El Niño Is About to End, Will Its Impact Still Linger?
Since the start of the flood season, the southern region of China has experienced one continuous heavy rainfall process after another. The National Climate Center predicts that this El Niño event will end in May, but its impact on the atmosphere will persist and will further manifest or amplify during this year's main flood season, especially during the plum rain season from June to July. There is a relatively high probability of abnormally excessive precipitation in the Yangtze River basin and other areas during the main flood season, and the flood control situation is not optimistic.
This Is the Strongest El Niño Event Since Complete Ocean Observation Data Became Available in 1951
On May 19, the south experienced its 14th round of relatively strong rainfall since the flood season began on March 21. The National Meteorological Center issued a yellow rainstorm warning on May 19, and the China Meteorological Administration activated a Level IV emergency response to address the heavy rain in the south.
Meteorological statistics show that as of May 18, the national average precipitation was 119.6 mm, 25 percent more than the same period in a normal year, the second highest for the same period since 1961. Since the start of the flood season, precipitation in the middle and lower reaches of the Yangtze River and most areas to its south has been 200-600 mm, while in southern Anhui, northeastern Jiangxi, northwestern Fujian, northern Guangdong, northern Guangxi and other places it reached 600-800 mm.
Zhou Bing, chief expert in climate monitoring at the National Climate Center, pointed out, "The frequent occurrence of heavy precipitation in the southern region is related to El Niño to some extent. This is mainly because, under the influence of El Niño, a high-pressure circulation system is triggered from west of the Philippines to the South China Sea, superimposing on the western Pacific subtropical high, making it abnormally strong and persistently positioned farther south. Moisture from the tropical Pacific is transported in greater amounts to the southern region of China, providing the moisture conditions for the frequent occurrence of heavy precipitation processes."
El Niño refers to a phenomenon in which sea water in the central and eastern equatorial Pacific remains abnormally warm over a large area for an extended period. When the sea surface temperature in that area remains 0.5 degrees Celsius higher than the same period in a normal year for more than 3 consecutive months, it enters an "El Niño condition." When the sea surface temperature remains 0.5 degrees Celsius higher than the same period in a normal year for more than 6 consecutive months, it is confirmed as an "El Niño event."
Zhou Bing explained in detail that the process by which El Niño affects China's weather and climate is realized through upstream effects. Generally speaking, after an El Niño event occurs, enhanced convective activity in the central and eastern equatorial Pacific due to rising sea temperatures causes updrafts to become abnormally strong. After the air rises to the upper levels, it diverges outward, the westerlies in the upper levels above the equatorial Pacific weaken, and anomalous easterlies prevail. The anomalous easterlies descend in the western equatorial Pacific and trigger an anomalous anticyclonic circulation in the lower levels above the western Pacific, strengthening the western Pacific subtropical high and extending it westward, which favors the transport of warm, moist southwesterly flow on the western side of the subtropical high to the southern region of China, resulting in above-normal precipitation there. In addition, several months after an El Niño event occurs, the tropical Indian Ocean begins to warm, which also triggers or strengthens an anomalous anticyclone near the western Pacific, enhancing the transport of warm, moist southwesterly flow to the southern region of China.
According to monitoring by the National Climate Center, this El Niño event began developing in September 2014 and has continued to date. According to the current operational standards of the National Climate Center, since 1951, a total of 14 El Niño events have occurred globally, 3 of which reached super-strong level, namely the 1982/1983 El Niño event, the 1997/1998 El Niño event and this event. From multiple perspectives, this El Niño event is the strongest since complete ocean observation data became available in 1951, and many sea temperature indicators have exceeded those of the previous two super-strong events. It is worth noting, however, that the tropical atmosphere's response to this super-strong El Niño has not been as significant as its response to the previous two super-strong El Niño events.
The Super-Strong El Niño Event Will End, but the Atmospheric Response Will Continue
Zheng Fei, deputy director and research professor of the International Center for Climate and Environment Sciences at the Institute of Atmospheric Physics, Chinese Academy of Sciences, said that the intensity of this El Niño event peaked in November last year and has been weakening since then, and it is expected to end in May-June this year.
Zhou Bing pointed out that because different countries and different institutions use different monitored target regions, ocean assimilation data and data processing methods, they obtain different monitoring results, but the trends should be consistent. So far, major foreign institutions have not yet released information on the end of this El Niño event.
Zhou Bing stressed, "Although the super-strong El Niño event is about to end, the atmospheric response to the super-strong El Niño event will continue, and because the amount of moisture transport increases in summer, the climate response during the main flood season, especially during the plum rain season from June to July, may further manifest or amplify. Therefore, the disaster prevention and mitigation situation is not optimistic."
Experts from the National Climate Center said that, based on historical statistical patterns, in the second year after an El Niño event peaks, the Yangtze River basin and regions south of the Yangtze River are prone to flood disasters in summer. The super-strong El Niño event of 1982/1983 led to persistent heavy rain in the middle and lower reaches of the Yangtze River from June to July 1983, with water levels at many gauging stations on the Yangtze River reaching historic highs. The super-strong El Niño event of 1997/1998 led to catastrophic floods in the Yangtze River basin and the Nenjiang and Songhua River basins in the summer of 1998.
"This El Niño event will affect the distribution of precipitation during China's flood season this year. The main rain belt during the flood season is expected to be located in the Yangtze River basin, with above-normal plum rain amounts and a high likelihood of flood disasters," said Gao Hui, chief forecaster at the National Climate Center.
According to forecasts by the National Climate Center, China's overall climate conditions during this year's flood season will be relatively poor, with above-normal precipitation and floods outweighing droughts. The middle and lower reaches of the Yangtze River, the Huai River, the Nenjiang River and the Songhua River basins should prepare for severe flood conditions. The southwestern region should strengthen defenses against flash flood and geological disasters such as landslides and mudslides. The southeastern and southern coastal areas should guard against rainstorm floods and wind disasters caused by strong typhoons.
Song Lianchun, director of the National Climate Center and research professor, reminded that all localities should closely monitor weather and climate changes and prepare for extreme weather and climate events. They should attach importance in thought, make arrangements in advance in action, and ensure that emergency response plans are practical and feasible.
Song Lianchun stressed that, unlike weather forecasting, climate prediction involves many uncertainties, and forecast opinions are mainly used for government decision-making and services. Regarding predictions of the position of the main rain belt during the flood season and meteorological disasters, meteorological departments will strengthen rolling consultations and promptly release the latest monitoring and prediction information.