"Autumn Tiger" takes over "Summer Heat", the situation of consecutive drought in summer and autumn in Jiangnan is not optimistic

Recently, the south has seen a lot of high temperatures, but the appearance of this "autumn tiger" is not unexpected. (For related predictions, please refer to "The High Temperatures of Midsummer Usher in the Last Madness")

Starting from September 5, high temperatures began to appear in various parts of the south in the middle and lower reaches of the Yangtze River. Judging from the number of high temperature days, the most significant areas are Jiangnan and northern South China, especially Hunan. In the past 20 days, most areas have experienced high temperatures of more than 35°C for 10 days.

Number of high temperature days nationwide since August 31

The highest temperature in China since August 31

In comparison, the high temperatures in Jiangbei were short-lived, and the number of high-temperature days in Hubei was less than 4 days. However, the highest temperature also reached 37℃, but there was no area above 40℃.

From the high temperatures in late August to early September, why did the "summer heat" turn into the "autumn tiger" after only a week's break?

The classic saying can be interpreted in terms of the progression of seasons. It can be simply concluded that the dog days of summer are over (this year, they start on August 25), and the subsequent heat is the autumn heat; or that June to August is summer, and September is autumn. There is nothing wrong with this, and this is the significance of the transition of seasons.

But in the eyes of forecasters, the difference between the two high temperatures is not just that, but has more significance.

The meteorological essence of "autumn heat" and "summer heat" is different

The biggest difference between the weather in the eyes of forecasters and the weather in the eyes of the public is that it not only includes the weather elements perceived by the public (such as temperature, humidity, clouds and rain, visibility, etc.), but also includes the atmospheric circulation conditions that cause these weather elements that the public is not aware of.

In terms of weather factors, if we only look at the temperature, there is almost no difference between the two, as both are high temperatures. Even though the temperature of this year's hot summer is much higher than that of the autumn heat due to its extreme nature, it is not enough to distinguish the two.

Although the maximum temperature is not enough to distinguish between the hot summer and the hot autumn, the daily temperature changes can actually be distinguished: the hot summer is also hot at night, while the hot autumn is much cooler at night.

In addition, the main weather factor that distinguishes the hot summer from the hot autumn is humidity. Simply put, the hot summer is stuffy/humid heat, while the hot autumn is dry heat.

Taking Hunan, where the autumn heatwave is most pronounced this year, as an example, we selected August 27 and September 5, two days with the highest temperatures closest to the hottest days (the highest temperatures were both around 37-38℃) for comparison:

Temperature (red) and relative humidity (white) at 15:00 on August 27, 2022

Temperature (red) and relative humidity (white) at 15:00 on September 5, 2022

The relative humidity on August 27 during the hottest days of the year was about 40%, while the relative humidity on September 5 during the hottest days of the year was about 20%. In fact, the relative humidity of about 40% in Hunan on August 27 was not very high, and was not a typical hottest day of the year, but it was still much hotter than the hottest days of the year.

The reason for this difference in humidity is that the air masses that control the two are different in nature: the hot summer days are actually controlled by oceanic air masses, while the hot autumn days are controlled by continental air masses. Traditionally, the former is often said to be warm and humid air flow, while the latter is cold air. But in this case, doesn't it mean that cold air brings hot autumn days?

Essentially, this is true, because when the cold air invades and moves southward, the high altitude sinks and the weather becomes sunny and sunny, and the dry and hot autumn weather is formed under the right conditions. The dry, wet, cold and warm properties of monsoons and air masses are actually quite complicated. To explain it in the simplest terms, for my country, when the continental air mass has an impact, it is represented by low-altitude northwest or northeast wind cold air, and dry and cold are the main properties; when the ocean air mass controls, it appears in the form of southwest or southeast summer monsoons, and warm and wet are the main properties.

Knowing this, the essential difference between the summer heat and the autumn tiger can be clearly seen from the low-level winds shown in the figure below.

850hPa wind field (about 1500 meters altitude) at 20:00 on August 27 (left) and 20:00 on September 5 (right)

The above description of the properties of the two air masses, the oceanic and continental, is actually only valid in winter, and is not strictly valid in other seasons. For example, in summer, at low and middle latitudes, the continental air mass is dry and hot, while the oceanic air mass is cold and wet. It is naturally more complicated when the seasons change. Therefore, the situation of "cold air causing autumn heat" may occur. This problem is more complicated because it involves many factors such as the degeneration of cold air, clear sky and sunshine under the control of cold air, sinking and warming under the control of cold air, etc., so I will not elaborate on it.

But no matter what, if we put aside temperature and talk about humidity, then the dryness of the continental air mass and the wetness of the maritime air mass are absolutely accurate. So, in summary, the difference between dryness and wetness is the essential difference between the summer heat and the autumn tiger.

Because of this, the following problems become prominent.

The situation of consecutive droughts in summer and autumn in Jiangnan region is not optimistic

The situation has been developing as predicted by the meteorological department in August. The end of the hot summer did not interrupt the meteorological drought in the south, and the arrival of the autumn tiger has made the situation more severe. The Central Meteorological Observatory has issued this year's drought warning for one month since its first release on August 18. The following figure is the meteorological drought monitoring map released yesterday afternoon:

National meteorological drought monitoring map on September 20, 2022 (National Climate Center)

The meteorological drought has lasted for more than a month, and the agricultural drought situation is not good either:

National meteorological drought monitoring map on September 20, 2022 (National Meteorological Center)

The agricultural meteorological forecast and suggestions given by the Central Meteorological Observatory are:

"In the next 10 days, rainfall will remain scarce in most areas of Jianghuai and Jiangnan, with rainfall generally less than 10 mm. Agricultural drought will continue or develop in some areas. Some areas in central and southern Jiangnan and southern China will experience high temperatures of 35 to 37 degrees Celsius, which is slightly unfavorable for the heading of late rice."

"The middle and lower reaches of the Yangtze River should still focus on drought prevention and control, scientifically dispatch water resources, seize the opportunity to do a good job in artificial rainfall, and ensure water for agricultural production; harvest mature autumn crops such as rice and cotton in time, and carry out rapeseed sowing and seedling raising work in a timely manner. The southwest region should seize the intervals between rainfall to speed up the autumn harvest, pay attention to ventilation and storage, prevent germination and mildew, and do a good job in autumn planting such as rapeseed sowing. The late rice producing areas in Jiangnan and South China should do a good job in water and fertilizer management in the late rice fields and monitoring and controlling pests and diseases, and adopt measures such as water temperature regulation and increased application of phosphorus and potassium fertilizers to promote the growth and development of late rice."

Agricultural meteorological focus from September 20 to 29, 2022 (National Meteorological Center)

Although the autumn heat wave in the south of the Yangtze River has temporarily moved southward to southern China today, it is expected that there will still be little rain in the south of the Yangtze River in late September, and there will be another wave of high temperatures in the middle and late part of the month.

In addition, according to the forecast of the National Climate Center, most of the areas of the above-mentioned meteorological drought will still have higher temperatures and less precipitation in early and mid-October.

Climate forecast bulletin released by the National Climate Center on September 19, 2022

To sum up, one thing is clear: in the next month, the drought in the meteorological drought areas, mainly in the Jiangnan region, will continue and may further develop.