Observed Long- and Short-lived North Atlantic Oscillation Events: Role of the Stratosphere

Abstract

Utilizing three different sets of reanalysis data, this study examines the long- and short-lived observed positive North Atlantic Oscillation (NAO) events (referred to as NAO+_LE and NAO+_SE) and long- and short-lived observed negative NAO events (referred to as NAO−_LE and NAO−_SE). Composite results indicate that the NAO-like circulation anomalies associated with the long-lived NAO events can reach the stratosphere, while they are primarily confined to the troposphere in the short-lived NAO events. Thus, the coupling/connection of stratospheric and tropospheric circulation anomalies is much better (worse) in the long-lived (short-lived) NAO events. A series of modified stratospheric initial-value experiments conducted with a simplified model indicate that a better (worse) connection between stratospheric and tropospheric circulation anomalies in the initial-value fields tend to gradually induce the NAO-like tropospheric circulation anomalies in the troposphere on the subsequent days, and thus naturally elongate (reduce) the lifetimes of the original NAO events by altering the tropospheric synoptic eddy vorticity flux over the North Atlantic region.

摘要

利用三套不同的再分析资料, 本文研究了观测的北大西洋涛动(NAO)的长生命和短生命事件。合成的结果表明, 长生命NAO事件的环流异常可以上探到平流层, 而短生命NAO事件的环流异常则主要局限于对流层。因此, 平流层和对流层的联系/耦合在长(短)生命的NAO事件中更为紧密(疏远)。利用一个简单大气模式运行的一组改变NAO事件的平流层初值场的实验表明, 在初值场中, 联系更为紧密(疏远)的平流层和对流层, 通过影响对流层天气涡旋强迫, 能够逐渐导致在随后的对流层中出现和原先NAO事件符号相同(相反)且空间结构类似的环流异常。这将自然地延长(缩短)原先NAO事件的生命期。