Observed Long- and Short-lived North Atlantic Oscillation Events: Role of the Stratosphere
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• Original Paper •
Observed Long- and Short-lived North Atlantic Oscillation Events: Role of the Stratosphere Jie SONG*1 and Jingjing ZHAO1,2 1State
Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
2College
of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China (Received 13 February 2020; revised 22 July 2020; accepted 2 September 2020) 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. Key words: North Atlantic Oscillation, lifetimes, stratosphere, eddy forcing Citation: Song, J., and J. J. Zhao, 2020: Observed long- and short-lived North Atlantic Oscillation events: Role of the stratosphere. Adv. Atmos. Sci., 37(12), 1338−1358, https://doi.org/10.1007/s00376-020-0021-y. Article Highlights:
• The lifetimes of observed NAO events are related to the connection between stratospheric and tropospheric circulation
anomalies in the initial-value fields.
1. Introduction Many studies have pointed out that the fundamental forcing that drives and maintains the formation of the North Atlantic Oscillation (NAO; Walker, 1924; Wallace and Gutzler, 1981) is the anomalous synoptic eddy forcing (Feldstein, 2003; Luo et al., 2007; Song, 2016). Thus, the physical essences of the NAO are considered as lying on intraseasonal time scales despite fluctuations of the NAO exhibiting a wide range of variation from intraseasonal to interdecadal time scales (Feldstein, 2000). Actually, interannual and interdecadal variabilities of the NAO, at least partly, simply reflect the aggregated effect of noisy intraseasonal fluctuations of the NAO (Stephenson et al., 2000). A better understanding about the intraseasonal lifecycle (growth and decay) of NAO events is valuable for us
* Corresponding author: Jie SONG Email: [email protected]
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