Importance of a vertically tilting structure for energizing the North Atlantic Oscillation

The North Atlantic Oscillation, a prominent mode of tropospheric variability responsible for weather fluctuations over a large part of Europe and Northeastern America, has been traditionally regarded as a low-frequency equivalent barotropic variability that is primarily driven and maintained through interactions with high-frequency transient disturbances. The analysis of the energetics of the NAO carried out in this work, however, reveals that baroclinic energy conversion from the background jets and planetary waves is primarily responsible for maintaining the energy of the NAO, owing to the vertically tilting baroclinic structure of the NAO anomalies. Eddy feedback processes, on the other hand, account for only a small net fraction of the NAO maintenance: high-frequency eddies typically remove available potential energy from the NAO anomalies and give it back in the form of kinetic energy, which results in a weak net contribution. Seasonality in the efficiency of the baroclinic energy conversion accounts for the larger amplitude of NAO-related circulation anomalies in winter, which is ascribed to the enhanced baroclinic nature of the NAO anomalies.