Apparent Vertical Propagation of the NAM Index Due to an Embedded Wave Structure

[摘要] The leading empirical orthogonal function (EOF) of the geopotential height is called the Northern Annular Mode (NAM). Stratosphere-troposphere connections seen in the multilevel NAM signal are often considered as due to a propagation of the annular structure. However, the tropospheric leading EOF contains not just an annular structure, but also a wave structure. This indicates that the NAM index is sensitive to changes in wave activity. A previous study showed that an apparent downward propagation of the NAM index can appear as a result of a reflection of wave 1 due to a stratospheric sudden warming. If this result is correct, we may also expect to find a reverse case. Here we show a case when an amplification of a tropospheric wave 1 leads to a stratospheric warming, an apparent upward propagation of the "Annular" mode signal, i.e., change in the amplitude of the NAM index appears due to a projection on the wave component of the tropospheric NAM.

[学科分类] 大气科学