“Double-peak La Niña” is coming, will you be frozen to tears this winter?

From November 4 to 8, my country just experienced a severe cold wave from west to east. Many areas in the north, including Beijing, experienced heavy rain and snow. Many cities have already started heating in advance. Is this related to the "double-peak La Nina" event?

Observations show that the sea temperature in the equatorial central and eastern Pacific has continued to drop since July this year, and entered the La Nina state in October. The National Climate Center predicts that a weak to moderate La Nina event will form in winter. Since a moderate La Nina event also formed last winter, it is called a "double-peak La Nina" event.

So what is a "La Niña" event, and what is a "double-peak La Niña" event?

A La Niña event refers to a cold water phenomenon in which the sea surface temperature in the equatorial central and eastern Pacific Ocean is colder than normal over a large area and its intensity and duration meet certain conditions (the sea surface temperature is more than 0.5 degrees Celsius lower and lasts for more than 6 months).

Schematic diagram of La Niña formation (Source: US Climate Prediction Center)

The La Nina phenomenon is the product of the combined effects of tropical ocean and atmospheric circulation. The ocean surface is mainly controlled by the sea surface wind. The prevailing easterly trade winds in the equatorial central and eastern Pacific Ocean blow a large amount of warm water from the surface to the equatorial western Pacific region, while the equatorial eastern Pacific Ocean is supplemented by the cold water below the sea surface. Therefore, the sea temperature in the equatorial eastern Pacific Ocean is lower than that in the western Pacific Ocean. When the easterly trade winds strengthen, the upwelling of deep sea water in the equatorial eastern Pacific Ocean becomes more intense, resulting in abnormally low sea surface temperatures, while the atmospheric sinking movement on the cold water ocean surface strengthens, and the atmospheric rising movement in the equatorial western Pacific Ocean intensifies, prompting the easterly trade winds to strengthen, which further intensifies the continuous development of cold water in the equatorial eastern Pacific Ocean, thus forming the La Nina phenomenon. On the contrary, when the easterly trade winds on the equator weaken, the El Nino phenomenon with warmer sea temperatures in the equatorial central and eastern Pacific Ocean, which is the opposite of La Nina, will occur.

The La Nina phenomenon can adjust the global atmospheric circulation system (such as "atmospheric bridge" or "teleconnection"), thus causing major catastrophic impacts on countries and regions in East Asia, South Asia, Africa, North America and other continents thousands of miles away, such as heavy rains, droughts, extreme heat, severe cold and super tropical cyclones. Usually, after a La Nina event in the first year, another La Nina event will occur in the second year, which is called a "double-peak La Nina" event.

In fact, "double-peak La Nina" events like this year are not rare in history. La Nina events usually tend to last for more than a year, with secondary cooling. Statistics show that in the 14 La Nina events from 1951 to 2019, there were 5 cases of La Nina events continuing to develop in the second half of the second year and forming La Nina events again, that is, "double-peak La Nina"; 3 cases developed into El Nino events; and 6 cases maintained a neutral state.

After the La Nina event, the cold air activities affecting my country are often more frequent and stronger than usual; the probability of overall cold winter is about 70%, about twice the probability of warm winter; the winter temperature in central and eastern my country is often low. Many people still remember the snow disaster in southern my country in 2008. Affected by the 2007/08 La Nina event, my country was hit by four low-temperature rain, snow and freezing weather processes from January 10 to February 2, 2008. Nearly 2/3 of the provinces (regions and cities) in the country, including Guizhou, Hunan, Hubei, Anhui, Henan, etc., were affected by this low-temperature rain, snow and freezing disaster.

In fact, it is not only my country that is prone to low temperature rain and snow disasters in winter. When most La Nina events occur, many other parts of the world will also experience extreme weather. For example, precipitation decreases in areas near the coast of South America, while precipitation increases in Indonesia and eastern Australia; droughts often occur in central Africa and southeastern United States, while floods are prone to occur in northeastern Brazil, India and southern Africa.

The abnormal impact of La Nina on the global winter climate (Source: Japan Meteorological Agency http://ds.data.jma.go.jp/tcc/tcc/products/climate/ENSO/lanina.html#DJF)

The impact of La Nina and El Nino on my country's climate (Image source: Meteorological Publicity and Science Center of China Meteorological Administration)

Generally speaking, the second year of a "double-peak La Nina" event will be weaker than the first year, but its climate impact may not be weaker than the first year, including the potential impact on my country's winter climate. Therefore, it is not ruled out that there will still be an outbreak of extreme cold waves and a period of cold winter this year. The National Climate Center also predicts that the temperature in central and eastern my country will be generally cold this winter, with strong cooling and heavy snowfall in the northeast, eastern Northwest China, and northern Xinjiang, while the south may experience periodic low-temperature rain, snow, and freezing weather.

But it is worth noting that not every La Nina year has a low average winter temperature in my country. Against the backdrop of global warming, the factors affecting my country's winter climate are becoming increasingly complex. In addition to La Nina events, it is also affected by other climate factors such as Arctic sea ice, changes in Eurasian snow cover, abnormal sea temperatures in other seas, and mid- and high-latitude circulation. In addition, in 2008, in addition to the low temperatures brought by the cold air caused by La Nina, there was also abundant water vapor in the south, so the probability of another extreme catastrophic event similar to the snowstorm in many places in the south in 2008 is still relatively low.

Image source: Meteorological Publicity and Science Center of China Meteorological Administration

Given that most La Niña years in history have had significant impacts on my country's agriculture, production, and life, it is necessary for industries such as agriculture, forestry, transportation, electricity, and energy to take necessary preventive measures in advance based on the forecast of frequent periodic cold air activities in my country this winter, and to improve early warning and energy security for possible low temperature rain and snow disasters in winter.

Image source: Meteorological Publicity and Science Center of China Meteorological Administration

Author: Liu Yunyun, National Climate Center, Beijing

This article was published by Tencent's "Everyone Loves Science" team

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