Will El Niño bring more extreme heat?

China Newsweek reporter/Huo Siyi

Published on July 2023, 7, the 10th issue of China Newsweek magazine

July 7 was the hottest day on record on Earth, according to the latest data from the National Oceanic and Atmospheric Administration (NOAA).

The World Meteorological Organization (WMO) said on the 4th that after seven years, El Niño reappeared, and it is expected that the temperature in most parts of the world will rise further in the future, and a record high temperature may occur within 5 years. According to the latest WMO projections, there is a 2023% chance that an El Niño event will last in the second half of 90, with an event of at least moderate intensity.

The Central Meteorological Observatory continued to issue a high temperature orange warning on July 7. The Beijing Meteorological Observatory upgraded to issue a high temperature red warning signal on July 6: the maximum temperature in most areas of the city is expected to rise above 7°C on July 6. The latest forecast of the National Climate Center's national climate trend in midsummer this year said that there may be phased high temperature processes in North China, central and southern China, and northeast of Southwest China.

Although it has not entered the height of summer, the national high temperature has obviously had the characteristics of "early occurrence, wide impact range, and significant extreme". Globally, Asia ushered in the hottest April on record, and in May, the global average temperature reached the highest since 4, and most countries in Southeast Asia have exceeded 5°C since June. Some analyses have suggested that these local climate anomalies may be related to El Niño.

After three consecutive years of La Niña, El Niño's return this year has become inevitable. As the world enters an "El Niño climate model", people are more worried about whether global warming combined with El Niño and extreme heat will become the "new normal" in the future?

"The Ultimate Weather Creator"

According to China's classification of extreme temperatures, high temperatures ≥ 40°C are "highly harmful". According to the National Climate Center, the high temperature in northern China this year is extremely extreme, and the high temperature has repeatedly appeared in succession, which is rare in history. Data show that on July 7, the maximum temperature in Beijing's southern suburbs observatory reached 5.40 °C, the fourth 9°C high temperature day this year. Previously, on June 40~6, the temperature of the Beijing Southern Suburbs Observatory reached or exceeded 22°C for three consecutive days, and the high temperature in the urban area exceeded 24 hours.

Is El Niño the "culprit" behind this round of extreme heat in North China?

Zheng Fei, a researcher at the Institute of Atmospheric Physics of the Chinese Academy of Sciences, has long been engaged in El Niño prediction and research, and he analyzed that the tropical Pacific Ocean is currently in the process of transforming from La Niña to El Niño, and the global atmospheric circulation will be adjusted accordingly.

El Niño occurs in vast areas of the equatorial Pacific Ocean in the east-central part of the ocean. As early as the 17th century, fishermen along the coast of South America found that every 2~5 years, the surface of the Pacific Ocean would appear abnormally warm seawater. More than 300 years later, oceanographers realized that this is not a local phenomenon off the coast of Peru, its effects span the entire tropical Pacific region, and it is a stage of longer-period SST oscillations, that is, the surface sea temperature in the central and eastern equatorial Pacific Ocean fluctuates between warm and cold, when SST is abnormally high, it is El Niño in the warm phase, and the abnormal decrease is the cold phase La Niña, which is a neutral phase close to the average temperature level. The entire natural cycle is known as the El Niño-Southern Oscillation (ENSO).

When El Niño occurs, the persistent, unusually warm waters of the central and eastern tropical Pacific Ocean continuously release more heat into the atmosphere above. Changes in the ocean lead to changes in the atmosphere, most affected by the Walker and Hadley circulations, and the "response" through atmospheric circulation patterns triggers a domino-like chain reaction. Local weather patterns around the equator and throughout the Pacific Rim change and even radiate to mid- and high-latitudes, eventually triggering global climate anomalies, including extreme weather events such as heat waves, droughts, floods, hurricanes and heavy rainfall. For this reason, El Niño is also known as the "ultimate weather creator".

Xie Shangping, a professor at the Scripps Institution of Oceanography at the University of California, San Diego, and the main author of the fifth assessment report of the United Nations Intergovernmental Panel on Climate Change (IPCC), explained to China Newsweek that El Niño originates in tropical seas, which is the largest precipitation area in the world, and releases a large amount of latent heat in the process of water vapor turning into water droplets, becoming an important "fuel" to drive the global ocean-atmosphere circulation. That is to say, the equatorial region is the "engine" of global atmospheric circulation, El Niño is "a state in which the engine becomes particularly strong", and the probability of global abnormal climate events will certainly increase significantly in El Niño years.

"Therefore, from the perspective of large-scale global circulation, the high temperature that is occurring in China is related to El Niño to a certain extent, but it cannot be said that El Niño is the direct cause of this round of high temperature in North China." Zheng Fei said.

He believes that the more direct impact on the high temperature in North China is the change of atmospheric circulation at middle and high latitudes, which is not entirely related to the chain reaction triggered by El Niño. This year, the cold air in northern China has become weaker after late spring, and there is no regular southward movement, and the warm and humid air in the south has not gone north smoothly, which makes the air in the entire north China in a dry state, and "the dry air warms up quickly." In Zheng Fei's view, the high temperature in North China is the result of the comprehensive influence of multiple factors, and El Niño is only one of them. In fact, this year is the year of El Niño's development, and at this stage, the "signal" of El Niño itself is not very strong.

Specifically, El Niño's impact on China is mainly through two weather/climate systems: one is the Northwest Pacific anticyclone, and the southwesterly wind anomaly on its west side is conducive to enhanced water vapor transport to southern China. Zheng Fei analyzed that the occurrence of El Niño may cause a significant increase in rainfall in South China and Southeast Coast. "The floods in the Yangtze River basin in 1998 and 2016 were largely related to the strengthening of anticyclones in the northwest Pacific under the influence of El Niño, and the large amount of warm and humid air brought by it from the sea converged with the plum rain front in the Yangtze River basin."

The other is the Western Pacific Subtropical High. Zhou Bing, chief expert of climate services at the National Climate Center, pointed out in a recent interview that in the summer of El Niño's development year, the sub-high in the western Pacific tends to be strong in the south, resulting in more precipitation in the southern part of China. "The recent frequent heavy precipitation in Jiangnan, South China and other places is one of the manifestations of the impact of El Niño status." At the same time, the southerly strength of the subtropical high is not conducive to the movement of the rain belt during the flood season.

The National Climate Center predicts that this summer, the southeast China, central and southern parts of China will be prone to more precipitation, and the risk of flooding will be great. The eastern part of the northwest region is prone to low precipitation, and the possibility of meteorological drought is high. Therefore, Zhou Bing reminded that this summer we need not only to be vigilant against waterlogging in southern cities, but also to pay close attention to the drought brought by northern places and other places. "The emergence of this El Niño event marks the return of China's precipitation in the traditional sense of 'flooding in the south and drought in the north'." He said.

According to short-term climate forecasts from the Institute of Atmospheric Physics of the Chinese Academy of Sciences, due to the development of El Niño, two main rain belts may form during the flood season in China this year, which are in South China and the Huanghuai-Northeast Eastern Region. The Yangtze River basin as a whole is still slightly dry, the upper reaches of Sichuan-Chongqing have normal or wet precipitation, the middle and lower reaches are slightly drier than in previous years, and the plum rain is basically flat or slightly less than usual.

In addition, there is a high possibility of drought in North China, especially before the rains in North China in mid-to-late July, and the high temperature and dry situation in North China will continue for some time. "Even in autumn, after the rain belt retreats, North China as a whole may still have high temperature and drought characteristics." Zheng Fei said that in addition to North China, high temperature and drought in parts of the northwest also need to be vigilant.

Zhou Bing pointed out that this year's El Niño background is similar to 2009, and from the fall of 2009 to the spring of 2010, the southwest region experienced the worst autumn, winter and spring drought since meteorological records began. Overall, the drought in the southwest has been ongoing since last year, and he worries that if the rainy season rains in the southwest do not alleviate the drought, the drought will become more pronounced after the autumn and winter.

However, Zheng Fei stressed that El Niño itself is a large climate impact factor, and the specific impact on a specific region involves a multi-scale complex dynamic mechanism, "such as a sudden strong typhoon, the Indian Ocean has changed the outbreak of the South China Sea monsoon, and even the impact of the polar vortex on the northeast region, etc., many weather and climate factors involved here are impossible to predict in advance for a long time, and many weather and climate factors affecting China will be combined to finally determine the distribution and amount of rainfall this summer." ”

In his view, the predictions for this summer's climate are in line with the typical distribution characteristics that may be presented in China's rain belt during the transition from La Niña to a later El Niño.

There is a high probability of an El Niño event this year

To be precise, the real El Niño event is not yet coming. The National Climate Center predicted at the end of June that the equatorial Middle East Pacific Ocean will remain El Niño for the next three months, with an eastern El Niño event of moderate or greater intensity forming this fall.

Status and events are judged differently. When detecting abnormal warming of sea surface in tropical Middle East Pacific waters, the deviation of SST from the perennial mean in key areas is often used as a baseline, which is the "Ocean El Niño Index (ONI)" that El Niño researchers are most concerned about. When the ONI exceeds the 0.5°C threshold, it can be considered to have entered an "El Niño state", but for a full-fledged "El Niño event" to form, the standard is much higher.

Zheng Fei introduced that the three-month sliding average of ONI exceeds 3.0°C and lasts for at least 5 months before it is defined as an El Niño event. "In this way, it basically requires that the abnormally high SST in the tropical Pacific region of the Middle East will last for about 5 months."

Why is this difference so important? Zheng Fei explained that the essence of El Niño's impact on the global climate is that it is an ocean-atmosphere coupling process, if the ocean is a gear, the atmosphere is another gear, only when the two gears are closely meshed together, there is a "clear, not short-lived" signal to prove that the interaction between the sea and the air has formed, such as the ocean has maintained an abnormally warm state for many months, the large-scale atmospheric circulation will really be driven, atmospheric changes will further affect the ocean changes, the two are complementary.

On June 6, the National Oceanic and Atmospheric Administration (NOAA) issued an "El Niño Alert", predicting that in the next few months, the probability of a moderate El Niño event is 8%, the probability of a strong event is 84%, and in the northern hemisphere winter of 56~2023, El Niño will gradually strengthen. NOAA said that the more intense the event, the greater the typical impact on a particular region. In NOAA's June 2024 report, the latest ONI index has climbed from 6.26°C to 0°C.

In fact, in El Niño's forecast, there is a "spring forecast hurdle" that has been difficult to get around for years. Zheng Fei explained that in the El Niño model forecast, it will be found that whenever the forecast of the model spans the spring, the forecasting capacity is greatly reduced. Because the atmosphere and oceans are in one of the most unstable stages in the spring, that is, the two gears do not bite well, "sometimes even counter-rotating or detached", so the indicative signal for the subsequent development of El Niño is not strong. Generally, after summer, the coupling of the ocean and atmosphere increases, and the forecast results are more reliable.

But this year is different from the past, Xie Shangping found that since April, the thermocline of the equatorial Pacific, that is, the transition layer between the warm current on the ocean surface and the cold current of the deep sea, has shown a trend of deepening, reflecting the abnormal increase in warm water in the equatorial Pacific. "Personally, I think this is one of the reasons why NOAA has a lot of confidence in predicting El Niño in early June."

Zheng Fei also said that from the comprehensive monitoring data of surface SST, subsurface SST, and atmospheric surface wind layer, the meshing trend of the two gears of the atmosphere and the ocean in the tropical Pacific Ocean has been clearly seen since this spring. "The International Institute for Climate and Society (IRI) in the United States synthesized about 20 prediction models internationally and calculated an overall probability of occurrence based on the different results of each model. In the past, due to the existence of spring forecast obstacles, the forecast results between various models usually varied greatly, and even the predicted SST would develop in different directions of cold and warm, but this year everyone is more consistent. He said it was relatively rare in previous El Niño events.

According to the prediction of Zheng Fei's team, there is a probability of El Niño events this year, which will enter the peak period in October ~ December, and begin to decline after the spring of the following year. A number of experts also pointed out that in the northern hemisphere this winter, El Niño can at least reach moderate intensity, Zheng Fei predicts that there may be a "strong El Niño event" with a 90% probability, corresponding to an abnormal temperature increase between 10.12 °C ~ 70 °C. "The impact is very large, because even if the temperature of the large ocean is only 1.5°C, it will release a lot of heat into the atmosphere."

But is it possible for China to usher in a "super El Niño event" that will warm more than 2°C this winter?

Experts have not yet reached a consensus on this, "It is difficult to predict at this time." "There are big differences in intensity forecasts among forecasting centers because El Niño peaks generally occur around December, and the initial conditions for forecasting are important." What might happen between now and December, no one knows. The atmosphere is a chaotic system, and a little disturbance can affect the future development of El Niño events, and the forecast may be clearer after August. ”

2024 may be the "hottest year"

A number of experts pointed out that if a super El Niño event does arrive, their biggest concern is the possibility of heavy rain flooding or extreme heat in the summer of 2024.

Zhang Ronghua, a professor at Nanjing University of Information Science and Technology's School of Marine Sciences, told China Newsweek that at present, El Niño has only just formed in the central and eastern waters of the equatorial Pacific Ocean, and it may reach its strongest around December this year, and to have a more direct and obvious impact on the East Asian climate, there will be at least a lag of at least two or three months. "Combined with historical data, it generally reaches its strongest in the spring and summer of the following year. So for El Niño, we need to be more vigilant about this time next year. He said.

Since the 20s of the 80th century, super El Niño events will occur about once every 15~20 years, and the last time a super El Niño phenomenon invaded China was in 2015~2016, which is the strongest event since 1951. In 2016, the middle and lower reaches of the Yangtze River experienced regional floods, some tributaries experienced extremely large floods, and this year also set the warmest record in the world to date. Zheng Fei said that the major floods in the Yangtze River basin in 1998 and 2006 both occurred in the second year after the development of El Niño, and if this winter's El Niño event meets the standard of a strong event, then the probability of flooding in the Yangtze River basin in the summer of 2024 is relatively large.

Several projections are more certain than uncertain floods that 2024 could be the "hottest year" to break historic high temperature records.

Experts point out that the impact of El Niño on global temperatures usually manifests itself within a year of development, so new record-breaking high temperatures are likely to be most pronounced in 2024. World Meteorological Organization (WMO) Secretary-General Petri Taalas said at a May 5 press conference that despite the cooling La Niña phenomenon over the past three years, the development of El Niño is likely to lead to new peaks in global warming and increase the chances of breaking temperature records. Zheng Fei predicted that the probability of setting a new global high temperature record for global average surface temperature in 3 "basically reaches 2024%", which is statistically estimated by combining historical event data.

Zhang Ronghua analyzed that in the more than 20 years since the beginning of this century, the surface waters of the equatorial sea in the Pacific Ocean have been cold as a whole, making La Niña appear more frequently; In contrast, El Niño is not easy to occur and weak, but the Pacific climate system still has decadal changes in a cycle of 10~20 years. In 2022, in the context of the cold decadal background, the global average temperature is still about 1°C above pre-industrial levels, almost similar to 2016.

He worries that in the next few years, under the "double warming effect" of El Niño combined with global warming, the global temperature curve will continue to rise. "Studies have shown that El Niño will increase the global average surface temperature by about 0.1°C, and if after the spring of 2024, El Niño enters decay and turns into La Niña, the temperature may also drop in the next few years, but if it does not change, the general trend will definitely be higher and higher."

Has global warming increased the intensity or frequency of El Niño?

In this regard, the team of Cai Wenju, academician of the Australian Academy of Sciences and chief scientist of the Institute of Oceanography and Atmosphere of the Australian Commonwealth Science and Industry Organization, found in a study published in 2023 that the observed ENSO SST variability in 1961~2020 exceeded the pre-industrial revolution simulation level of 97.5%. This suggests that it is unlikely to be caused by natural fluctuations within ENSO alone. Cai Wenju et al. speculated that the upper oceans of the tropical Pacific Ocean may warm faster than the lower ones due to global warming, and the coupling efficiency between the ocean and the atmosphere increased. "The magnitude of ENSO changes is likely to have increased by about 10% over the past century, and while 10% may not seem like much on the surface, it is actually accompanied by strong El Niño and strong La Niña events occurring more frequently and translating into more extreme and frequent droughts, floods, heat waves, wildfires, etc."

However, several experts interviewed pointed out that the link between global warming and El Niño is still much controversial in science. "There are about twenty or thirty climate models around the world studying this question, and is El Niño itself stronger or weaker due to climate change on large timescales? Some patterns say they are strengthening, some say they are weakening, and some patterns say nothing has changed. Xie Shangping said.

But amid all the uncertainties, at least one thing is certain: in the context of global warming, El Niño will affect the intensity and frequency of extreme weather outbreaks around the world. "The world is facing a hotter, drier and more waterlogged future, and extreme heat has become a 'new normal'." Zhou Bing said.

China Newsweek, Issue 2023, 25

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