The cold wave is coming, what does a double La Niña year mean?

According to the National Climate Center on October 22, the sea temperature in the equatorial central and eastern Pacific has continued to drop since July 2021. It is expected to enter a La Niña state in October and form a weak to moderate La Niña phenomenon in winter.

On October 17 this year, the lowest temperature in Beijing fell below zero degrees Celsius for the first time in the second half of the year, about 20 days earlier than the average date in normal years. On November 6, Beijing ushered in a local snowstorm. People shivering in the cold wind couldn't help but ask, is this the magic attack of La Niña? In fact, this round of large-scale cooling in central and eastern my country is the prelude to the La Niña phenomenon.

Changes in sea temperature anomalies in key La Niña regions from October 2019 to September 2021

Figure | National Climate Center

Anomaly changes in the surface 300-meter sea temperature in the equatorial Pacific from August 11 to October 5, 2021

The horizontal axis of this vertical profile is longitude, the vertical axis is sea depth, and the sea temperature increases from blue to red.

Comparison of historical data averages from 1981 to 2010

Photo | NOAA National Oceanic and Atmospheric Administration

Everyone should have a full understanding of the "Holy Child" El Nino phenomenon, but as the noun corresponding to El Nino, the "La Nina" phenomenon seems to have a slightly lower presence. La Nina is the transliteration of the Spanish "La Niña", which means little girl. It is the opposite of the El Nino phenomenon and refers to an abnormal drop in water temperature in the eastern Pacific near the equator, which is manifested as a significant cooling of the eastern Pacific, accompanied by global climate anomalies.

Above: Differences in Pacific SST anomalies during the peak of the strong La Niña phenomenon (December 1988)

Below: Differences in Pacific Ocean Surface Temperature Anomalies during the peak of the strong El Niño phenomenon (December 1997)

Photo | NOAA National Oceanic and Atmospheric Administration

——Why did the little girl appear? ——

La Nina is a typical low-latitude Pacific climate anomaly. Under the direct influence of the sun, there are southeast trade winds and northeast trade winds on the north and south sides of the equator respectively. The trade winds continuously drive the higher temperature surface water to flow from east to west, forming the South Equatorial Warm Current and the North Equatorial Warm Current. The low-latitude areas in the western Pacific Ocean become a warm water pool with a higher temperature. When the eastern surface water moves westward, the adjacent and lower seawater comes to supplement and form a low-temperature compensation current, making the eastern Pacific Ocean relatively cold. This continuous exchange of sea-air heat forms an atmospheric circulation called the Walker Circulation.

When the trade winds strengthen, the upwelling of deep water in the eastern equatorial Pacific becomes more intense, resulting in abnormally low sea surface temperatures, which causes the air currents to sink in the eastern equatorial Pacific, while the upward movement of the air currents in the west intensifies, which is conducive to strengthening the trade winds. The cold water in the eastern equatorial Pacific continues to develop in this positive feedback cycle until the La Niña phenomenon is formed.

The circulation during the peak period of La Nina shows that the Pacific Ocean is cold in the east and hot in the west

Key areas for monitoring sea temperature during El Nino/La Nina events in the equatorial Pacific

Figure | National Climate Center

It is not difficult to see that El Nino and La Nina are inseparable. They are two sides of the same coin, one hot and one cold. They have different impacts on the climate and are two different phases in the El Nino-Southern Oscillation (ENSO) cycle. La Nina can be seen as an overcorrected climate effect after El Nino. The two are like the left and right tilt imbalances of a gyroscope. The stronger the El Nino, the more likely it is to form La Nina, but its frequency and intensity are generally not as high as El Nino.

——The impact of La Nina on production and life——

As the most significant change characteristics of the Pacific Ocean on a multi-year time scale (approximately 2 to 8 years), La Nina and El Nino cover a range close to the planetary scale. Both can affect the global climate and interfere with agricultural production to a certain extent through large-scale precipitation and temperature anomalies. For example, crops such as corn and soybeans in South America will face more severe droughts, and the risk of wildfires in California will remain high. On the other side of Asia, coffee and rubber production on the island of Java in Southeast Asia may suffer from floods.

In addition to the above-mentioned agricultural problems, in the context of large-scale emission reduction, carbon restriction and capacity reduction, the world is generally short of energy today. The La Niña that appears at this time may superimpose and amplify the adverse effects. The cold weather will put great pressure on energy and other related industries, exacerbate market concerns about winter energy shortages, promote irrational increases in the prices of bulk energy commodities, and affect the rhythm of economic production.

Comparison of agricultural product price trends during La Niña and non-La Niña periods

During the La Nina period, except for sugar, the prices of the other five agricultural products all showed obvious fluctuations.

The data is the monthly price average since 1980.

Figure | Industrial Bank Research

Especially in recent years, when the global industry has been worried about supply and demand imbalance, we should be more vigilant about abnormal extreme weather. For example, the shrinking production capacity and supply shortage of automotive chips in 2021 are related to the Texas cold wave at the beginning of the year. The 200-fold surge in electricity prices has paralyzed industrial centers. Such cases are enough to sound the alarm for global industrial production.

It should be said that the impact of La Nina on different regions and fields is different. It does not bring harm to all. When most parts of the world are adversely affected, some local industries may benefit. For example, southern Africa will receive abundant moisture, and more rainfall will be beneficial to local sugar and corn production.

——"Little Girl" and China——

China is located on the northwest coast of the Pacific Ocean. Although it is far away from the key area where La Niña forms, the La Niña phenomenon is still closely related to us. Due to the active Walker circulation, the surface water temperature in the western Pacific Ocean rises, which usually leads to stronger autumn typhoons in La Niña years and more in number. The mid-latitude atmospheric circulation is strengthened in the meridional direction in winter and spring, and cold air and dust weather may also frequently visit my country. In addition, there are regional differences in the impact on precipitation in my country, which roughly shows the distribution characteristics of drought in the south and flood in the north.

The percentage of composite precipitation anomalies during La Niña years since 1951 shows that precipitation in the middle and upper reaches of the Yangtze River is higher than normal.

Data | NOAA Precipitation Reconstruction over Land (PREC/L)

Figure | National Climate Center

Floods in the Yangtze River Basin generally occur in the year following El Niño. The most typical example was the 1998 Yangtze River flood, which was largely related to the rapid transition from El Niño to La Niña in 1997 and was one of the reasons for the sudden increase in rainfall in the Yangtze River Basin.

The movie "Changjin Lake" which was popular during the National Day in 2021 showed us the heroic war scenes of the volunteers in the ice and snow. At that time, East Asia was widely affected by La Niña, and cold waves and wet and cold air currents jointly invaded the Gaema Plateau in North Korea, making the Battle of Changjin Lake surpass the Battle of Moscow in World War II and become the coldest battle in human war history since clear records.

Against the backdrop of global warming and increasing volatility, the factors affecting my country's winter climate have become more complex. my country's winter climate is still mainly affected by the East Asian monsoon circulation. In addition to La Niña, there are also factors such as changes in Eurasian snow cover, melting of Arctic sea ice, and abnormal sea temperatures in other seas.

Record of La Niña events since 1950

Excerpt from the National Climate Center's El Nino-Southern Oscillation Historical Event Table

Figure | National Climate Center

Looking back at the 15 La Niña years recorded in my country since 1951, winter temperatures were low in 10 years and high in 5 years. This year is the 16th La Niña year. Whether my country will usher in a cold winter is still unknown. The concept of a cold winter has a strict definition in terms of time and space. Only by the end of this winter can we determine whether it is a cold winter through measured data. At present, the temperature forecasts of East Asia in winter by research institutions of various countries generally tend to be high overall, and the possibility of a cold winter is low.

It is worth noting that although the five double La Niña years in history were all colder, and this year's La Niña is closer to typical events that have occurred in the past, due to the complexity of the climate system, each La Niña has some differences, so there is still great uncertainty in the mid- and late-term evolution trend of this year's climate. Under the combined effects of other factors, such as global warming and the Arctic amplification effect, even if last year's warm winter continues, the possibility of extreme cold waves and short-term cold weather cannot be ruled out. In addition to the public taking good cold-proof and warm-keeping measures, improving the accuracy of event warnings and improving the disaster prevention and mitigation system are still the main themes of our meteorological work.