The Subseasonal North Atlantic Oscillation is a Quasi-Semiannual, Propagating Disturbance

Samuel Smith , Jian Lu, Paul W Staten

The North Atlantic Oscillation (NAO) is a well-studied mode of regional climate variability, associated with fluctuations in sea-level pressure (SLP), storm tracks, and the North Atlantic jet. These fluctuations have been perceived as a seesawing between two climatic phases, one corresponding to a more poleward jet and the other to a more equatorward. However, recent work has shown that zonal wind anomalies also propagate poleward at interseasonal timescales. Using reanalysis data, this work demonstrates for the first time that the subseasonal NAO propagates with a 145-day period, explaining the recently discovered long-term predictability of the NAO. This propagation period can be predicted from a reduced-order model of zonal wind dynamics. This propagating behavior is fundamental to the NAO, representing the true “dynamic mode” of North Atlantic jet variability, and removing this propagation in the reduced-order model decreases the NAO’s predictability. Furthermore, the NAO’s climate anomalies, including SLP, propagate along with the wind; SLP exhibits the same low-frequency periodicity as the wind. This suggests the NAO has two under-studied phases in quadrature with the “seesaw” phases. Finally, our work suggests that the North Atlantic response to uniform climate forcing will include both a poleward jet shift and a jet strengthening.