欧洲地区夏季热浪的特征及其与阻塞环流的联系

选取了一个热浪指数,利用地面2 m气温场和500 h Pa位势高度场的美国环境预报中心和国家大气研究中心(NCEP/NCAR)再分析资料,通过聚类分析发现欧洲大陆容易产生6类热浪:西欧型(WE)、俄罗斯型(RU)、东欧型(EE)、斯堪的纳维亚半岛型(SC)、北海型(NS)、伊比利亚半岛型(IB);这些热浪事件都与欧洲大陆阻塞的位置有关。同时我们发现这6类热浪发生的频率出现明显的年代际变化,特别在20世纪80年代以后欧洲大陆热浪发生频率明显的增多趋势可能与欧洲大陆增暖背景有关,而欧洲大陆热浪发生频率的年代际变化可能是夏季北大西洋涛动(NAO)的年代际变化的结果。夏季NAO偶极子通过欧洲地区的阻塞异常对欧洲大陆气温有重要的调制作用。当夏季NAO指数处于正位相阶段时,欧洲大陆容易产生高纬度热浪,反之则容易产生低纬度热浪,并且欧洲大陆增暖趋势并不影响NAO对欧洲气温的调制作用。同时还发现:大西洋夏季NAO事件可以是欧洲热浪发生的前期条件,欧洲大陆阻塞异常落后于NAO事件1~5 d,其中IB型和WE型与NAO同期相关,其余4类型热浪对应阻塞落后于NAO 4~5 d。另外,也发现大西洋—欧洲大陆定常波列正距平的位置通过对欧洲阻塞的影响,而影响欧洲热浪发生的频率和位置。

Characteristics of Summer Heatwaves over Europe and Their Link with European Blocking Circulations

Based on daily surface air temperature at 2 m and 500-hPa geopotential height fields from the NCEP/NCAR reanalysis dataset, a heat wave index over Europe is calculated in this paper. Spatial characteristics of heat waves over Europe and associated atmospheric circulations are investigated using the K-mean clustering analysis method. It is shown that the heat waves over Europe can be classified into six types with different spatial patterns:Western Europe (WE),Russian (RU), Eastern Europe (EE), Scandinavian (SC), North Sea (NS), and Iberian (IB). The frequency of heatwave events over Europe is found to have increased significantly after the 1980s, which may be associated with the global warming. The decadal variability of the summer North Atlantic oscillation (SNAO) is possibly a reason for the decadal variability of European heatwaves. While the global warming can increase the frequency of European heat waves, the atmospheric internal processes that affect the strength, position, and frequency of European heat waves seem to be dominant. Composite of geopotential height anomalies shows that the occurrence region of European heat waves is determined by the position of the blocking anticyclone over Europe that is tightly tied to positive stationary wave train anomalies over Euro-Atlantic region and the phase of NAO. Low latitude-type heat waves are more likely to take place when the SNAO index decreases to a negative value, while the high latitude-type heat waves often occur during the period when the NAO index is in a positive phase. Moreover, it is found that the NAO event precedes the European blocking by 1-5 days, suggesting that the phase of NAO can modulate the European heat waves.