Are we heading for the coldest winter yet?

The first snow in the second half of 2024 seems to have come earlier than before. As early as early September, many places in the north, such as Xinjiang, Inner Mongolia, and Heilongjiang, had snowfall, more than a month earlier than in previous years. The phenomenon of early first snow also occurred in Beijing. During the National Day holiday not long ago, small snowflakes were floating in the mountainous areas of Beijing. It is expected that this week, most parts of the country will also usher in two strong cold fronts, and the temperature will experience a "cliff-like" drop.

In mid-September, Xinjiang Hemu welcomed snowfall

(Photo source: Xinjiang Radio and Television)

On October 2, residual snow on Baihua Mountain in Mentougou District, Beijing

(Photo credit: Wei Ke) The early arrival of the first snow indicates a cold winter. Will the coming winter be colder and come earlier than before?

Part 1 Why is it so cold in winter? Where does the cold air come from?

Let's start with the mechanism of winter cooling. China is located in the middle latitudes as a whole, and most areas have four distinct seasons. The cold and hot conversion from summer to winter is due to the seasonal movement of wind belts and pressure belts caused by the movement of the direct point of the sun. In winter, the northwest wind brings strong cold air from Siberia into China and sweeps across China, causing a significant drop in temperature and strong winds. Contrary to what most people think, the real "nest" of cold air is not Siberia, but the Arctic. In the winter of the northern hemisphere, due to the low solar radiation received, cold high pressure is formed near the ground in the Arctic, and low pressure corresponds to high altitude. Under the action of the Coriolis force and mountainous terrain, a polar vortex is formed, which is the warehouse of cold air. When the vortex weakens, the air mass from the low latitude will squeeze the cold air in the north and move it southward. According to statistics, 95% of the cold air in Eurasia passes through the central Siberia (70°-90°E, 43°-65°N) area, where it accumulates and strengthens. We call it the "cold wave key area." After passing through the key cold wave areas, cold air usually enters my country through three typical routes: central, eastern or western. Under the influence of cold air again and again, the temperature in various parts of my country has dropped significantly, gradually entering the autumn and winter season. Each cooling process is often accompanied by rainfall, so there is a saying that "an autumn rain brings a chill."

(Photo source: China Meteorological Administration Meteorological Publicity and Science Center) In terms of time, cold air usually starts to "strike force" in September. From the path of the cold air, we can see that the northwest region of my country is the first to welcome the arrival of cold air, so snowfall in September is not uncommon, but this year's snowfall is a little earlier than usual. For Hebei, Inner Mongolia and Northeast China, although snowfall in September is not common, it is also a normal phenomenon. When the cold air is strong and the 0℃ layer of the atmosphere is close to the ground, snowfall may occur.

Part.2 The first snow comes early, will this winter be colder?

We often say that the winter of a certain year is a warm winter or a cold winter, based on personal feelings or abnormal weather in a few days. In fact, my country has clear national standards for "cold winter" and "warm winter" (GB/T 21983-2020 Warm Winter Grade and GB/T 33675-2017 Cold Winter Grade), which are mainly defined by the average temperature of the three months of winter across the country, and are judged by the difference (anomaly) and standard deviation (statistic describing the deviation of the sequence from its average condition) between the average temperature of the winter of that year and the average temperature of many years. In terms of space, it is divided into two major ranges: single station and national. The national warm winter depends on the proportion of warm winters at a single station.

National standards for determining cold/warm winters

(Photo source: China Meteorological News Agency)

As for whether the coming winter will be colder, this is a short-term climate forecast, which is very difficult to predict. It is not possible to draw a conclusion based on the early or late first snow. Whether the winter is a "warm winter" or a "cold winter" is affected by many factors such as the El Nino-Southern Oscillation (ENSO) and the Arctic Oscillation (AO). Among them, the most concerned is the ENSO phenomenon with the strongest interannual impact. ENSO refers to the periodic climate fluctuation phenomenon in the tropical Pacific region, which is divided into two types: El Nino and La Nina. El Nino refers to the phenomenon of abnormally high sea surface temperature in the central and eastern equatorial Pacific, which usually lasts for several months to about a year. It has a significant impact on global climate and weather patterns, especially in the Pacific Rim. During the El Nino period, the abnormally high sea temperature in the central and eastern Pacific usually leads to global warming. For China, El Nino generally makes the winter tend to be warmer, especially in the southern region, causing rising temperatures and reduced precipitation. The main feature of the La Nina phenomenon is the abnormally low sea surface temperature in the central and eastern equatorial Pacific, which often leads to global cooling. La Niña usually makes China's winter colder, especially in the northern region, where lower temperatures and more snowfall may occur. Therefore, forecasting the development and evolution of ENSO is very important for predicting China's winter temperature. In the face of climate anomalies, we can closely observe changes in sea temperature and use various algorithms such as statistical models, climate models and artificial intelligence to forecast ENSO. According to the latest report released in October by the International Climate Research Institute (IRI) of Columbia University in the United States, it is expected that there is a 60% chance that La Niña will occur in October 2024 and will last until the end of spring 2025. Therefore, this winter may be more likely to be a "cold winter" than in previous years.

Causes and impacts of La Nina and El Nino

(Image source: NOAA)

Part.4 Why is the winter colder than before despite global warming?

This brings up a new question that everyone is concerned about: Isn't it global warming? How can there be a "cold winter"? In fact, "global warming" is a long-term climate concept, while "cold winter" and "warm winter" describe climate states on a smaller time scale. The former is a long-term, average level change, referring to the warming of the global average temperature; while the latter is a change between years. As mentioned above, "cold winter" and "warm winter" have multiple scales in space. This definition is not as easy as everyone imagines, using one word to define the cold and warm conditions of the whole of China. In the whole Chinese region, the cold and warm conditions of different regions in the same winter are often different. For example, the winter of 2022-2023 showed an uneven situation of colder north and warmer south; from a temporal point of view, even in the same winter, the previous winter and the next winter may also show different cold and warm conditions due to changes in weather systems and atmospheric circulation. Our conclusions are meaningful to science and society only when we describe the average cold and warm conditions of weather at multiple time and space scales. Therefore, "global warming" and "cold winter" and "warm winter" belong to climate concepts of different time scales, and the two do not conflict. In addition to the "cold winter", you may often hear descriptions such as "once-in-a-century cold wave". This extreme cold event does not conflict with global warming, because it is just a fluctuation in the temperature of the weather within a few days, and the time scale is shorter. So, is there any relationship between global warming and extreme cold events? To answer this question, we still have to emphasize the concept of "unevenness". Although evidence shows that the frequency, intensity and duration of extreme cold events have decreased significantly with global warming, the frequency of occurrence of extreme cold events of different intensities varies. In the Northern Hemisphere, the frequency and duration of weak cold events have decreased significantly, but the changes in strong cold events are not obvious, and even show an increasing trend in Siberia and Canada. Therefore, it is not surprising that extreme cold events occur at the same time as global warming.

Changes in the frequency (a), duration (b), maximum intensity (c), projected area (d), average intensity (e), average area (f), total magnitude (g) and total area (h) of extreme cold events from 1959 to 2020. (Image source: Reference [1])

So how will China's extreme cold events change? Studies have shown that the frequency, intensity and duration of extreme cold events in China over the past century have been declining, but in different regions, "cold events" often occur. For example, the extreme cold event in the north in 2023 can be called the "most" since 1959. The regional average temperature in North China, Huanghuai, Northeast China and Inner Mongolia is the lowest in the same period since 1961. In mid-December, the lowest temperature at 78 national meteorological observation stations across the country fell below the historical extreme value in December, and Beijing experienced the longest continuous low temperature since 1951. Weather phenomena are closely related to both society and individuals in our lives. For the country and the government, predicting the "warm winter" and "cold winter" phenomena can have a macro and overall grasp of weather phenomena, so as to introduce climate policies that are convenient and beneficial to the people. For us personally, weather fluctuations and warning information at our doorstep are more important than the overall cold or warm winter. Whether the winter of 2024 will usher in a "cold winter" is affected by many complex factors. But when the existing forecast information reminds us that a cold winter may be coming, I hope everyone will pay more attention to the warning information, wear warm clothes and cover themselves with enough quilts, and don't catch a cold!

References:

[1] He, Y., Wang, X., Zhang, B. et al. Contrast responses of strong and weak winter extreme cold events in the Northern Hemisphere to global warming. Clim Dyn 61, 4533–4550 (2023). https://doi.org/10.1007/s00382-023-06822-7

[2] Wang Bingjie, Sun Ying, Hu Ting, et al. Changes of extreme cold events in China over the past century based on reanalysis data[J]. Advances in Climate Change Research, 2023, 19(04): 403-417.

Source: Science Institute