"Autumn comes overnight", the temperature drop in Chongqing is heart-pounding!

On the eve of National Day, Chongqing finally experienced a drop in temperature after a long summer. However, this drop in temperature was a big dive, with the temperature dropping from 35℃ to 16℃ overnight, almost overnight from midsummer to late autumn. Foreign friends who came to Chongqing for National Day in advance may think that such weather is magical, but Chongqing people have gotten used to it over the years and always joke that "diving-style temperature drop is very Chongqing!"

Why does Chongqing always experience autumn overnight? Why does the temperature always plummet?

First of all, let's talk about this year's situation. When talking about this strong temperature drop, we have to talk about this year's high temperature. This year, Chongqing has experienced a record-breaking long and hot summer season.

Before mid-August, the high temperature in Chongqing this year was relatively weak compared with previous years. However, when the beginning of the end of summer came, the high temperature and drought in Chongqing began to take effect. This summer, the representative of Chongqing meteorological stations, Shapingba National Basic Meteorological Station, had 19 days with temperatures reaching 40°C, exceeding that in 2006 and second only to 2022.

The total number of high-temperature days in summer has accumulated to 77 days, which not only broke Chongqing's own annual high-temperature day record in 2011, but also broke the annual high-temperature day record of provincial capital cities in my country held by Fuzhou. The average monthly temperature in Shapingba in September this year was 32℃, setting a record for the highest average temperature in September in the country's history. The comprehensive intensity of Chongqing's summer high temperature this year is second only to that in 2022, but the duration exceeds that in 2022. Such a strong high temperature has laid a solid foundation for the big drop in temperature.

To end this severe high temperature, very strong cold air is needed to completely expel the hot air from Chongqing, and the cold air that moved south at the end of September this year came with this mission.

First of all, its path is very conducive to cooling down Chongqing: it is authentic western cold air.

Generally speaking, the cold air entering our country is divided into western, central and eastern routes. Chongqing belongs to the western region. Compared with the cold air from other routes, the cold air from the west can best break through the barrier of Qinling Mountains and Daba Mountains, bringing significant impact to Chongqing.

The main paths of cold air invading my country | Source: Meteorological Publicity and Science Popularization Center of China Meteorological Administration

Secondly, this cold front is extremely strong, almost the strongest force that can be dispatched from the Arctic region at present.

This year's cold air came from the cold air source 1 in the above picture, that is, the Barents Sea-Novaya Island area in the Arctic Ocean. As early as September 22-24, when Chongqing was still in high temperature weather controlled by the subtropical high pressure, the cold vortex in the Barents Sea began to develop and led to the gathering of polar and Ob River cold air masses in the key cold wave area of ​​western Siberia. This process brought in most of the cold air in Siberia.

At the same time, the European high-pressure warm ridge moved eastward, and with the replacement of troughs and ridges of the Rossby wave (the trough is called a trough, and the peak is called a ridge) in the westerly belt, the Ural Mountains warm ridge was formed in the Ural Mountains. After the 25th, the cold vortex degenerated into a southwest-northeast trough during the critical period of the cold wave. After the 28th, the Ural barrier collapsed, and the trough moved eastward and turned vertical, guiding a large number of cold air troops to march southward into my country.

Therefore, given the high base temperature, the western path of the cold air and its strong intensity, a sharp drop in temperature in Chongqing is foreseeable.

Image source: provided by the author

Because of the drastic drop in temperature, Chongqing is always ridiculed as "having no autumn, only summer and winter", but it is still a long time away from the actual winter (average daily temperature below 10℃).

However, Chongqing's "fall with a cliff-like temperature drop" has a certain pattern. For example, in 2021 and 2022, the temperature first rose during the National Day holiday and then plummeted, and around September 20, 2023, the temperature dropped from over 30℃ to 20℃ overnight. So why does this happen almost every year now?

First of all, this is determined by the transition from the summer dry season to the autumn rainy season in Chongqing. From mid-to-late July to early to mid-September every year is the summer dry season in Chongqing. During this period, Chongqing is easily controlled by the subtropical high pressure in the northwest Pacific, resulting in continuous high temperature weather.

The autumn rain season in Chongqing is from mid-to-late September to October every year. During this period, Chongqing is in the low pressure area between the Northwest Pacific Subtropical High Pressure and the Iranian High Pressure. The southwesterly airflow on the west or northwest side of the Northwest Pacific Subtropical High Pressure continuously transports warm and humid air from the South China Sea and the Indian Ocean. At the same time, the cold air activity in the north begins to strengthen, and the cold and warm air frequently meet, forming the Huaxi Autumn Rain here. Chongqing is one of the cities that is most significantly affected by the Huaxi Autumn Rain.

The characteristic of Huaxi Autumn Rain is continuous autumn rain. Continuous rainy days will keep the temperature at a relatively low level, which is in sharp contrast to the high temperature and drought season in the previous period. This change of seasons is very sudden, and it often takes a strong cold air to drive the subtropical high pressure out of Chongqing. At this time, the temperature drop is very drastic and obvious.

Second, the Sichuan Basin has a lot of heat accumulation, so slow cooling won’t work very well there. We know that the Sichuan Basin is protected by the Daba Mountains and the Qinling Mountains, and is flanked by the Yunnan-Guizhou Plateau to the south, which not only makes it difficult for cold air from the north to enter, but also makes it difficult for the high temperature and heat to recede in summer. Seasonal changes are all done by cooling once and for all.

The third point is global warming. Global warming makes summers longer and hotter. In recent years, not only have high temperatures become more severe, but the average temperature in autumn and winter, when there is no high temperature, has also been rising. However, the frequency of cold waves has not decreased despite the increase in average temperature (this is mainly because global warming has led to more intense cold and warm exchanges in the polar and low-latitude regions). This has led to higher temperatures when there are no cold waves. The base temperature is high, and the temperature drops more dramatically when cold waves or cold air come, so we feel that the temperature is dropping drastically.

In short, in the future of climate change, we may experience more such cliff-like temperature drops. Everyone remember to add clothes in time to keep warm and avoid catching a cold!

Source: Chongqing Science and Technology Museum

Author: Deng Feng, popular science author

Audit expert: Li Chunli

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