Why is the wind so strong this year? Is it blowing everyone into the same hairstyle as Nezha?

Produced by: Science Popularization China

Author: Chen Dexiu (Institute of Atmospheric Physics, Chinese Academy of Sciences)

Producer: China Science Expo

Last week, Beijing was hit by extreme gale, and some areas even had a rare gale of 13 on land. Unexpectedly, the first express that the workers received after the Spring Festival holiday in the Year of the Snake was the North Wind Express. Someone even joked: "When this wind blows, it's easy to get the same look as Da Zha."

Today, we will use science to unravel the secrets behind this "extreme cold delivery" and see how this level 13 gale was delivered to our doorstep.

1. Air pressure difference drive - the engine of wind

Wind is the flow of air from high-pressure areas to low-pressure areas. When there is a difference in temperature on the surface of the earth, hot air rises to form low pressure, and cold air sinks to form high pressure. The pressure difference drives the horizontal flow of the atmosphere on the surface of the earth. Common examples include uneven heating by solar radiation between the equator and the poles, which drives global atmospheric circulation; the temperature difference between the ocean and land during the day and night, which forms sea and land breezes. During the day, the land warms up quickly, and the wind blows from the ocean to the land (sea breeze). At night, the ocean has a larger specific heat capacity, and the temperature decreases less, so the wind blows to the warmer ocean (land breeze); the temperature difference between the east and west of the tropical Pacific Ocean (the temperature difference is about 4-6℃, 28℃ in the west vs 22℃ in the east) causes the Walker circulation in the atmosphere.

Figure 1 Basic weather analysis chart of China at 14:00 (left: February 6; right: February 11)

(Photo source: China Meteorological Observatory)

Recently, the first cold wave of the Year of the Snake hit, and the drastic pressure difference between the Siberian cold high pressure and the mid-latitude low pressure system became the core driving force of this strong wind. The central pressure of the cold high pressure is as high as 1060 hPa, while the pressure in the North China Plain to the middle and lower reaches of the Yangtze River drops to about 1020 hPa, forming a steep pressure gradient. Compared with the weather maps of the 6th and 11th (Figure 1), the pressure contour lines (blue lines) on the afternoon of the 6th are more dense, which is like a layer of steps. The more steps there are, the greater the pressure gradient (slope) within the same distance. This powerful gradient force is like a "super wind pump" spanning thousands of miles, pushing the cold air to move south at a rapid speed. The cold wave process is accompanied by strong winds, and the ground wind speed quickly increases to level 8-10 (20-25 m/s).

2. Terrain Difference Acceleration - Wind Speed ​​Amplifier

Rough surfaces (forests, etc.) act like brake pads, increasing friction near the ground and slowing down the wind. However, when there are mountains on both sides, this terrain difference will form a narrow channel. When the airflow encounters the mountain barrier, it will choose to "squeeze" through this channel quickly. This phenomenon is called the narrow channel effect. Common examples of narrow channel effects in life include: pinching the outlet of a water pipe will make the water spray farther, draughts, etc.

Figure 2 Narrow tube effect

(Image source: self-made by the author)

Let us further explain what the narrow pipe effect is and how it amplifies the wind speed by combining two basic principles in fluid mechanics: According to the law of conservation of mass, in the case of an ideal fluid (the air flow through any cross section of the pipe per unit time is the same), therefore, when the cross section of the pipe becomes smaller, the speed of the airflow must be increased to ensure that the same amount of air passes through per unit time. In addition, the Bernoulli principle in fluid mechanics tells us that under ideal conditions, assuming no frictional dissipation, the sum of the kinetic energy and potential energy of the unit volume of fluid at any cross section of the same flow tube remains unchanged, that is, there is an energy conservation relationship between speed, pressure and height. Therefore, when air flows through a narrow channel, the height of the fluid decreases and the speed is significantly enhanced.

Figure 3 China regional topographic map

(Photo source: Tiandi Map)

Looking at my country's topographic map (Figure 3), when the cold air from the northwest moves southward, it is restrained by the Taihang Mountains and the Yanshan Mountains and is concentrated in the narrow corridor from Zhangjiakou to Beijing. In particular, the wind is strong in areas such as Yanqing. During this cold wave, observation stations such as Yanqing Erhaituo experienced a 13-level gale. In cities, high-rise buildings are densely packed, and local strong gusts are easily formed through the narrow channel effect.

3. High-altitude booster - the second engine of strong wind

In addition to the steep pressure difference on the ground and the narrow terrain, this cold wave and strong wind also includes the confrontation between the high altitude and the ground. There is also an "invisible engine" at an altitude of more than 5km that is fueling the cold wave and strong wind. The driving force behind this cold wave and strong wind is the linkage between the cold vortex army moving southward from the Arctic and the westerly jet stream.

Vortex from the Arctic: Go forward, go forward, go forward!

Figure 4 Basic weather conditions of the 500hPa isobaric surface in the Eurasian region from February 3 to 6 (Source: China Meteorological Observatory, drawn by the author)

Almost all weather occurs in the troposphere. Strong winds are blowing on the ground. What happens in the middle and upper troposphere? From the weather map at 500hPa (about 5.5 km altitude) (Figure 4), we can see that in early February, there was a high pressure block (red circle) in Eurasia extending toward the Arctic. This monster-like shape squeezed the cold air in the Arctic and brought it down.

A cold vortex (bright circle) from the Arctic moved southward so quickly that by February 5, the cold vortex was divided into two. The cold vortex in the west was like a high-altitude "cold air bomb", carrying extremely cold air towards my country (Figure 4). The cold vortex continued to rotate while moving southward as a whole, like a spinning top. When it rotated at high speed, it threw a strong westerly airflow on its south side. Large-scale cold air in the high altitude was transported horizontally with strong airflows and poured into the North China region, which had already warmed up during the Spring Festival. The cold air sank heavier and the warm air rose, forming a strong vertical mixing movement, thus paving a transmission channel for cold air from high altitude to the ground. Comparing the wind speed at high altitude with the ground, the strengthening of the wind speed in the high-altitude jet stream caused the wind speed to increase sharply with altitude, and the vertical gradient change was also conducive to the downward transfer of momentum, further enhancing the strong wind on the ground. The vortex that was advancing rapidly and the westerly airflow that was strengthened in power filled the fuel tank for this cold wave and strong wind.

Knowledge card: A cold vortex refers to a cold vortex in the middle and upper layers of the troposphere thousands of meters above the ground. We can imagine it as a "cold air gyroscope" floating in the sky and rotating counterclockwise. When the lower layer is heated (such as afternoon solar radiation and ground temperature rise), it is easy to form an unstable stratification with a top-heavy vertical direction, which in turn triggers strong convective activities such as short-term heavy rainfall, thunderstorms, gale, and hail.

4. Cold wave breaks the mountain of prejudice

1. The Arctic polar vortex: It may be related to me, but don’t mistake it for the vortex

In the news about this cold snap and strong wind, the "Arctic vortex" frequently comes into everyone's view. As a giant cold storage in the sky, is it because its door is not closed tightly that affects this cold snap and strong wind? Can we simply equate the polar vortex with the cold snap?

Knowledge card: Why is the polar vortex called a giant cold storage in the sky?

The Arctic vortex is a persistent large low-pressure vortex system that is entrenched in the sky above the Arctic and surrounded by a strong westerly jet stream. We can think of the westerly jet stream as a high-speed rotating wall that traps the cold air in the Arctic. Therefore, the Arctic vortex is like a natural cold storage in the sky.

The specific location of the wall generally depends on the maximum wind speed, but the wall is not horizontal or vertical. From the troposphere to the stratosphere, the edge contour of the wall has obvious temporal and spatial discontinuities. Therefore, scientists believe that these are two independent vortices [1], namely the tropospheric polar vortex and the stratospheric polar vortex (Figure 5). They are two climate systems with different structures and characteristics (Table 1). Since there is still controversy in the academic community about the "tropospheric polar vortex" [2], we will only discuss the stratospheric polar vortex in the Arctic.

Figure 5 Schematic diagram of the polar vortex in the stratosphere and troposphere

(Image source: Reference 1)

Table 1 Differences between the stratospheric and tropospheric polar vortices (generated by DeepSeek)

As mentioned earlier, weather mainly occurs in the troposphere, but the abnormal stratospheric polar vortex does have the potential to affect the outbreak of tropospheric cold waves. When the stratospheric polar vortex becomes sick (for example, when there is an explosive warming event in the stratosphere, the originally stable wall begins to shake and the stratospheric polar vortex collapses), the diseased air will spread downward to infect the troposphere (through the complex stratosphere-troposphere coupling, causing abnormal atmospheric circulation in the troposphere), indirectly affecting the surface weather (Figure 6).

Figure 6 Schematic diagram of the Arctic vortex abnormal disturbance

(Image source: NOAA)

But this is not necessarily the case. For example, in early 2019, the weakening characteristics of the stratospheric polar vortex were difficult to transmit below 200hPa and could not affect the tropospheric circulation. The cold wave in the United States in early January this year remained relatively stable and normal because the bottom of the stratospheric polar vortex did not deform toward North America (Figure 7), which also reduced the suspicion of infection.

Figure 7 The three-dimensional structure of the stratospheric Arctic vortex on January 5, 2025 cold wave in the United States

(Image source: NOAA)

Figure 8 Changes in the Arctic vortex one week before this cold wave (10hPa)

(Image source: earth.nullschool)

It takes time for the “disease” to spread, usually about 7-10 days [6]. Looking back at the stratospheric polar vortex a week before this cold wave (Figure 8), the vortex structure was normal in the early stages, but gradually stretched out to both ends of Eurasia and North America, from an ellipse to a long and thin strip to a peanut shape (“8” shape), indicating that it became sick when the cold wave approached. Perhaps this cold wave event is related to this change.

At present, the causal relationship between the stratospheric polar vortex and weather processes such as cold waves has not been clarified, and more research is still needed to reveal its specific mechanism. Therefore, there is no need to take out thick clothes when you see the changes in the Arctic vortex. You should still follow the weather forecast. Since the splitting process of the polar vortex is very similar to the changes in the cold vortex mentioned above, I would like to speak up for the stratospheric polar vortex: Don’t mistake it for a vortex, we are not on the same level!

2. Global warming: I can be warm in nature but also cold in temper.

“Isn’t 2024 the hottest year?”, “Why is it so cold in a warm winter?”…

Unlike the global average warming trend, weather is the "emotional fluctuations" of the atmosphere on a short time scale, which is affected by many factors. Under the background of global warming, the global climate system has become more unstable and prone to turbulence, and the frequency and intensity of extreme weather and climate events have increased.

For example, as the world warms, the warming trend in the Arctic region far exceeds the global average, narrowing the temperature difference between the polar regions and mid-latitudes. At this time, disturbances in the westerly belt tend to develop rapidly, forming large fluctuations that span the mid- and high-latitudes from north to south and half the globe from east to west. Near the trough of the fluctuation, cold air from the polar regions and high-latitudes moves south, and blizzards and cold waves are the most severe; while near the ridge of the fluctuation, warm air from the mid- and low-latitudes can move directly into the high-latitudes and even into the polar regions, causing the temperature in the polar regions to exceed that in the mid-latitudes [7].

References:

[1] Waugh, DW, Sobel, AH, & Polvani, LM (2017). What Is the Polar Vortex and How Does It Influence Weather?. Bulletin of the American Meteorological Society, 98(1), 37-44. https://doi.org/10.1175/BAMS-D-15-00212.1

[2] Manney, GL, Butler, AH, Lawrence, ZD, Wargan, K., & Santee, ML (2022). What's in a name? On the use and significance of the term “polar vortex”. Geophysical Research Letters, 49, e2021GL097617. https://doi.org/10.1029/2021GL097617

[3] Understanding the Arctic polar vortex (https://www.climate.gov/news-features/understanding-climate/understanding-arctic-polar-vortex)

[4] The polar vortex is going to make you put on a sweater. Be afraid. Be very afraid. (https://www.climate.gov/news-features/blogs/enso/polar-vortex-going-make-you-put-sweater-be-afraid-be-very-afraid)

[5] Is the polar vortex causing the US cold air outbreak? (https://www.climate.gov/news-features/blogs/polar-vortex/polar-vortex-causing-us-cold-air-outbreak)

[6] Baldwin, MP, & Dunkerton, TJ (2001). Stratospheric harbingers of anomalous weather regimes. Science, 294, 581−584. https://doi.org/10.1126/science.1063315

[7] It’s been a warm winter, so why is the recent cold wave still so severe? (https://mp.weixin.qq.com/s/koZ3D4SPEA4ymQlI57eJhg)