Large-scale tropospheric wave activity involved in the winter NAO-related variability

In the last decades, there has been an ongoing discussion whether the winter North Atlantic Oscillation (NAO) is a zonally-symmetric hemispheric mode of variability driven by transient eddies and strongly coupled to the stratosphere [Arctic Oscillation/Northern Annular Mode (AO/NAM)]; or a regional mode of variability forced locally by transient eddies (NAO) but with associated hemispheric anomalies related to stationary eddies in the Circumglobal Waveguide Pattern (CWP). We revisit this question using zonal wavenumber decomposition of NAO-related circulation anomalies in reanalysis (ERA5, NCEP-NCAR). At upper-tropospheric levels, the NAO exhibits a wave-like structure that resembles the CWP, where wavenumber 3 seems to dominate at subpolar latitudes and  wavenumber 5 is more prominent at suptropical latitudes. Wavenumber 4 does not significantly contribute to the NAO pattern in the total field. The wave activity flux of NAO-related variability reveals downstream propagation and splitting of wave energy which appears to be consistent with the meridional component of the wind and with theoretical arguments of stationary wavenumber based on the background flow. These results support the relevance of large-scale stationary waves in the hemispheric signature of the NAO. To further diagnose the tropospheric propagation of NAO-related anomalies, additional analysis using ray tracing and experiments with a linear barotropic model are performed.