The driving force behind polar cold waves: How does the jet stream shape the Earth’s climate and weather?

Tuchong Creative

The westerly jet stream is a powerful air current in the Earth's atmosphere. As global warming intensifies, its changes may have a profound impact on weather patterns, extreme weather events, and aviation. Hello everyone, I am Wei Ke. There are various weather phenomena in our atmosphere, one of which is rarely paid attention to by the public, but is very important, that is the westerly jet stream.

1. What is the jet stream?

The westerly jet stream is a strong and narrow airflow in the upper troposphere or stratosphere of the mid-latitude westerly belt. It is generally hundreds of kilometers wide horizontally, thousands of meters thick vertically, and thousands of kilometers long. It meanders from west to east around the hemisphere. Some areas of this westerly jet stream are called the westerly jet core area, where the westerly airflow can reach 100~150 m/s or above, which has exceeded the wind speed of the strongest typhoon and tornado on the surface.

The formation of the westerly jet stream is related to the rotation of the earth and the thermal difference of the atmosphere. The equatorial region is directly exposed to the sun, and the air is warmer and rises. After reaching high altitudes, it moves toward the poles. Due to the Coriolis force generated by the rotation of the earth, the airflow moving toward the poles deflects to the right in the northern hemisphere and to the left in the southern hemisphere. Further driven by heat and vortices, the wind speed increases, thus forming a westerly jet stream in the subtropical region. This is the subtropical jet stream. In addition, the temperature difference between the north and the south leads to a pressure difference, which further increases the wind speed.

In the 1920s, humans began to understand the westerly jet stream and developed the Rossby wave theory in the westerly belt, which became the basic theory of atmospheric dynamics and the theoretical basis of weather forecasting. In the late World War II, Japan used the westerly jet stream to carry out large-scale long-distance bombing. They made and launched about 9,300 balloons carrying bombs and incendiary bombs. The Japanese called them "wind ship bombs", which means balloon bombs. Each balloon is about 10 meters in diameter and carries 15 kilograms of high-explosive bombs and incendiary bombs. According to Japanese estimates, these balloons can reach the mainland of the United States within 3 days along the westerly jet stream, which can cause large-scale explosions and forest fires in the mainland of the United States, leading to panic. According to subsequent statistics, about 300 balloons arrived in North America, but due to the wartime US news blockade, Japan was unable to evaluate the effect of the action, and due to the shortage of resources, it ended the action hastily. Despite this, these balloons still caused some damage and casualties, but did not achieve the strategic effect expected by Japan.

2. The impact of the westerly jet stream on flight

The jet stream has a significant impact on flying. It can be a friend or a troublemaker to pilots. When an aircraft flies along the jet stream, the actual speed increases, thereby shortening the flight time and reducing fuel consumption. In early 2024, CBS News reported that some flights flying with the wind exceeded 800 miles per hour (1287.472 kilometers per hour), which is much faster than commercial flights, which usually fly at speeds of 500 miles per hour (804.67 kilometers per hour) to 600 miles per hour (965.604 kilometers per hour). This has enabled many flights to arrive dozens of minutes earlier. (1 mile is approximately equal to 1.60934 kilometers)

Of course, the plane has to fly back after it has flown there. If you fly in the westerly jet, the actual speed of the plane will be reduced, which will increase the flight time and fuel consumption. But the biggest trouble is that the airflow inside the westerly jet is not uniform, and sometimes strong turbulence and wind shear will form, which will cause the plane to bump, causing discomfort to passengers and even damage the aircraft structure.

Therefore, airlines and pilots need to plan routes based on the location and strength of the westerly jet stream to maximize tailwinds or avoid headwinds and turbulent areas.

3. The jet stream and climate change

Changes in the westerly jet stream can have a significant impact on local weather and climate. On the one hand, the intensity of the westerly winds will affect extreme weather in mid-latitudes. When the westerly jet stream is strong, it is easier to confine cold air in high latitudes and polar regions. In mid-latitudes, there is less cold air activity and the temperature is milder. When the westerly jet stream weakens, it is more likely to fluctuate, which may cause polar cold air to spread southward, triggering extreme weather events such as cold waves and blizzards in mid-latitudes. The existence of the westerly jet stream is like a spinning top. When the top spins faster, it is more stable, and when the top spins slower, it is more likely to fall to the ground.

On the other hand, the position of the jet stream also affects the global and local climate. When the North Atlantic and European westerly jet streams move northward, they bring warm, stormy and humid weather to Europe, but lack rainfall in areas south of latitude. However, when the jet stream in this area moves southward, it will cross the Atlantic Ocean directly into the Mediterranean region, bringing abundant heavy rains to these southern regions.

In the process of climate change, when the global temperature rises, the tropical regions expand towards the poles, and the westerly jet stream expands towards the poles. When the global temperature drops, the tropical regions shrink, and the westerly jet stream moves towards the equator. Michael Mann wrote in the book Fragile Moment that when the temperature in the ancient climate drops and the glaciers are more widely distributed, the expanding ice sheet drives the atmospheric jet stream to move towards the equator, cooling and aridifying the subtropical and tropical regions. Conversely, when the ice age ends and the interglacial period begins, global warming causes the atmospheric jet stream to move towards the poles, and the temperature and precipitation in many subtropical regions begin to increase, and vegetation begins to recover.

As global warming intensifies, temperatures in polar regions are rising faster than in low-latitude regions, a phenomenon known as "Arctic amplification," which can lead to a decrease in the temperature difference between the poles and the equator. This decrease in temperature difference may affect the strength and position of the westerly jet stream. Some studies have shown that as global warming intensifies, the westerly jet stream may become more unstable, leading to an increase in the frequency and intensity of extreme weather events. For example, fluctuations in the westerly jet stream may trigger more frequent southward movement of polar air masses, leading to severe cold weather in mid-latitudes. At the same time, changes in the westerly jet stream may also affect the transport of water vapor in the atmosphere, thereby affecting the distribution of precipitation modes.

In addition, in terms of aviation safety, studies have shown that wind shear associated with the westerly jet stream has continued to increase over the past few decades. For example, between 1979 and 2017, the wind shear intensity of the high-altitude jet stream in the North Atlantic region increased by 15%, which increased the duration and intensity of severe clear-air turbulence in the region. As global warming continues, this situation will become more serious. Climate model simulations show that when carbon dioxide concentrations double, the median intensity of high-altitude clear-air turbulence in the North Atlantic in winter will continue to increase by 10-40%, with severe turbulence increasing by 149%. East Asia is also the region with the most significant increase in clear-air turbulence frequency, which is not a good thing for the aviation industry.

In general, the westerly jet stream is a powerful air current in the Earth's atmosphere that has an important impact on weather and climate. As global warming intensifies, changes in the westerly jet stream may have a profound impact on weather patterns, extreme weather events, and aviation. Understanding and studying the westerly jet stream is crucial to predicting and responding to the challenges posed by climate change.

This article is a work supported by the Science Popularization China Creation Cultivation Program

Author: Wei Ke

Reviewer: Dai Yunwei, Senior Engineer, China Meteorological Administration

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.