Why is the winter of 202 1 colder than in previous years?

Because 202 1 is a double La Nina year.

The data show that after La Nina event, the probability of overall cold in winter in China is about 70%, and the probability of warm is about twice.

La Nina event refers to a large-scale cold water phenomenon with a certain intensity and duration caused by the abnormal sea surface temperature in the equatorial Middle East Pacific Ocean.

Since the beginning of this century, the whole Pacific Ocean has been in a cold state, which also makes La Nina events more likely to happen.

In the context of La Nina, this year's colder, warmer, wetter and drier extreme weather continues to be staged.

Reasons for the formation of double La Nina year

So how was La Nina formed? El Nino is related to the warming of sea surface temperature and the weakening of trade winds in the equatorial Middle East Pacific, while La Nina is related to the cooling of sea surface temperature and the strengthening of trade winds in the equatorial Middle East Pacific. So La Nina is actually the product of the interaction between tropical ocean and atmosphere.

The motion of the ocean surface is mainly constrained by the sea surface wind. The existence of trade winds makes a lot of warm water blown to the equatorial western Pacific Ocean, and the warm water is blown away in the equatorial eastern Pacific Ocean, mainly supplemented by cold water below the sea surface. The SST in the equatorial eastern Pacific is obviously lower than that in the western Pacific.

When the trade winds strengthen, the upwelling phenomenon of deep seawater in the equatorial eastern Pacific becomes more intense, resulting in abnormally low sea surface temperature, which makes the airflow in the equatorial eastern Pacific sink, while the upward movement of the airflow in the equatorial western Pacific intensifies, which is conducive to the strengthening of the trade winds, thus further aggravating the development of cold water in the equatorial eastern Pacific and triggering the so-called La Nina phenomenon.

Trade winds refer to the winds blowing from tropical regions to equatorial regions at low altitude, which are called "northeast trade winds" in the northern hemisphere and "southeast trade winds" in the southern hemisphere. Spaniards living in South America used this persistent easterly wind to sail to Southeast Asia for business activities. So trade winds are also called trade winds.