To say what this year's keyword is, that is, "snow", in addition to the Winter Olympics, that is, the south and the north round after round of blizzards, occupying the front page of the news, many southerners have said: how many years have not seen such snow.
At the same time, many people have questioned about this, is it still so cold in the spring, and is it normal to snow at both ends of the three days?
In the lunar calendar, we are accustomed to using spring as the date when spring begins and winter ends, but this is not the case in astronomy, where winter begins at the winter solstice and ends at the vernal equinox.
Of course, this classification in astronomy is more scientific, because the change of the Earth's seasons is caused by the 23.5-degree tilt between the Earth's axis of rotation and the Sun's orbit.
Picture note: Winter time for the next few years
It is only because the Earth's orbit is elliptical, so the time of the winter solstice and vernal equinox is not fixed, but in any case, winter is around December 20th of each year to around March 20th of the next year.
Therefore, we are still in the winter, it is very normal to snow, and sometimes there may be a "cold spring".
But what's going on when it snows so frequently, and there's more snow in the south than in the north?
Although it is normal to snow now, the pattern of snow this year is indeed a bit strange, and many meteorologists say that it is difficult to explain it with a single factor, but it also reflects the complexity of the weather system.
Why is there so much snow in the South this year?
As we all know, snow must be cold enough, which needs to be supported by cold air in the north, but only cold air is not enough for snow, because cold air is relatively dry, so another key factor in snow is that there must be enough water vapor.
This year, the South Branch Low Pressure Trough on the southern edge of the Tibetan Plateau has been unusually active, transporting water vapor from the Bay of Bengal into southern China, while the water vapor of the South China Sea is also moving north with warm and humid air currents.
Cold air from the north and warm and humid air currents from the south meet just over the south, which is why snow continues to fall in the south this year.
But why is this the case this year?
This year the cold air and warm and humid airflow are relatively abnormal!
With the onset of winter, the sunshine level in the Arctic is decreasing, so that the temperature in the Arctic continues to decline, but there is a huge temperature difference between the equator and the poles, which forms a low-pressure area rotating from west to east over the Arctic, which is called the polar vortex.
The lower the temperature of the Arctic means that the greater the temperature difference, the stronger the polar vortex, and the more stable it is.
We often say global warming, but winter seems to be quite cold, which is actually because global warming leads to the weakening of the polar vortex strength, which eventually leads to the abnormal phenomenon of cold air moving south.
However, this year's cold air anomaly is not only due to global warming, but also a more important reason – that is, La Niña.
According to the National Weather Service, last winter was also a La Niña year, which means that this year is the second consecutive year that the La Niña phenomenon has occurred, and the La Niña phenomenon itself is relatively rare, and it is even rarer for two consecutive years.
La Niña and El Niño are the opposite, they alternate, but La Niña is less frequent, and its typical feature is that the continent's winters will be colder, that is, "cold winters" (El Niño will lead to warmer continental winters).
Because La Niña favors the longitude of the mid-latitude atmospheric circulation, that is, cold air is more likely to move south, so in the La Niña year, cold air tends to be more frequent.
Therefore, as far as cold air is concerned, this year will indeed be different from previous years, and it will obviously increase.
However, winters affected by La Niña often lead to less water vapor delivered from the south to the north, which means that the south of the La Niña year should have less precipitation.
Image source: Chinese weather
But the truth is that the recent precipitation in the southern part of the mainland is generally high, just think of the clothes you are about to change, and many areas have even reached an all-time high – such as many in Hunan Province.
Xie Yijun, chief expert of climate prediction at the Hunan Provincial Climate Center, said that this year's precipitation pattern and the impact of La Niña are obviously inconsistent, indicating that the influencing factors of the winter climate are very complex [1].
In terms of warm and humid air flow, with the arrival of spring, the warm and humid air flow from the South China Sea will increase significantly, so under normal circumstances, spring precipitation will generally increase.
Perhaps because of la niña's weaker strength this year, its restrictions on warm and humid airflow are not particularly obvious, or perhaps there are many other factors that cause this year's warm and humid airflow to be more than in previous years.
Therefore, even under the influence of La Niña, the precipitation has not decreased.
At last
In any case, the complexity of the climate is far greater than imaginable, and no single weather system can explain all weather phenomena.
However, it should be explained here that the phenomenon of continuous snow after spring this year is not unique, such as 2008 and 2018, which are similar, of which 2008 is also a year influenced by La Niña.
In addition, today may be the last day of snow Oh, the next temperature is going to warm up, so cherish the last snow scene!
Resources:
[1] How often does it snow in Changsha? The reason is found! -Released in Changsha