北极海冰与夏季欧亚遥相关型年际变化的联系及对我国夏季降水的影响

本文利用NCEP/NCAR等再分析资料和CAM3.1数值模式研究了夏季欧亚中高纬遥相关型年际变率与前期春季北极海冰变化的联系及其对我国夏季降水影响的可能机制.结果表明，夏季北大西洋-欧亚中高纬地区500 hPa位势高度场自然正交分解第二模态表现为"-+-+"遥相关波列，其中格陵兰岛-北大西洋和乌拉尔山地区为异常高空槽区所控制，而欧洲和贝加尔湖附近地区则为异常高压脊区，这种波列分布与欧亚中高纬EU型遥相关型十分类似.当遥相关波列为"-+-+"（"+-+-"）型分布时，前期春季巴伦支海北部和巴芬湾一带海冰偏少（多），同期夏季巴伦支海北部一带海冰亦持续偏少（多），同时在我国东北北部地区、长江和黄河之间地区降水明显偏少（多）.深入分析发现，巴伦支海北部和巴芬湾一带海冰偏少后，由于该地区湍流热通量明显偏强，在动力过程影响方面会形成异常Rossby波源，准定常Rossby波活动通量将向东亚地区传播，使得夏季欧亚中高纬"-+-+"遥相关波列出现.另外，海冰异常偏少后，在热动力过程影响方面，4-5月欧亚中高纬乌拉尔山-贝加尔湖以北地区积雪会出现"西少东多"偶极子型异常分布，其通过影响后期土壤湿度及下垫面热通量异常，也有利于夏季欧亚中高纬遥相关波列的维持.伴随着欧亚中高纬"-+-+"遥相关波列的出现，乌山阻塞高压偏弱，东亚槽偏浅，且亚洲副热带急流随之加强，贝加尔湖以北的副极地地区出现西风异常，东亚副热带急流北侧出现东风异常，贝加尔湖以南地区为异常反气旋控制，南下冷空气活动减弱.受到上述环流形势影响，我国东北北部地区、黄河和长江之间地区降水明显偏少.当巴伦支海北部和巴芬湾区域海冰偏多时，结论则反之.最后，基于春季海冰指数和晚春偶极子型积雪指数，我们建立了江淮流域夏季降水的预测模型，回报结果表明其对江淮流域夏季降水的年际变率具有较高的预测技巧.

Relationship between the interannual variations of Arctic sea ice and summer Eurasian teleconnection and associated influence on summer precipitation over China

This work investigates the relationship between Eurasian mid-high latitudes teleconnection in summer and the interannual variations of Arctic sea ice concentration (SIC) from preceding spring to summer and the associated influence on East Asian summer precipitation during the period of 1980-2015 by using NCEP/NCAR reanalysis datasets and the Community Atmospheric Model version 3.1. Results show that the second Empirical Orthogonal Function mode of the summer 500 hPa geopotential height field over mid-high latitudes of the North Atlantic-Eurasian continent shows a clear "-+-+" pattern with an anomalous trough over the region from Greenland to the North Atlantic, Ural Mountains, and a anomalous ridge over Europe and regions around Lake Baikal, which is similar to the Eurasian (EU) teleconnection pattern. When the summer Eurasian teleconnection exhibits a "-+-+" ("-+-+") pattern, the light (heavy) SIC occurs in regions over Northern Barents Sea and Baffin Bay in preceding spring and over Northern Barents Sea in simultaneous summer, leading to deficit (excessive) summer precipitations over most regions to north of the Yangtze River valley, especially northern Northeast China and the region between Yellow and Yangtze River valleys. Further study shows that the persistence of light SIC tends to change the local surface turbulent heat flux from spring to summer, triggering the anomalous Rossby wave sources and the propagation of quasi-stationary Rossby wave activity flux southward and eastward from Arctic to East Asia. These physical processes ultimately result in the development of anomalous summer Eurasian teleconnection. In addition, corresponding to the influence of light SIC condition, the "-+" dipole pattern of anomalous Eurasian snow water equivalent (SWE) in April-May is observed from the Ural Mountains to north of Lake Baikal. This dipole SWE pattern then changes the soil moisture and associated heat flux anomalies in the following summer, further resulting in the enhancement of the summer Eurasian teleconnection. Accompanying with occurrence of the "-+-+" Eurasian teleconnection, the Ural blocking and the East Asian trough are weakened. Meanwhile, the summer Asian subtropical jet stream is accordingly accelerated, with increase of the westerly wind over subpolar regions and decreasing to the north of the East Asian subtropical jet stream. This favors formation of anomalous anticyclonic circulation in south of Lake Baikal, leading to weaker cold air activities. The aforementioned circulation system ultimately results in deficit of summer precipitation in northern Northeast China and the regions between Yellow and Yangtze River valleys. Relative to heavy SIC conditions, the influences are inversed. On the basis of the spring SIC index and late spring SWE index, we also establish a prediction model for the summer precipitation in the Yangtze-Huaihe River valley. The hind-cast results show that our prediction model can well predict the variation of summer precipitation in the Yangtze-Huaihe River valley.