An Asymmetric Spatiotemporal Connection between the Euro-Atlantic Blocking within the NAO Life Cycle and European Climates

Abstract

This paper examines an asymmetric spatiotemporal connection and climatic impact between the winter atmospheric blocking activity in the Euro-Atlantic sector and the life cycle of the North Atlantic Oscillation (NAO) during the period 1950–2012. Results show that, for positive NAO (NAO⁺) events, the instantaneous blocking (IB) frequency exhibits an enhancement along the southwest–northeast (SW–NE) direction from the eastern Atlantic to northeastern Europe (SW–NE pattern, hereafter), which is particularly evident during the NAO⁺ decaying stage. By contrast, for negative NAO (NAO⁻) events, the IB frequency exhibits a spatially asymmetric southeast–northwest (SE–NW) distribution from central Europe to the North Atlantic and Greenland (SE–NW pattern, hereafter). Moreover, for NAO⁻ (NAO⁺) events, the most marked decrease (increase) in the surface air temperature (SAT) in winter over northern Europe is in the decaying stage. For NAO⁺ events, the dominant positive temperature and precipitation anomalies exhibit the SW–NE-oriented distribution from western to northeastern Europe, which is parallel to the NAO⁺-related blocking frequency distribution. For NAO^- events, the dominant negative temperature anomaly is in northern and central Europe, whereas the dominant positive precipitation anomaly is distributed over southern Europe along the SW–NE direction. In addition, the downward infrared radiation controlled by the NAO’s circulation plays a crucial role in the SAT anomaly distribution. It is further shown that the NAO’s phase can act as an asymmetric impact on the European climate through producing this asymmetric spatiotemporal connection with the Euro-Atlantic IB frequency.

摘要

本文分析了1950-2012年冬季大西洋-欧洲大气阻塞与北大西洋涛动(NAO)在生命周期(天气尺度)内的时空非对称关系. 结果表明, 对于NAO正位相(NAO+)事件, 瞬时阻塞频率(IB)主要分布在大西洋东部至欧洲东北部, 呈现出西南-东北向(SW-NE)的舌状分布, 并且在NAO+事件的衰弱期最为明显. 同时, 对于NAO负位相(NAO-)事件, IB的空间分布相比NAO+事件则表现出非对称的特征, 主要表现出东南-西北(SE-NW)向的舌状分布, 自欧洲中部延伸至北大西洋和格陵兰地区. 此外, 还研究了NAO与阻塞对气温和降水的时空非对称影响. 对于NAO-(NAO+)事件, 最大的温度负(正)距平出现在NAO的衰减期. 温度和降水的空间分布也呈现出与IB空间分布相匹配的特征. 同时, NAO与阻塞, 温度和降水存在时间上超前滞后的非对称关系. 最后, 诊断分析表明, NAO对向下红外长波辐射的调控可以显著地影响表面气温的变化. NAO通过对欧洲-大西洋阻塞的非对称的调控, 进而在天气尺度上对欧洲气候产生时空非对称影响.