover plays an important role in land-atmosphere interaction and has a feedback effect on precipitation.

Yunnan's early extensive economic operations led to vegetation degradation, deforestation, soil erosion and excessive consumption of water resources. In recent years, with the efforts of the province, Yunnan's vegetation coverage has improved, and is about to achieve a leap from 24.9% in 1975 to 60% (expected) in 2020.

The trend of vegetation coverage in Yunnan Province from 2001 to 2016, with red representing a decrease and green representing an increase (Image source: Reference 10)

However, due to factors such as unreasonable forest structure, a large proportion of artificial forests and a large number of near-mature forests, the quality of forest resources here is not high. For the sake of economic development, deforestation and tea planting in nature reserves often damage the ecology, and there are also cases of replacing forests with economic forests. Studies have found that the rapid expansion of rubber planting in Xishuangbanna has accelerated soil acidification, which will lead to a series of ecological problems such as a significant reduction in the area of ​​tropical rainforests and a decline in water conservation capacity. The spread of the "green desert" will inevitably pose a major threat to the surrounding ecology.

Changes in vegetation coverage of tropical rainforests in Xishuangbanna between 1980 and 2016 (Image source: Earth Quantum)

In addition, the destruction of forests in countries or regions surrounding Yunnan (such as Thailand and Laos) will damage the "resting place" along the way of water vapor from Yunnan's summer monsoon, further causing drought in Yunnan.

Without the pristine forests to produce water vapor condensation nuclei and retain water vapor to conserve water resources, Yunnan, which has lost its forest precipitation, can only rely on "God's gift".

Yunnan has suffered from drought for a long time. Will it get better in the future?

As mentioned earlier, 2020 is not the beginning of the drought in Yunnan.

Drought has visited Yunnan every year in the past decade. In May last year, China Weather pointed out that since 2019, Yunnan's weather and climate have shown two characteristics: first, the temperature in winter, spring and early summer has continued to be high; second, the precipitation in early summer is significantly less, and drought is prominent. The province (marked by 60% of the stations) entered the rainy season on June 24, 29 days later than usual, setting a record for the latest in history.

The drought in 2019 was difficult to alleviate due to the low rainfall in the rainy season. Yunnan's water reserves have been depleted, causing the adverse effects of the drought to "quietly" grow. In 2020, water use for agricultural production increased sharply in spring, precipitation decreased significantly, and water resources were naturally "in urgent need of water resources."

Comprehensive drought monitoring results on April 20, 2020 (Image source: China Climate Center)

The good news is that precipitation in Yunnan has increased significantly since April 25. However, spring is a dry season, and even if there is a short-term, large-scale heavy rainfall, the drought in Yunnan will still be difficult to control in the short term. The drought may develop to a deeper level and continue until the beginning of the early summer rainy season.

Associate Researcher Wang Lin and his collaborators evaluated the potential future changes in drought in southwestern my country based on the CMIP5 framework and concluded that the drought risk in the region is almost twice that of other parts of the country. However, the study also pointed out that current climate models cannot simultaneously capture the location and intensity of sea temperature anomalies and circulation anomalies, making it difficult to establish an effective prediction method and the prediction results may not be accurate.

Although the climate is unpredictable, the meteorological community has not stopped its research. But we can be sure that only by strengthening infrastructure construction, improving the serious water shortage in some areas caused by the extremely uneven temporal and spatial distribution of precipitation, increasing the development and utilization rate of water resources, and reducing the area of ​​rocky desertification, can the threat of severe drought be alleviated.

At the same time, we should continue to strengthen the protection and management of forest resources, give priority to ecological development, and turn barren forests into green forests, so that Yunnan can retain water in the future. Only by predicting and detecting the occurrence of drought as early as possible can we take more timely and effective prevention and rescue measures.

Against the backdrop of global warming, the Yunnan drought is not the first local extreme climate event, nor will it be the last. Climate can create the beautiful "South of the Colorful Clouds", but it can also severely damage its beautiful mountains and rivers. Climate research explains why Yunnan is drying up, but it cannot answer when it will stop drying up. I hope that through the joint efforts of scholars and the general public, a no-longer "extreme" Earth can soon reappear.

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