Cold winter vs. warm winter, should you wear thicker or thinner clothes this winter? Read this article!

This year, my country experienced the hottest summer since 1961. After such a hottest summer in history, two predictions for this winter's temperature appeared on the Internet:

Prediction 1: This summer was so hot, so the winter will probably be very cold, so it will be a cold winter;

Prediction 2: This summer’s heat wave will continue into winter, so it will be a warm winter.

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So which of these two predictions is true? Can we infer the winter climate trend based on summer temperatures? To truly understand the winter climate, we must first understand what climate prediction is, how to make climate predictions, and the limitations of climate predictions.

01

How to predict the climate?

The weather forecast and climate prediction released by the China Meteorological Administration are the most authoritative scientific forecasts and predictions in our country. Major universities, research institutes, ministries, enterprises, etc. also have personnel majoring in meteorology engaged in science-based forecasting work.

The process and principles of scientific weather forecasting are very complex . They involve not only the physical chemistry of the Earth's atmosphere, but also the physical chemistry of factors that affect the atmosphere, such as the sun, rocks, oceans, organisms and even human activities. They also involve the means of detecting these physical and chemical properties, such as mechanics, electronics, communications and computer science, etc. Today's weather forecasts are the result of human scientific and technological development in all these fields.

It is also important to understand that there are many factors and aspects that affect temperature and climate, such as:

Physical reasons that directly affect the ground temperature: such as solar radiation, geothermal radiation, cold air inflow, and sinking of high-altitude airflow, all of which directly affect the ground temperature;

Atmospheric circulation: Subtropical high pressure, Siberian cold high pressure, etc., these are weather systems in the atmospheric circulation, they will indirectly affect the temperature;

Factors outside the atmospheric system: External forcing factors of the climate system such as La Nina and snow accumulation on the Qinghai-Tibet Plateau will affect atmospheric circulation and indirectly affect temperature through the long-term accumulated slow-changing effect. Therefore, temperature and climate prediction is a very complicated matter, and it is impossible to infer the future climate trend based on a single factor .

02

Modern scientific weather forecast

Weather forecasting has existed since ancient times, but the history of weather forecasting based on modern meteorological science is very short, only a hundred years or so. The history of more accurate weather forecasting, marked by numerical weather forecasting, is only a few decades old. The scientific principles and technical means of modern weather forecasting are mainly reflected in the following aspects.

1. Define weather

Weather phenomena such as wind, clouds, rain and snow are produced in the Earth's troposphere, mainly the result of various complex changes in the physical and chemical properties of the tropospheric atmosphere. If you want to predict the weather, you must first understand it, which requires observing or detecting the various properties of the atmosphere. The properties of the atmosphere are many and complex, and the more important ones are air quality (density or air pressure), internal energy (temperature), movement (wind field) and the amount of water vapor contained in the air (humidity). Only by scientifically measuring these properties can we have a more accurate forecast of the weather .

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2. Recognize the rules

With the description and observation of the properties of the atmosphere, scientific analysis will reveal that the various properties of the atmosphere have some characteristics in time and space, which is the atmospheric circulation . Atmospheric circulation basically determines all weather, and other factors contained in the atmosphere, such as the land, ocean, human activities, and external factors such as solar radiation, will determine and affect the characteristics of atmospheric circulation.

500hPa isobaric weather map used to describe atmospheric properties

As for how atmospheric circulation determines the weather, how is it formed and changes? These all need to comply with the laws of physics. After hundreds of years of efforts, meteorological science has gradually figured out the various links and various causal relationships. Yes, it is only a rough approximation, because it involves a lot of inevitable simplifications , such as ignoring the impact of air vibrations such as sound on the weather, which cannot be calculated, or it is meaningless to calculate.

3. Calculate future changes in atmospheric circulation

Science has learned everything from atmospheric properties to weather, and then simulated its future changes. In the past, this was mainly based on the scientific knowledge and experience of forecasters, and in modern times, it is based on numerical weather forecasting .

Numerical weather forecasting uses high-performance computers to input the current state of the atmosphere into a set of mathematical and physical equations that can describe the laws of atmospheric operation, and then solve the future state of the atmosphere to obtain various physical elements of the atmosphere, such as wind, air pressure, temperature, humidity, etc., and then calculate various weather phenomena, such as rain and snow, visibility, and sky cloud cover.

Overview of the numerical forecasting business system of the China Meteorological Administration

03

The limitations of modern weather forecasting

After hundreds of years of efforts, modern weather forecasting capabilities are still limited , which is directly reflected in the fact that the longest weather forecast on our mobile weather apps is generally only 15 days. And you will find that the weather on the 15th day you see today may be very different from the weather when the 15th day actually arrives.

This is the limitation of modern weather forecasting.

Since the input data in the weather forecast calculation is approximate, it is also processed approximately, there are errors that cannot be eliminated, and the numerical forecast output results are reprocessed using objective forecasting methods (AI is one of them) and subjective forecasting experience, the final weather forecast results are only approximate simulations of the future and cannot be completely accurate. And this approximate simulation will increase as the forecast needs become more sophisticated and as the forecast time is extended .

Such a forecasting system results in weather forecasts being time-sensitive and unable to provide accurate forecasts at fixed times, fixed locations and fixed quantities. It can only provide forecasts for approximate time and geographical ranges. Generally speaking: it is basically reliable within 3 days, highly referenceable within 10 days, not very reliable beyond 10 days, and absolutely impossible beyond 20 days .

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04

Weather Forecast and Climate Prediction

Since the weather beyond 10 days can basically be classified as unreliable, what about the seasonal "weather" forecasts released by the Meteorological Bureau?

In fact, the "weather" that the public understands is divided into two situations in the field of meteorological science: weather and climate . They are related to each other but different. You can feel it from the following three points:

In terms of external manifestations: weather is the atmospheric conditions at a certain moment and the instantaneous result of its short-term evolution; climate refers to the average atmospheric conditions over a period of time and the average result after a longer period of evolution;

In terms of information acquisition or expression: weather is the meteorological element detected by various instruments, and climate is the result of weather calculation;

For example, weather refers to whether it is windy and rainy and the temperature on a certain day. Climate refers to the overall rainfall over a period of time, as well as the deviation from the average of many years or places, and whether the temperature is lower or higher. Therefore, it is actually impossible to predict the weather on a specific day in the long term, but it is possible to predict the climate state .

For example, let's assume that we can predict the climate conditions in October with 100% accuracy in August. We know in advance the actual deviation of the temperature in October from the average conditions as shown in the figure below. We can find that except for North China and Northeast China, where the temperature does not change much, the temperature in other regions will be slightly higher.

Actual temperature anomalies in October 2022

But even so, we still could not predict that Henan, Hubei, Anhui and other places would experience such a rare cold wave and cooling process during the National Day holiday.

The largest temperature drop in the country from September 30 to October 5

It can be seen that weather is a rapid change in atmospheric properties . When predicting such a rapid change process, slow-changing factors such as changes in sea surface temperature in the equatorial eastern Pacific (La Nina for colder weather and El Nino for warmer weather) or global carbon dioxide concentration can be ignored. However, for long-term climate conditions, such as a few months later, the cumulative impact of sea temperature may play an important role.

In addition to the sea temperature in the equatorial eastern Pacific, there are many other factors that also play an important role in the long-term state of atmospheric circulation, such as the sea temperature in the Indian Ocean, the glacier and snow on the Qinghai-Tibet Plateau, changes in the sea ice range in the Arctic, and even the burping and farting of Australian cattle (affecting the methane content, which is a more ferocious greenhouse gas than carbon dioxide), etc., all of which will affect the atmospheric circulation and thus affect the climate.

Methane produced by cows' farts can have an impact on the climate.

Even if we do not consider all these so-called "external forcing" factors of the atmosphere, the dynamic evolution of the atmospheric circulation itself is also very complicated. This makes climate prediction more difficult than weather forecasting, and its uncertainty is greater than that of weather forecasting .

It can be seen that using scientific data analysis to forecast weather and climate is a very complicated matter. Therefore, the statement that "summer temperature is high" and "predicts" that winter will be cold or warm is more of a "rumor" than a "prediction" .

05

In the final analysis

What will the temperature be like this winter?

In fact, the National Climate Center of the China Meteorological Administration has recently made a forecast for this winter's temperature: this winter's temperature in most parts of the country will be close to or slightly higher than normal, with obvious stage characteristics of temperature changes, with the first winter being warmer and the second winter being colder . So it is neither a cold winter nor a warm winter.

According to the forecast of the National Climate Center, the temperature in various regions of my country this winter is as follows:

The temperature in the eastern and western Inner Mongolia, northern Northeast China, most of South China, southeastern Southwest China, and central and eastern Northwest China is lower than the same period of previous years. The temperature in northeastern Inner Mongolia, Heilongjiang, western Shaanxi, central and eastern Gansu, northeastern Qinghai, and Ningxia is 1-2℃ lower .

The temperature in other parts of the country is close to or slightly higher than the same period of previous years. Among them, Shanghai, Jiangsu, Zhejiang, Anhui, northeastern Jiangxi, Shandong, eastern Henan, northeastern Hubei and other places are 1 to 2 degrees Celsius higher .

Average temperature anomaly forecast from December 2022 to February 2023, image source: National Climate Center

The stage characteristics of winter temperature changes are obvious :

From December 2022 to mid-January 2023, the intensity of the cold air affecting my country is relatively weak, and the temperature in most parts of the country is higher than the same period of previous years;

From late January to February 2023, the intensity of cold air gradually increased, and the temperature in most parts of Inner Mongolia, most parts of Northeast China, northern North China, Central China, western South China, eastern and northern Southwest China, and most parts of Northwest China was lower than the same period of previous years. Even the National Climate Center only made a "forecast" for this winter's temperature, which shows how great the uncertainty of climate forecasts is.

For questions with simple answers such as "cold winter", "warm winter", or "both cold and warm winter", there is a high probability that you will guess correctly, but compared to the results, the process of scientifically forecasting weather and climate is more important .

Author: Zhang Tao, Chief Forecaster of China National Meteorological Center

Review | Wei Ke, Associate Researcher, Institute of Atmospheric Physics, Chinese Academy of Sciences

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