Hemispheric stationary-eddy activity of the winter North Atlantic oscillation

Abstract

The winter North Atlantic Oscillation (NAO), key driver of climate variability in the Euro-Atlantic sector, remains unsettled as to its spatio-temporal characteristics in the Northern Hemisphere. A wavenumber decomposition of NAO-related circulation anomalies is performed using reanalysis data to explore its hemispheric signature, which exhibits a non-annular, wave-like structure at all levels. In the troposphere-stratosphere system, the NAO variability yields a wavenumber 2 pattern tilting westward and increasing in amplitude with height, suggesting upward propagation. Wavenumber 1, instead, shows no statistical significance in the middle stratosphere, unlike the El Niño-Southern Oscillation teleconnection where is dominant. In the upper troposphere, the NAO variability displays a wavenumber 5 pattern at middle-subtropical latitudes reminiscent of the Circumglobal Waveguide Pattern, and a wavenumber 3 pattern at middle-subpolar latitudes which is phase-locked with climatology in the lower troposphere. These findings emphasize that interannual NAO dynamics not only include transient, synoptic-scale eddy activity in the North Atlantic but also stationary, large-scale wave activity far beyond, having marked implications for climate predictability.