The Intraseasonal North Atlantic Oscillation as a Quasi-Semiannual Propagating Disturbance

The North Atlantic Oscillation (NAO) is a well-studied mode of regional climate variability over the Northern Atlantic ocean typically associated with fluctuations in sea-level pressure, storm tracks, and the North Atlantic jet. These fluctuations are classically understood to be a kind of see-sawing between one climatic phase, corresponding to a more poleward North Atlantic jet, and the other, a more equatorward North Atlantic jet, making the NAO a regionalized annular mode. However, recent work has shown that annular modes are only sometimes quasi-stationary oscillations of the jet around its time mean position; at many other times they display a “propagating” regime hallmarked by a slow, poleward, meridional propagation of zonal wind anomalies. Using reanalysis data, this work demonstrates that the intraseasonal NAO also propagates with a period of about 140-days from the equator to the pole. We then develop a novel, regionalized budget of the eddy momentum forcing responsible for the NAO, and we proceed to utilize it to understand both the NAO’s stationary and propagating regimes. Propagation occurs because of an interaction between the first and second EOFs of zonal wind which results in a slow shift in critical latitudes for wave breaking, and a lower-level response to upper-level changes which reinforces this propagation. Latent heating opposes this propagation and acts to prolong the NAO’s persistence. The surface anomalies associated with the NAO also propagate along with it, providing further evidence that the intraseasonal NAO should not be understood as a stationary mode of climate variability.