北极涛动主模态下北极冷空气的优势路径和影响地区的研究

摘要

本文以拉格朗日观点分析北极涛动 (Arctic Oscillation, AO) , 也被称为北半球环状模 (Northern Hemisphere Annular Mode, NAM) 的指数异常事件中北极近地面冷气团的活动路径, 直接地表现出了异常事件中冷气团运动的优势路径, 从而反映出AO/NAM对地面气温的直接调控作用.在正AO/NAM指数异常事件中, 极区近地面冷气团活动轨迹以纬向环流为主, 表现为环绕北半球中高纬地区的冷气团活动轨迹特征明显.而在负AO/NAM指数异常事件中, 极区冷气团以反气旋式轨迹流出极区后, 流入中纬度海洋上的低气压区, 这种由极区向中纬度地区流动的经向运动轨迹特点显著.并且在指数下降的中后期出现两种强烈影响欧亚大陆的运动轨迹.正负事件中冷气团运动轨迹很好地解释了传统公认的AO/NAM对北半球不同地区冬季气温的影响.特别是对中国冬季气温的影响上, 正AO/NAM指数异常事件中的中低层冷气团活动有利于南支槽加深, 进而为南方地区冰冻雨雪天气提供了有利条件;而负事件中的极地近地面冷气团可直接影响东北地区, 形成寒潮降温天气.%The Arctic Oscillation (AO) /Northern Hemisphere Annular Mode (NAM) has a strong influence on weather and climate on different time scales, of which the intrinsic time scale is about 1020days.The synoptic time-scale variable of the AO/NAM determines its intrinsic and direct influence on weather.And usually extreme weather events occur corresponding to the abnormal AO/NAM indices.This is because the AO/NAM can modulate the movement of nearsurface cold air in the Arctic that can affect the atmospheric circulation in the North Hemisphere and is the direct cause of cold surges and cold air outbreaks.Therefore, it is of great importance to investigate the motions of the near-surface cold air over the Polar under the modulation of the AO/NAM.Based on previous studies, we track trajectories of the near-surface air parcels pole ward of 60°N during AO/NAM events from the Lagrangian viewpoint.The dominant trajectories during the AO/NAM events are direct reflections of the AO/NAM′s modulation to surface air temperature.During positive AO/NAM events, the near-surface atmospheric masses are restricted in the regions pole ward of 45°N.The air parcels converge in the regions of Aleutian islands and Norwegian Sea consistent with low pressure.The prominent feature of the cold air parcels during positive AO/NAM events is that latitudinal circulation prevails at middle and high latitudes, which limits the cold surge outbreak.During negative AO/NAM events, the air parcels flow out anticyclonically from the polar to the northern Pacific and Atlantic, respectively, especially in the valley of the AO/NAM index.Of particular importance, there are two dominant trajectories strongly influencing the Eurasian continent during the middle and later phases of the negative AO/NAM index decreasing stage.The trajectories of the near-surface air parcels over the polar match the correlation map of wintertime surface air temperature with the AO/NAM index very well and the results give a visualized interpretation of the AO/NAM′s influence on weather.Also, near-surface air parcels during the positive and negative AO/NAM events exert different effects on northern and southern China.During positive AO/NAM events, the south-branch trough strengthens, which favors the transfer of cold air from mid-and high-latitude areas of Europe as well as water vapor from the Bay of Bengal to southern China, which results in conditions that favor the formation of cold waves and even frozen rain and snow in southern China.In comparison, during negative AO/NAM events, near-surface cold air in the eastern New Island and northern Iceland affect most of Europe and the easterly movement of these cold air masses directly affect northeastern China and key cold-wave regions.With the Siberian High and the East Asian Major Trough, the latter can cause temperature decreases, e.g., cold waves and strong winds, and further influence temperature in other regions of China.