Journal of Meteorological Research

, Volume 33, Issue 4, pp 593–608 | Cite as

Why Is the East Asian Summer Monsoon Extremely Strong in 2018?—Collaborative Effects of SST and Snow Cover Anomalies

  • Lijuan Chen
  • Wei GuEmail author
  • Weijing Li
Forecasting Forum


In 2018, summer precipitation was above normal in North and Northwest China and below normal around the Yangtze River valley, due to an extremely strong East Asian summer monsoon (EASM). The atmospheric circulation anomalies in East Asia and key external forcing factors that influence the EASM in 2018 are explored in this paper. The results show that there existed an anomalous cyclonic circulation near the Philippines, while the western Pacific subtropical high was located more northward than its normal position. In the mid-high latitudes, a negative geo-potential height anomaly center was found near the Ural Mountains, suppressing the blocking activity. Under such a circulation pattern, precipitation near the Yangtze River valley decreased because local divergence and subsidence intensified, whereas precipitation in northern China increased due to large amounts of water vapor transport by anomalously strong southerly winds. Further analyses reveal that the strong EASM circulation in 2018 might result from the joint influences of several external forcing factors. The weak La Nina event that started from October 2017, the positive North Atlantic Tripolar mode (NAT) in spring 2018, and the reduced snow cover over the Tibetan Plateau in winter 2017/18 all collaboratively contributed to formation of the cyclonic circulation anomaly near the Philippines, leading to the extremely strong EASM. Especially, the positive NAT and the reduced Tibetan snow cover may have caused the negative geopotential height anomaly near the Ural Mountains, in favor of a strong EASM. The above external factors and their reinforcing impacts on the EASM are further verified by two groups of similar historical cases.

Key words

East Asian summer monsoon external forcing factors La Niña North Atlantic Tripolar mode Indian Ocean basin mode snow cover of the Tibetan Plateau 


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We thank the four anonymous reviewers for their constructive comments that led to improvements to this paper.

Supplementary material

13351_2019_8200_MOESM1_ESM.pdf (3.6 mb)
Why Is the East Asian Summer Monsoon Extremely Strong in 2018?—Collaborative Effects of SST and Snow Cover Anomalies


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Copyright information

© The Chinese Meteorological Society and Springer-Verlag Berlin Heidelberg 2019

Authors and Affiliations

  1. 1.Laboratory for Climate Studies, National Climate CenterChina Meteorological AdministrationBeijingChina
  2. 2.Collaborative Innovation Center on Forecast and Evaluation of Meteorological DisastersNanjing University of Information Science & TechnologyNanjingChina

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