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

作　　者：Lijuan CHEN Wei GU Weijing LI 

机构地区：[1]Laboratory for Climate Studies,National Climate Center,China Meteorological Administration,Beijing 100081 [2]Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters,Nanjing University of Information Science&Technology,Nanjing 210044

出　　处：《Journal of Meteorological Research》2019年第4期593-608,共16页气象学报（英文版）

基　　金：the National Key Research and Development Program of China(2018YFC1506006);National Science and Technology Support Program of China(2015BAC03B04);National Natural Science Foundation of China(41805067 and 41275073)

摘　　要：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 geopotential 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 Ni?a 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.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 geopotential 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 Ni?a 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.

关 键 词：East Asian summer monsoon external forcing factors La Nina North Atlantic Tripolar mode Indian Ocean basin mode snow cover of the Tibetan Plateau 

分 类 号：P73[天文地球—海洋科学]