The Role of Underlying Boundary Forcing in Shaping the Recent Decadal Change of Persistent Anomalous Activity over the Ural Mountains

Abstract

Observational analyses demonstrate that the Ural persistent positive height anomaly event (PAE) experienced a decadal increase around the year 2000, exhibiting a southward displacement afterward. These decadal variations are related to a large-scale circulation shift over the Eurasian Continent. The effects of underlying sea ice and sea surface temperature (SST) anomalies on the Ural PAE and the related atmospheric circulation were explored by Atmospheric Model Intercomparison Project (AMIP) experiments from the Coupled Model Intercomparison Project Phase 6 and by sensitivity experiments using the Atmospheric General Circulation Model (AGCM). The AMIP experiment results suggest that the underlying sea ice and SST anomalies play important roles. The individual contributions of sea ice loss in the Barents-Kara Seas and the SST anomalies linked to the phase transition of the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) are further investigated by AGCM sensitivity experiments isolating the respective forcings. The sea ice decline in Barents-Kara Seas triggers an atmospheric wave train over the Eurasian mid-to-high latitudes with positive anomalies over the Urals, favoring the occurrence of Ural PAEs. The shift in the PDO to its negative phase triggers a wave train propagating downstream from the North Pacific. One positive anomaly lobe of the wave train is located over the Ural Mountains and increases the PAE there. The negative-to-positive transition of the AMO phase since the late-1990s causes positive 500-hPa height anomalies south of the Ural Mountains, which promote a southward shift of Ural PAE.

摘要

观测分析表明,乌拉尔环流正持续异常事件(PAE)在2000年前后呈现出年代际增加,并表现出发生位置偏南的特征。这些年代际变化与欧亚大陆上空的大规模环流变化有关。通过第六阶段耦合模式比较计划的大气环流试验,探讨了下垫面异常对乌拉尔PAE和相关大气环流的影响。结果表明,下垫面海冰和海表温度异常对乌拉尔PAE和相关大气环流起着重要作用。通过大气环流模式的敏感性试验,分别研究了巴伦支海-喀拉海海冰减少和与太平洋年代际振荡(PDO)和大西洋多年代际振荡(AMO)位相转变相关的海温异常的单独贡献。巴伦支海-喀拉海海冰减少在欧亚中高纬度地区激发了大气波列,在乌拉尔上空出现了正异常,有利于乌拉尔PAE的发生。PDO向负相位的转变激发了从北太平洋向下游传播的波列,波列的一个正异常中心位于乌拉尔山地区,增多了该地区的PAE。20世纪90年代末AMO位相由负变正导致了乌拉尔山脉以南500hPa的正高度异常,促进了乌拉尔PAE的南移。