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

Abstract

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.

Multi-decadal modulations in the Aleutian-Icelandic Low seesaw and the axial symmetry of the Arctic Oscillation signature, as revealed in the 20th century reanalysis

“…Thus, they proposed that the seasonality of the AO signature could be interpreted as a superposition of the AIS signature upon the robust, dominant signal of NAO. Moreover, Kuroda (2005) and Kodera and Coughlin (2007) pointed out that AIS represents the 'tropospheric AO' (Kodera and Kuroda, 2000) since the 1970s, in light of the similarity in their formation processes.…”