Zonal wavenumber characterization of the North Atlantic Oscillation-associated anomalies

In the last decades, there has been an ongoing discussion about whether the winter NAO has a deep barotropic and annular signature, being the stratosphere a key player of this variability, or, on the contrary, the NAO presents a wave-like behavior with its perturbations originating in the troposphere and propagating upward and downstream.

In our work we tackle this discussion through zonal wavenumber decomposition of the NAO-related anomalies in both reanalysis (ERA5 and NCEP-NCAR) and climate models of different complexity and resolution (SPEEDO/L8, CAM5.3/L46 and EC-EARTH3.3/L91). We find that the NAO exhibits a non-annular, wave-like structure in all levels, particularly in the upper troposphere (300 hPa) but also in the middle stratosphere (30 hPa). Indeed, we identify that the troposphere-stratosphere coupling associated with the NAO includes a wavenumber 2 structure tilting westward and increasing in amplitude with height, indicating upward propagation and forcing of the polar vortex. This recurrent NAO-related wavenumber 2 anomaly could contribute to the characteristic wavenumber 2 pattern of the climatological stratospheric circulation. Wavenumber 1, on the other hand, shows no statistical significance in the stratosphere; while in the troposphere it appears to be an artifact of the method, reflecting the prominence of regional dynamics in the North Atlantic. At upper-tropospheric levels, wavenumber 3 seems to dominate the wave-like structure of the total field at mid/high latitudes, whereas wavenumber 5 is more prominent at lower latitudes. Finally, wavenumber 4 does not significantly contribute to the NAO-related anomalies. These results are robust in reanalysis and in the climate models, although some differences are noticeable, likely linked to model biases in the mean flow.

To further interpret the vertical propagation of long waves (wavenumbers 1 and 2) and the horizontal propagation of small waves (wavenumber 3 and 5), additional diagnostics such as eddy heat flux and wave activity flux will be analyzed.