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At present, the large-scale continuous high temperature weather in the south has basically ended. The unusually strong subtropical high in the western Pacific is the most direct cause of the high temperature weather this summer. This is related to the general trend of global warming, and also to the abnormal SST in the northwest Pacific Ocean on the seasonal scale.
At 18: 00 on August 30, the Central Meteorological Observatory lifted the yellow warning of high temperature, and the large-scale continuous high temperature weather in the south basically ended. This round of high temperature is the strongest comprehensive high temperature since there was a complete meteorological observation record in 1961, and its duration, range above 40 degrees Celsius and extremes have set a new historical record.
This year, the western Pacific subtropical high is extremely strong, which is the most direct cause of persistent high temperature weather. What is a subtropical high? Why is the subtropical high in the western Pacific unusually strong this year?
The anomaly of subtropical high is the most direct cause of persistent high temperature weather.
Recently, Sichuan Basin, Shaanxi, Chongqing and other places have continuously ushered in heavy precipitation, and the high temperature has finally subsided.
This summer’s high-temperature heat wave event started early, lasted for a long time, affected a wide range and had strong comprehensive intensity. Meteorological monitoring shows that from June 13th to August 30th, the regional high temperature event lasted for 79 days, covering more than 5 million square kilometers. There were 23 provinces with high temperatures above 40 degrees Celsius, and the daily maximum temperature of 366 national weather stations reached or exceeded the historical extreme.
Due to the long duration of high temperature, some rivers dry up during the main flood season. In the Yangtze River basin, the water levels in many places have been the lowest in the same period since the record was set, and Dongting Lake and Poyang Lake "dried up" ahead of schedule. At the same time, high temperature weather has also brought various adverse effects to people’s production and life, such as the surge in demand for electricity, the decline in hydropower production, the obstruction of agricultural production, and forest fires.
Chen Tao, the chief forecaster of the Central Meteorological Observatory, said that this year’s western Pacific subtropical high has a large range and strong intensity, which is the most direct cause of persistent high temperature weather. Under its control, the southern part of China is controlled by downward flow as a whole, with clear skies and partly cloudy. During the day, under the influence of sunshine radiation, the near-surface heating is intense, and the hot air stays on the ground, which can’t be blown away or dispersed, so there is a wide range of persistent high temperature weather.
Not only in China, but also in the northern hemisphere this summer, the western Pacific subtropical high, the Atlantic subtropical high and the Iranian high have all been strengthened in stages, forming a large-scale global warm and high-pressure belt and forming high-temperature weather in many parts of the northern hemisphere.
"The occurrence of high-temperature heat wave events in summer is normal from the perspective of climate state. However, the duration, intensity and influence range of this summer’s high-temperature heat wave event have reached a super level. " Chen Lijuan, chief forecaster of National Climate Center, analyzed that persistent atmospheric circulation anomaly is the "culprit" of persistent high temperature. This year, the western Pacific subtropical high is unusually strong and extends westward, and the west reaches the eastern part of the Qinghai-Tibet Plateau. At the same time, the southern and northern boundaries of the subtropical high are both extended, and the area is very large. In summer, the high temperature events in most parts of southern China last for a long time, and the end time is obviously later than normal.
The strong subtropical high zone blocks the transport of water vapor from the low-latitude ocean to the south.
What is the subtropical high? Why does it often appear in high temperature and summer events? Why is it so powerful this year?
Zhang Daquan, a senior engineer in the Climate Prediction Office of the National Climate Center, said that the well-known subtropical high generally refers to the subtropical high in the northwest Pacific Ocean in the northern hemisphere, which has a great impact on China. It exists all the year round and is located in the middle and lower troposphere of the atmosphere. It is a stable and warm deep system with little movement. Due to the strong solar radiation in the equatorial region, the atmosphere is heated up and flows to the north and south poles with lower temperature. Under the influence of the earth’s rotation, it settled around 30 degrees north and south latitude, and then the air flow returned to the equator, forming an atmospheric circulation circle called Hadley circulation. The subtropical high is formed when the airflow sinks in the subtropical area near 30 degrees north and south latitude.
Affected by land and sea distribution and other factors, the subtropical high belt in the northern hemisphere splits into several high-pressure units, namely, the North Pacific High, the North Atlantic High and the Iranian High. Among them, the western part of the North Pacific High-the Western Pacific Subtropical High has an important impact on China’s weather and climate, and its staged "northward jump" is directly related to the start and end time of the rainy season and the location of the rain belt in the eastern monsoon region of China, while abnormal advance and retreat and stable maintenance are important factors leading to abnormal weather and climate in China. After the "Meiyu" every year, the main body of the West Pacific subtropical high directly controls the middle and lower reaches of the Yangtze River, and the downward flow leads to sunny and hot partly cloudy, with strong solar radiation, which usually leads to high temperature and summer drought.
This year, the subtropical high is extremely strong, resulting in continuous high temperature and little rain in the Yangtze River basin, which is more and stronger than the perennial high temperature and summer drought weather. From the perspective of influencing factors, on the one hand, it is related to the overall trend of global warming, on the other hand, it is also related to the thermal anomaly of the Qinghai-Tibet Plateau, La Nina phenomenon and SST anomalies in the Indian Ocean and the Northwest Pacific Ocean on the seasonal scale.
This summer, the heating effect of the Qinghai-Tibet Plateau was remarkable. The South Asian high over the Qinghai-Tibet Plateau was unusually strong, and the West Pacific subtropical high strengthened its westward extension and opened up with the Iranian high in the west of the plateau. The strong subtropical high zone blocks the transport of water vapor from the low-latitude ocean to the southern part of China, and the area controlled by the subtropical high continues to have high temperature and little rain. Judging from the marine conditions, this summer is in La Nina state, the SST in the equatorial Middle East Pacific is abnormally low, and the SST in the western Pacific and the South China Sea is high. The tropical SST anomaly is conducive to the strong and large subtropical high in the west Pacific through the air-sea interaction, which leads to a wide range of high temperature this year, including Huanghuai, Jianghan, Sichuan and Chongqing.
Raise awareness of adapting to climate change and actively respond.
Chen Lijuan believes that the extreme characteristics of high temperature events this year are closely related to the background of climate change.
The sixth assessment report of the United Nations Intergovernmental Panel on Climate Change (IPCC) pointed out that under the background of global warming, many extreme weather and climate events have been observed since the middle of the 20th century, among which the frequent occurrence of high-temperature heat waves is a very significant feature.
"This is a warning to us, prompting us to have a deeper understanding of climate change and improve our ability to adapt to climate change in all aspects." Zhang Daquan said, "In recent years, China’s ability to cope with climate risks and reduce and prevent disasters in agricultural production, floods and architectural design has been improving. On the other hand, we should also raise the awareness of the whole society to adapt to climate change, use climate prediction products to cope with climate risks in advance, and strive to minimize social and economic impacts and losses. "
Experts explained that in the context of climate warming, this summer’s high-temperature heat wave broke the historical record and has certain extremes. However, from the perspective of climate periodicity, because the earth’s climate system has self-regulation function, the increase in the probability of extreme events does not mean that the same type of extreme events will occur every year.
On the other hand, with the development of social economy, the demand for meteorological services by the country and all walks of life is increasing, which requires meteorological departments to further improve the accuracy of forecasting, and at the same time expand the forecasting objects to the downstream, and develop climate risk forecasting based on disaster risk assessment, so as to facilitate decision-making departments and the public to deal with climate change more scientifically and effectively.
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