Recently, many people have discussed that this year is a cold winter. The reason is that the sea surface temperature in the equatorial Middle East Pacific is low this year, and there is a La Nina phenomenon. Because last winter is also La Nina, it appears continuously this year, so China's temperature is low and it will be colder. Is this really the case? Lanina's appearance for two consecutive winters is actually very common La Nina is a phenomenon of cold sea surface temperature in the equatorial Middle East Pacific. It is not uncommon for La Nina phenomenon to occur in winter for two consecutive years. La Nina often occurs for two consecutive winters, while El Nino (almost the opposite of La Nina) rarely occurs for two consecutive years. For example, the three consecutive winters of 1973 / 74, 1974 / 75 and 1975 / 76 are La Nina, and the two consecutive winters of 1983 / 84 and 1984 / 85, 1998 / 99 and 1999 / 2000, 2007 / 08 and 2008 / 09, 2010 / 11 and 2011 / 12 are examples of La Nina. The continuous occurrence of La Nina phenomenon is related to the climate state of the equatorial Middle East Pacific and the formation mechanism of La Nina phenomenon. Is climate prediction really that simple and crude Many people hold the view that when La Nina phenomenon occurs, China's winter temperature is low. Even some meteorological departments and official media think so, mainly based on the emergence of La Nina phenomenon. However, this understanding of China's climate change is to simplify complex problems, and does not recognize the complexity of China's climate and the uncertainty of short-term climate prediction. First of all, China has a large land area, spanning the tropical, subtropical, warm temperate, middle temperate and cold temperate zones, and there are vast Gobi desert and Qinghai Tibet Plateau in the West. There are many factors affecting the winter climate anomalies in this region, including not only the SST in the tropical Middle East Pacific, but also the SST in the Indian Ocean and Atlantic Ocean, as well as the SST in China's coastal waters. In addition, it is also affected by Arctic sea ice in middle and high latitudes, Eurasian snow cover and land surface conditions. The role of different impact factors in each region is different, and the weight of each region is not the same. Therefore, it is easy to generalize only by using La Nina to discuss China's climate. As far as the eastern region of China is concerned, the weights of influence factors in the north and South regions are also different. Among them, the southeast coast is vulnerable to tropical ocean processes, including the form of SST in the Middle East Pacific (La Nina or El Nino phenomenon) and the form of Ocean Dipole in the Indian Ocean. Although in La Nina years, it is easy to form cyclonic circulation over the equatorial western Pacific and the South China Sea, and the northerly wind anomaly on the west side of the cyclonic circulation is conducive to the cold air activities in the southeast coastal areas, there is basically no completely consistent La Nina - each time is different, The intensity and shape of the cyclonic circulation over the equatorial western Pacific and the South China Sea are also different, so it is not suitable to apply the statistical law. For the north by east of China, it is more affected by the circulation anomalies in the middle and high latitudes, among which the Arctic oscillation is the most important factor. For the middle and high latitudes, the predictability is lower than that in the tropics, because these regions are affected by the middle latitude weather process, and the time period of the weather process is generally 7 ~ 10 days. These processes can be regarded as the "noise" part for the whole winter (November ~ March, and some studies only analyze December ~ February). In a winter, this process occurs at least 8 ~ 10 times, and the temperature fluctuates at least 10 ℃ each time. It is very challenging to identify the seasonal signal of average temperature (~ 1 ℃) in this fluctuation. In addition to frequent cold air activities in winter, the winter climate in eastern China also has a certain unique nature. For example, the temperature changes inversely between the northeast northwest and the south. When the temperature in the northeast and northwest is high, the temperature in the south is often low. On the contrary, when the temperature in the northeast and northwest is low, the temperature in the south is often high; The difference between the former Winter and the latter winter. When the temperature in the current winter (November ~ December) is high, the temperature in the latter winter (February ~ March) is easy to be low, and vice versa. Therefore, we can not use the overall "warm winter" or "cold winter" to discuss. "Cold cry" last year was a warm winter The forecast even shows that it may be warmer this year Taking the winter of last year (2020 / 21) as an example, many people still remember it as "crying cold", "the lowest temperature in the 21st century" and "the lowest temperature record in 20 years". However, the temperature change this winter is like a roller coaster. In December 2020, the national temperature is generally on the high side. The cold wave comes in early January 2021, and the temperature drops sharply. However, the national temperature rises from late January, and rises sharply and warms up rapidly in February. The daily maximum temperature from North China to South China quickly exceeds 20 ℃, and even reaches 29 ~ 30 ℃ in some areas of Henan and Hebei. According to the average winter temperature from December to February, the whole country is actually a proper warm winter (determined by the official announcement of the National Climate Center of the China Meteorological Administration on March 3, 2021). The La Nina phenomenon also occurs in the winter of 2020 / 21, but the extreme cold wave events go hand in hand with the warmer winter, which makes people who say cold and warm find a reason, and makes people who say cold and warm unable to fully justify themselves. Making the climate forecast for the next quarter one or two months in advance belongs to short-term climate forecast, or seasonal forecast. At present, seasonal prediction is very challenging. Many institutions at home and abroad are continuously carrying out a lot of analysis and prediction work. The most important prediction tool is S2S prediction system, that is, predicting the average state from next season to season. According to the prediction of various research institutions at home and abroad, the La Nina phenomenon in this winter is basically a certainty. For the temperature in East Asia, more studies tend to predict that the overall temperature is high. The range of high temperature is larger in northern China, which can reach more than 1 ℃, while in southern China, the increase is limited and basically close to normal, so the possibility of the legendary "cold winter" is relatively low. Winter survival rule: Paying attention to weather fluctuations is more meaningful than cold winter and warm winter "Cold winter" or "warm winter" is the consideration of the whole winter. For example, according to the current standards of China Meteorological Administration, it is necessary to investigate the average temperature of the whole winter (December ~ February) to determine the deviation of each station from the historical data, the total number of cold and warm stations and the size of cold and warm area. This is actually a very academic concept, which has limited significance to the public. As discussed above, in each weather process in winter, the up and down range of temperature is often greater than 10 ℃, while the temperature anomaly in cold winter and warm winter is generally only 0.5 ~ 1.0 ℃. Even if it is finally confirmed that winter is cold, as long as the temperature is stable as a whole, the impact of low temperature is very limited; During the warm winter, if there are short-term extreme cold air activities, such as the extremely cold weather in the past 2020 / 21 warm winter, the impact will also be very serious. For the general public, instead of focusing on whether this winter is a "cold winter" or a "warm winter", it is more important to deal with weather fluctuations, pay attention to weather forecast and early warning at any time and deal with the disastrous weather in winter in time. The accurate prediction of an extreme cold or warm event one or two months in advance has basically exceeded the current global prediction ability. If an organization or individual claims to have this ability, it can be a question mark. (author Wei Ke from Institute of Atmospheric Physics, Chinese Academy of Sciences)