Floods in the south and droughts in the north, why is the weather so strange this year?

In the past month, heavy rainfall occurred in many places in South China and Jiangnan, causing serious floods in Fujian, Guangdong, Guangxi, Hunan, Anhui, etc. At the same time, many places in North China and Central Plains suffered large-scale high temperature heat waves, causing serious droughts, with parts of southern North China, northern and western Henan, southwestern Shanxi, and northern Shandong reaching 40 to 42 degrees Celsius.

What is going on with this extreme weather? Is there a connection between floods in the south and droughts in the north? Will this kind of extreme weather become more frequent in the future?

Distribution map of total precipitation in China since June 2024 (unit: mm). Image source: China National Climate Center

Since June 2024, the national temperature has deviated from the normal average for the same period (unit: ℃). It can be seen that many places in North China and Central China are significantly higher. Image source: China National Climate Center

Heavy rain in the south

What is the direct cause?

The most direct and important reason for the large-scale and continuous heavy rainfall in the south is the strong and southerly subtropical high pressure in the northwest Pacific. A large amount of water vapor carried by the summer monsoon from the tropical Indian Ocean and Pacific Ocean is further transported to the southern China region through the southerly wind and southwesterly wind on the west and northwest edges of the subtropical high pressure. When the water vapor meets the dry and cold air mass in the north, large-scale rainfall occurs.

More specifically, due to the persistent and significantly stronger subtropical high pressure, not only the water vapor transport volume is further increased, but also the cold air and warm and humid air currents are stably converged on its north side. This results in not only strong rainfall but also long-term lingering in a certain area, causing extreme cumulative precipitation and flood disasters.

Since June 17, the subtropical high pressure in the northwest Pacific has extended westward and northward to a certain extent. Correspondingly, the heavy rainfall belt has also moved northward to a certain extent - from the Nanling Mountains and Fujian mountainous areas to the middle and lower reaches of the Yangtze River coastal areas, but it is still quite strong, and has further caused heavy rainfall in southern Anhui and other places.

In addition, mountainous areas are widespread in the south. When warm and humid air currents encounter complex mountainous terrain, effects such as terrain uplift will occur, leading to further intensification of local rainfall. This is also one of the important reasons for the recent mountain torrents and geological disasters in many places.

High temperature process in North China and Central Plains

What is the direct cause?

The high temperature heat wave process in North China and Central Plains is concentrated in June and the first half of July. The cause is mostly the warm high pressure (ridge) in the middle and upper troposphere or the northwest airflow in front. When in this situation, there will be sinking movement and clear, less-cloudy weather. Not only can the ground absorb a large amount of solar radiation and warm up, but the adiabatic warming during sinking and compression can further develop and strengthen the dry hot air mass.

The high temperature events since early June this year also conform to the above situation. If we analyze the period with the most concentrated high temperatures from June 9 to 15, we will find that in Northwest my country, North China, Central Plains and other places, there was a significant downward airflow in front of the strong warm high pressure in the middle troposphere from Central Asia to the Ural Mountains, and a significant warm air mass appeared in the lower troposphere close to the ground in North China, causing this high temperature process.

In addition, urban heat islands have some topographic effects. For example, when the northwest airflow passes over the Taihang Mountains, the North China Plain further sinks and warms on the leeward slope, which also intensifies the high temperature heat wave.

This year's floods in the south

Is it related to the high temperature heat wave in North China and Central Plains?

Is there any connection between the huge difference between the "floods in the south" and the "drought in the north" this year? Normally, this weather pattern is mainly affected by the seasonal distribution characteristics of atmospheric circulation, which is the case all year round. But the difference this year is that the El Nino event from summer 2023 to spring 2024 also affects this year's climate characteristics.

The El Niño event is not only a phenomenon in which the sea surface temperature in the equatorial central and eastern Pacific is significantly higher (more than 0.5°C higher for 5 consecutive months), but this warmer sea water will further affect the atmospheric circulation and more distant oceans, becoming one of the most important factors affecting the global climate over a period of months to years.

Although the El Niño event has shown a significant weakening since this spring and has now basically ended, my country was most significantly affected by its subsequent impacts when the El Niño event was weakening. For example, it caused warming in the tropical Atlantic and Indian Oceans, which, through many mechanisms, caused the subtropical high pressure in the northwest Pacific to be significantly stronger and more southerly. The warmer tropical Indian Ocean also increased ocean evaporation and water vapor, which together led to heavy rainfall in many parts of the south.

At the same time, due to the southward movement of the subtropical high pressure in the northwest Pacific, the rainfall belt where cold and warm weather meet on its north side also moved significantly southward, staying in the south for a longer time, making the north more susceptible to the influence of the warm high-pressure ridge in the northern westerly belt. This not only caused multiple high-temperature heat waves, but also caused long-term low rainfall in many places in North China and Central Plains, resulting in obvious drought.

Is global climate change related to

Is it related to the increase in extreme weather and climate events?

Since the beginning of this century, the global average surface temperature has risen significantly and continuously refreshed the historical records of observations. Climate change represented by this has increasingly entered the public eye. Is global climate change also related to such extreme weather and climate events?

The answer is yes. The impact of global climate change does not necessarily play a role in single heat waves or extreme rainstorm events. However, based on observations and model simulations in recent decades, it is believed that the overall characteristics of increasing intensity and frequency of extreme events are closely related to climate change.

The impact of climate change on the world is not only the rise in average temperature, but also the significant increase in the extreme deviation of temperature from normal (statistically measured by variance/standard deviation); and according to the Sixth Assessment Report conducted by the Intergovernmental Panel on Climate Change (IPCC) of the United Nations in 2021, with the current global average temperature being 1°C higher than in the second half of the 19th century, the frequency of heat wave events is already about 2.8 times that of the second half of the 19th century; and if it warms further by 0.5°C, the frequency will be 4.1 times that of the second half of the 19th century.

The IPCC Sixth Assessment Report mentioned the changes in the frequency and intensity of high temperature heat waves in 1850-1900 (baseline), the current (1°C above the benchmark) and different possible scenarios in the future. Image source: IPCC

Although we cannot accurately predict the region, duration and extent of each flood and heat wave in the coming decades, it is clear that under such climate change, the overall frequency of such extreme heat and precipitation events will increase in the world in the coming decades, and the degree of extremeness will also increase significantly, which will have a significant impact and even threaten more people. Faced with such a broken future, we have no choice but to stay out of it or avoid it, and we can only face it together.

Author: Fengyun Mengyuan, PhD student of Department of Atmospheric and Oceanic Sciences, Fudan University

Review丨Zhang Tao, Chief Forecaster of China Central Meteorological Observatory (National Meteorological Center)