北极—欧亚温度偶极型对应环流的非对称特征及可能成因

基于1950—2021年美国国家环境预报中心/大气研究中心(NCEP/NCAR)再分析资料以及英国哈德莱中心提供的海温、海冰密集度资料,分析了年际尺度上北极—欧亚温度偶极型模态及其对应环流的非对称特征和可能形成机理。结果表明,这种模态自身即相关地表气温异常的位置和强度在其正负位相下非对称性都较弱,其中暖北极—冷欧亚(Warm Arctic-Cold Eurasia,WACE)事件下的温度异常分布略偏北,在欧亚大陆的冷异常相较于冷北极—暖欧亚(Cold Arctic-Warm Eurasia,CAWE)事件暖异常的强度略强。然而,WACE模态正负位相对应的环流有明显的非对称性,WACE事件对应着弱的北大西洋涛动(North Atlantic Oscillation,NAO)正位相以及欧亚大陆上空的强大的高压异常,而CAWE事件则对应着强NAO负位相。进一步研究表明,这种环流的非对称性特征与热带太平洋海温调控的非对称性有关,当La Niña事件出现,尤其海温的负异常中心偏西时,欧亚大陆中高纬地区地表气温易呈现WACE的形态,而CAWE事件则主要由大气内部变率调控,与热带太平洋海温的联系不明显。

Asymmetric characteristics and possible mechanisms of the circulation corresponding to the Arctic-Eurasian temperature dipole pattern

Based on the atmospheric reanalysis data from NCEP/NCAR and the sea surface temperature and sea ice concentration data from 1950 to 2021 provided by the Met Office Hadley Centre, this paper concentrates on the Arctic-Eurasia temperature dipole mode on the interannual timescale and the asymmetric characteristics of its corresponding circulation and possible mechanisms. Results show that the asymmetry of the surface air temperature anomalies of the WACE (Warm Arctic-Cold Eurasia) pattern is weak from both aspects of location and intensity. The temperature anomalies during the WACE events are distributed slightly more northward, and the cold anomalies in Eurasia are slightly stronger compared with the intensity of the warm anomalies during the Cold Arctic-Warm Eurasia (CAWE) events. However, the corresponding circulation of the WACE pattern has obvious asymmetry in its warm and cold phases. The WACE events are usually accompanied with weak positive phase of the North Atlantic Oscillation (NAO) and strong high-pressure systems over Eurasia, while the CAWE events correspond to strong negative phase of NAO. Further analyses show that the asymmetric characteristics of this circulation are related to the asymmetric modulation of the sea surface temperature in the tropical Pacific. When a La Niña event occurs, especially when the center of negative sea temperature anomalies is displaced westward, the surface air temperature anomalies in the mid-high latitudes of Eurasia tend to present a WACE-like pattern. In contrast, CAWE events are mainly controlled by atmospheric internal variability, which are not closely related to the tropical Pacific Sea surface temperature anomalies.