Shulou(Shulou.com)11/24 Report--

This article comes from the official account of Wechat: Earth knowledge Bureau (ID:diqiuzhishiju), text: Feng Yun Meng Yuan, revised manuscript: gu Hanying / Editor: chestnut

In the past two months, our country has encountered wave after wave of high temperature heatwave, which has been testing people's physiological limit for a long time.

According to the data, the number of days in which the average temperature in China exceeded 35 ℃ in the first half of this year reached 4.1 days, 1.9 days more than that of the same period in other years, which is about double, reaching the highest in the history of the same period since 1961.

▼

From a sub-regional point of view, North China, the north of East China, the west of South China and the south of southwest China are the most significant. Especially in North China, the number of days of high temperature reached an astonishing 9.8 days, 5.2 days more than in the same period of the year.

Anomalous distribution map of high temperature days in China from January 1 to June 30, 2023

(photo: China Meteorological Administration) ▼

The direct cause of extreme high temperature in North China this year, the high temperature in North China is concentrated on June 14-17 and June 21-30. The latter, in particular, is simply not the hottest, only hotter. 22 stations such as Beijing Tanghekou (41.8 ℃) and Tianjin Dagang (41.8 ℃) have reached or exceeded the historical extreme value. From the 22nd to the 24th, the temperature of the southern suburb observatory in Beijing was 40 ℃ (greater than or equal to 40 ℃) for three consecutive days, of which the highest temperature on the 22nd day was 41.1 ℃, the second highest since the founding of the people's Republic of China.

People are going to melt. I really can't stand it.

(photo: central Meteorological Station) ▼

The high temperature in early July was equally extreme, with another two hot days above 40 ℃ in Beijing. Since the founding of the people's Republic of China, Beijing has had a total of 11 days of high temperatures of more than 40 ℃, and five days so far this year, accounting for nearly half of the total.

Beijing breakthrough 40 ℃ is easy to be discussed by the whole network, while the Hebei city next door, the sense of online presence is much smaller, but their high temperature is no less, along the line of Taihang Mountain, from Shijiazhuang, Xingtai and Handan in Hebei Province to Anyang, Xinxiang and Jiaozuo in Henan Province, and at the eastern foot of Taihang Mountain, there are also significant high temperature heatwaves, with 43.7 °C in Pingshan, Hebei Province on July 6 and 43.3 °C in Wuan, Hebei and Linzhou, Henan Province on the 7th.

Taihang Mountain keeps out the high temperature on its own.

(photo: central Meteorological Station) ▼

As a matter of fact, the high temperature in North China this year is not only related to El Nino and global warming, but also closely related to the "atmospheric abnormal activity" as far away as the Atlantic Ocean. Multiple factors are superimposed, acting on the westerly belt over North China. Created this extremely hot summer that broke through historical extremes.

It's time to come or come.

(figure: wmo) ▼

As we mentioned in the previous article, the causes of summer high temperatures in southern and northern China are different. The summer drought in the Yangtze River basin after the plum rainy season is controlled by the western Pacific subtropical high, while the high temperature heat wave in North China is concentrated from June to the first half of July, mainly controlled by high pressure ridges or continental abnormal highs formed by fluctuations in the westerly belt.

For example, last year's drought in Jiangxi and Hunan lasted for several months.

Is caused by the subtropical high belt that surrounds the entire northern hemisphere.

(photo: central Meteorological Station) ▼

Let's take June 22-25 this year as an example, from North China to Heilongjiang, it is controlled by abnormal warm high pressure, and the northern part of North China, including Beijing and Tianjin, is its center. Under its control, sunny and cloudy weather occurred in the northern part of North China, and solar radiation directly heated the low altitude, coupled with the subsidence and warming of the upper atmosphere, resulting in a rapid rise in surface temperature. Beijing and Tianjin both exceeded 40 ℃, and the high temperature above 35 ℃ spread all over North China.

In addition, some areas are also affected by the underground surface of the bureau, such as urban heat island, Lee slope subsidence and valley warming, which will make local temperatures higher and more unbearable.

From June 22 to 25, the temperature anomalies in the lower troposphere (850hPa) and the height field anomalies in the middle troposphere (500hPa) are ▼.

This warm high over North China is actually part of a larger ridge of high pressure in the westerly belt. We know that the westerly belt is not always straight, and when large fluctuations occur, it forms a ridge of high pressure followed by a trough of low pressure. The areas controlled by high pressure ridges are characterized by atmospheric subsidence, sunny and rainless weather.

Schematic diagram of contours and geostrophic wind on 500hPa

(the left shows a straight westerly belt, and the right shows the trough ridge formed after disturbance.) ▼

The hot weather in North China in mid-late June is due to the ridge of high pressure.

(500hPa somatosensory temperature on June 15) ▼

Whether the westerly belt will fluctuate greatly depends on, on the one hand, the temperature difference between the polar regions and the low latitudes, which is the driving force behind the formation of the westerly belt.

On the one hand, this kind of fluctuation needs a "disturbance source". When an abnormal high or low pressure appears in a certain area of the westerly belt, it can gradually stimulate the long wave of the westerly belt by affecting the downstream link, and then change the distance beyond the skyline. This phenomenon is often called teleconnection.

The large trough and ridge produced by the long waves in the westerly belt largely determine the weather in the northern hemisphere.

On the other hand, short waves can be superimposed on long waves to deepen or weaken ▼.

In the middle and high latitudes of the northern hemisphere, there are usually two most prominent centers of activity, the North Pacific and the North Atlantic. The "source of disturbance" of the weather process on the Asian continent often sets out from the North Atlantic and spreads eastward along the westerly jet.

Rapids are narrow, strong winds that usually blow all over the world from west to east.

(figure: NOAA) ▼

In order to understand the long waves of the westerly belt and the relationship between the upstream and downstream of the westerly belt, we have made the following picture.

It reflects the "teleconnection" between June 22-25, 500hPa altitude (about 9000 meters), and the upper troposphere in different regions.

The warm color indicates the abnormal high pressure, the cool color indicates the abnormal low pressure, and the arrow is the "wave flux" of the long wave in the westerly belt.

Analysis of 500hPa stream function anomaly and wave action flux from June 22 to 25 ▼

It can be seen that there are a series of abnormal centers from the North Atlantic to Northeast Asia, including abnormal low pressure in Greenland and the south of Iceland, abnormal high pressure in Europe, abnormal low pressure in West Siberia, and abnormal high pressure from North China to the Sea of Okhotsk.

The long wave propagation indicated by the arrow shows that the North Atlantic is the "source of disturbance", and the long wave from here presents an arc path of "to the northeast and then to the southeast", which finally leads to the formation of abnormal warm high pressure in North China.

Analysis of 500hPa stream function anomaly and wave action flux from June 22 to 25 ▼

The influence of El Ni ñ o above is the analysis of the direct cause of this year's high temperature event. But from a broader point of view, there are more important factors behind the frequent high temperatures across the country this year: including the developing El Ni ñ o event and global warming dominated by human activities for more than a century.

In addition, the sea surface temperature in the tropical Indian Ocean and extra-tropical Pacific Ocean, the ups and downs of Arctic sea ice, and the rise and fall of snow cover in Eurasia, especially the Qinghai-Tibet Plateau, also affect the climate of North China and even the whole country in summer to a certain extent.

El Ni ñ o under the blessing of global warming

Raise the upper limit of global temperature again and again

Challenging the limits of human tolerance, ▼.

So, how much impact does this year's El Ni ñ o have on the high temperature in North China?

The answer is that there is a certain impact, but there is a lot of uncertainty.

We summarize the "summer wind field" of El Ni ñ o development stages in different years in history. Although El Ni ñ o is not the only factor affecting the wind field, as long as there are enough samples, the commonness of these samples can indicate the influence of El Ni ñ o more clearly. (the composite picture below)

Combining the 500hPa wind chart of the El Ni ñ o development period after 1979, it can be seen that the most prominent feature of Northeast Asia is the cyclone circulation (blue frame) entrenched in the northeast region to the Sea of Japan, and a weak anticyclone (red frame) in the area of Lake Baikal.

Summer of El Nino event development

Anomalous wind field in the middle troposphere (500hPa) synthesizes ▼

On the other hand, North China is located between the two. On the one hand, it is affected by the downdraft on the west side of the cyclone, and at the same time, sandwiched between the western anticyclone and the eastern cyclone, it will produce "upper negative vorticity advection". The result is sunny and less rain, atmospheric subsidence and frequent high temperature.

However, because most El Nino events do not reach their peak in winter, the intensity is still limited and the response signal strength is weak in the summer of the development year, so it can only explain part of the cause of the high temperature.

In the summer of El Nino development

The temperature in most parts of China is on the high side, ▼.

The ultimate BOSS of extremely high temperatures compared with the ups and downs of El Ni ñ o and La Nina, global warming hidden behind the scenes is the largest BOSS. Although global warming does not necessarily play a role in individual high temperature events, it has become obvious in recent decades that global warming is getting stronger and more frequent.

In addition to the higher average temperature, there is also a significant increase in deviating extreme temperatures, including extreme high temperatures and extreme low temperatures, of which extreme high temperatures are more frequent.

Temperature anomaly distribution curve of summer stations in the Northern Hemisphere

Remarkable changes in different ages ▼

There are many explanations for the mechanism of the correlation between global warming and extreme high temperatures, one of which is that Arctic sea ice melts sharply under climate change, and the exposed sea and land surfaces are much darker than when they were previously covered with ice, resulting in lower albedo and absorbing more heat, so the warming in the Arctic is more severe than in low latitudes.

Rising temperatures cause sea ice to melt

And the melting of sea ice promotes the rise of temperature.

This vicious cycle causes sea ice to melt faster and faster, ▼.

Arctic sea ice is below average in winter 2023

(figure: NASA) ▼

This directly leads to a narrowing of the temperature difference between the polar and tropical regions.

As we mentioned earlier, the temperature difference between the polar regions and the tropics is the driving force of the westerly belt, and it is the wind driven by this temperature difference that is transformed into a strong westerly jet under the force of geostrophic deflection, just like a natural graben blocking between the polar regions and low latitudes, making the air masses on both sides of the north and south insurmountable.

The greater the temperature difference between the two sides of the jet stream, the stronger the jet stream.

The smaller the temperature difference, the weaker the jet stream.

(figure: eoas.ubc.ca) ▼

When this temperature difference narrows, the westerly jet weakens accordingly, and the obstruction of cold and warm air to the north and south becomes smaller, which is more likely to cause extreme weather events everywhere. Among them, there are large trough ridges formed by large fluctuations in the westerly belt, which is also an important climatic background in which North China is controlled by high pressure ridges.

It can be seen that the warming in the Arctic has been the most significant in the past 40 years.

(figure: NASA) ▼

Different states of westerly jet and heat exchange in north-south direction

Always accompanied by extreme cold and warm events.

(figure: NOAA) ▼

It is not yet possible to accurately predict the region, extent and duration of each high temperature heat wave in the future. But what is clear is that under such climate change, in the coming decades, such extremely high temperatures will only become more and more extreme, and the threat to human life and even survival will become more and more significant.

In the face of such a grim situation, mankind has no choice but to join hands to reduce greenhouse gas emissions in order to minimize the impact of global climate change, but how difficult it is to do so.

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