This summer, not only my country’s Jiangnan, South China and other places suffered from prolonged high temperature weather, but many places in the northern hemisphere were also attacked by abnormally high temperature weather. The United Kingdom, France and South Korea issued high temperature warnings, and many places in Japan and the United States broke records. What is the meteorological cause behind this extreme weather? Why have heat waves become more common in recent years?
Direct cause 1: downdraft
There are various views that the reasons for the abnormally high temperature in the northern hemisphere are diverse, but the abnormal atmospheric circulation must be one of the important factors.
During this time, a rare global warm high pressure belt appeared in the northern hemisphere. According to the analysis of the National Climate Center, since June, over the subtropical region of the northern hemisphere, the subtropical high pressure in the western Pacific, the Atlantic high pressure and the Iranian high have all strengthened in stages, thus forming a large-scale global warm high pressure belt. Under the control of the warm high pressure zone, downdraft air becomes prevalent.
What are the effects of the presence of downdrafts? We all know that with the increase of altitude, the temperature shows a downward trend. Generally, the temperature decreases by 0.6 °C for every 100 meters of increase. Therefore, when an air mass continues to rise under the action of external force, its temperature continues to decrease, so that the water vapor inside it can condense into clouds and produce rainfall. In the same way, when the air mass is sinking, the temperature inside it will continue to rise, and it is not easy to form clouds at the same time. These characteristics are conducive to ground warming, and the reduction of clouds also makes it easier for solar radiation to reach the ground, resulting in frequent and strong high temperatures.
Direct Cause 2: The “Hot Dome” Phenomenon
Many people remember that as early as 2021, there was a super heat wave event in western North America. In recent days, Wang Chun, a researcher at the South China Sea Institute of Oceanography, Chinese Academy of Sciences, published a research analysis on last year’s super heat wave event in western North America. This latest research result clarifies the relationship between the “heat dome” and super heat wave events from the perspective of the physical mechanism.
”Hot dome” refers to the fact that the thermal high pressure in the upper atmosphere is stagnant for a period of time, and the atmospheric circulation between the high pressure and the nearby low pressure forms a stable “Ω” shape. At the same time, cold air is prevented from entering, making the temperature inside the “hot dome” higher and higher, which can lead to a heat wave or super heat wave event.
How is such an “Ω” shaped cover formed? Wang Chun pointed out that after the low-level air near the ground is heated by the ground, it rises in the “hot dome”, but because the upper layer is high pressure, it is pressed back to the ground by the high pressure of the upper layer. This cycle causes the temperature in the hot dome to increase. Come higher. At the same time, the stable “Ω”-shaped atmospheric circulation prevents cold air from entering the hot dome, and the temperature inside the hot dome cannot be lowered, thus triggering a continuous super heat wave event under the hot dome.
Root Cause: Climate Change
If the anomaly of the atmospheric circulation is contingent to a certain extent, then the long-term climate change is the fundamental reason for the frequent occurrence of heat waves.
Past meteorological data are the best evidence for the real existence of climate change. The Sixth Assessment Report on Climate Change (IPCC AR6) recently released by the Intergovernmental Panel on Climate Change pointed out that compared with the 1850-1900 and 2001-2020 average global surface temperature increases by 0.99 (0.84-1.10) °C.
Analysis of the causes of temperature rise from 1850 to 2020 (the black line represents the temperature change curve under the combined influence of natural and human factors, and the blue line is the temperature change curve under the influence of natural factors alone) Image source: IPCC “Climate Change 2021: Natural The World Meteorological Organization said on the 19th of this month that the high temperature weather that swept Europe this time may become a “standard” for European summer in the future. “Heat waves will occur more and more frequently in the coming decades. Negative trends caused by climate change will continue until at least 2060, and people are slowing climate change,” WMO Secretary-General Petri Taalas said in the release. What has been achieved will not change this trend.” The World Meteorological Organization expects the global trend of glacier loss to continue for hundreds of years or more.
Scientifically speaking, the main ways that can cause climate change can be roughly divided into three categories: natural internal variability, natural external forcing (such as the sun, volcanoes) and man-made external forcing. In the long run, human activities dominated by greenhouse gas emissions are not only the main reason for the daily temperature increase, but also greatly increase the probability of similar heat wave events.
Carbon dioxide emitted by human activities is an important greenhouse gas. A large amount of carbon dioxide is like a quilt on the earth, locking more solar heat irradiating the surface in the near-surface layer. Faced with increasingly severe heat waves, it is imperative for all mankind to take effective countermeasures. Among them, a very important and effective measure is to reduce carbon dioxide emissions.
Compared with the randomness of the effects of natural variability, the effects of human activities are relatively deterministic. Therefore, in the face of the threat of climate change, human beings should do something.