El Nino is leaving. Will the impact last?

Since the beginning of the flood season, the rainstorm weather process in southern China has occurred one after another. The National Climate Center expects the El Ni ñ o event to end in May, but its impact on the atmosphere will continue and will be further revealed or magnified during this year's main flood season, especially during the June-July rainy season. The possibility of abnormal precipitation in the Yangtze River Basin and other places in the main flood season is greater, and the situation of flood control is not optimistic.

This is the strongest El Nino event since complete ocean observations were available in 1951.

On May 19, the south ushered in the 14th round of heavy rainfall since the start of the flood season on March 21. The Central Meteorological Observatory issued a yellow warning for heavy rain on the 19th, and the China Meteorological Administration launched a level 4 emergency response to heavy rain in the south.

Meteorological statistics show that as of May 18, the country's average precipitation was 119.6 mm, 25% more than that of the same period in a normal year and the second largest since 1961. Since the beginning of the flood season, the precipitation in the middle and lower reaches of the Yangtze River and most areas to the south of the Yangtze River has reached 200 Mel 600 mm, while that in southern Anhui, northeast Jiangxi, northwest Fujian, northern Guangdong and northern Guangxi has reached 600 Mel 800 mm.

Zhou Bing, chief expert on climate monitoring at the National Climate Center, pointed out that "heavy precipitation occurs frequently in the south, which is related to El Ni ñ o." This is mainly due to the influence of El Ni ñ o, which stimulates a high-pressure circulation system from the west of the Philippines to the South China Sea, which is superimposed on the western Pacific subtropical high, making it abnormally strong and southward. Water vapor from the tropical Pacific is transported to southern China, providing water vapor conditions for the frequent occurrence of heavy precipitation. "

El Ni ñ o refers to the phenomenon that the sea water is abnormally warm in a large area in the east-central equatorial Pacific Ocean. When the sea surface temperature of the sea area lasts for more than 3 months, which is 0.5 degrees Celsius higher than that of the same period of the normal year, it enters the "El Nino state". When the sea surface temperature lasts for more than 6 months, it is 0.5 degrees Celsius higher than the same period of the normal year, it is confirmed as an "El Nino event".

Zhou Bing specifically explained that the process of the influence of El Ni ñ o on China's weather and climate is realized through the upstream effect. Generally speaking, when the El Ni ñ o event occurs, the updraft becomes unusually strong due to the rise of sea surface temperature in the equatorial Middle East and Pacific Ocean. After rising to the upper level, the air flow diverges outward, the westerly wind at the upper level over the equatorial Pacific weakens, and the abnormal easterly wind prevails. The abnormal easterly sinks in the equatorial western Pacific and triggers the abnormal anticyclonic circulation in the lower layer over the western Pacific, which strengthens and extends westward the West Pacific subtropical high, which is beneficial to the transport of the southwest warm and humid air flow on the west side of the subtropical high to southern China, resulting in more precipitation in the southern region. In addition, a few months after the El Ni ñ o event, the tropical Indian Ocean will begin to warm up, and abnormal anticyclones will be triggered or strengthened near the western Pacific to enhance the transport of southwest warm and humid air to southern China.

According to the National Climate Center, the El Ni ñ o event began to develop in September 2014 and continues to this day. According to the current operational norms of the National Climate Center, there have been 14 El Nino events in the world since 1951, of which 3 have reached the super-strong level, namely, the 1982 / 1983 El Nino event, the 1997 / 1998 El Nino event and this event. From many aspects, this El Nino event is the strongest since the complete ocean observation data were available in 1951, and many SST indicators have exceeded the previous two super-strong events. However, it is worth noting that the response of the tropical atmosphere to super-strong El Nino is not as significant as the previous two super-strong El Ni ñ o events.

The super El Nino event will end, but the atmospheric response will continue.

Zheng Fei, deputy director and researcher of the International Climate and Environmental Science Center of the Institute of Atmospheric Physics of the Chinese Academy of Sciences, said that the intensity of the El Nino event peaked in November last year and has been weakening since then, and is expected to end in May-June this year.

Zhou Bing pointed out that different monitoring results will be obtained due to different monitoring target areas, marine assimilation data and data processing methods adopted by different countries and different institutions, but the trend should be consistent. So far, major foreign institutions have not released information about the end of the El Nino incident.

Zhou Bing stressed, "although the super El Nino event is coming to an end, the atmospheric response to the super El Ni ñ o event will continue, due to the increase in water vapor transport in summer." In the main flood season, especially in the rainy season from June to July, the climate response can be further revealed or magnified, so the situation of disaster prevention and reduction is not optimistic. "

According to experts from the National Climate Center, according to the law of historical statistics, in the second year after the El Nino event reached its peak, floods are easy to occur in the Yangtze River basin and the south of the Yangtze River in summer. The super strong El Nino event in 1982 / 1983 led to the continuous torrential rain in the middle and lower reaches of the Yangtze River from June to July 1983, and the water level at many stations of the Yangtze River reached the highest in history, while the super El Nino event in 1997 / 1998 led to severe floods in the Yangtze River, Nenjiang and Songhua River basins in the summer of 1998.

"this El Nino event will have an impact on the distribution of precipitation in this year's flood season in China. It is expected that the main rain belt in the flood season is located in the Yangtze River basin, with more plum rainfall and prone to flooding. " Gao Hui, chief forecaster of the National Climate Center, said.

According to the forecast of the National Climate Center, the overall climate situation in China is deviated in this year's flood season, with more precipitation and more waterlogging than drought. The middle and lower reaches of the Yangtze River, the Huaihe River, the Nenjiang River and the Songhua River Basin should be prepared for serious flood. The southwest region should strengthen the prevention of mountain torrents and geological disasters such as landslides and debris flows. The coastal areas of Southeast China and South China should pay attention to preventing rainstorms, floods and strong winds caused by strong typhoons.

Researcher Song Lianchun, director of the National Climate Center, warned that all localities should closely monitor weather and climate change and prepare for extreme weather events. Pay attention to it ideologically, arrange the action in advance, and the emergency plan should be feasible.

Song Lianchun stressed that unlike the weather forecast, there are many uncertainties in the climate forecast, and the forecast opinions are mainly used for government decision-making and services. For the prediction of the location of the main rain belt and meteorological disasters in the flood season, the meteorological department will strengthen rolling consultation and release the latest monitoring and forecasting information in a timely manner.