On the Origin of the Solar Cycle Modulation of the Winter North Atlantic Oscillation

Abstract

AbstractUsing data from recent 64 years, we found that the zonal wind signal associated with the winter‐mean North Atlantic Oscillation (NAO) tends to extend into the upper stratosphere when solar activity is high, but the vertical extension is limited when solar activity is low. To examine the physical source of this phenomenon, the momentum and wave‐energy budget are analyzed using the 63‐winter record of the Japanese Reanalysis data set from 1958 to 2021. These analyses show that (a) the zonal‐mean momentum acceleration is driven by eddies throughout the entire depth of the atmosphere around 60°N, (b) the barotropic energy conversion from eddies to zonal‐mean flow is centered in the middle stratosphere around 55°N, and (c) the baroclinic energy conversion from zonal‐mean flow to eddies from the surface to the middle stratosphere around 60°N tends to be enhanced from December to February associated with the positive winter‐mean NAO index in High Solar (HS) activity years. These energy transfers are closely related to enhanced zonal wind. These analyses suggest that the structural modulation of the NAO due to the solar cycle comes from the enhanced sensitivity of the wave‐mean flow interaction in the stratosphere in HS years, which is broadly shared with the solar cycle modulation of the late‐winter Southern Annular Mode (SAM).