Dynamical feedbacks and the persistence of the NAO

The persistence of the North Atlantic Oscillation (NAO) is studied using observations of the three-dimensional

vorticity budget in the Atlantic sector. Analysis of the relative vorticity tendency equation shows that convergence

of eddy vorticity flux in the upper troposphere counteracts the effect of anomalous large-scale

divergence at the upper level. At low levels, the convergence associated with this large-scale vertical circulation

cell maintains the relative vorticity anomaly against frictional drag. The eddy vorticity flux convergence

thus acts to sustain the vorticity anomaly associated with the NAO against drag and increases the persistence

of the NAO vorticity anomaly. The adiabatic cooling associated with the rising motion in the vorticity

maximum also sustains the thermal structure of the NAO anomaly. This constitutes a positive eddy feedback

that helps maintain the NAO. The positive eddy feedback occurs only in the mid-latitude region and is

strongest when the Atlantic jet is displaced toward the equator, with a high pressure anomaly to the north

and a low pressure anomaly to the south. The stronger feedback demonstrated in the case where the jet is

displaced toward the equator is consistent with the greater persistence observed for this phase of the NAO.

The positive feedback appears to be associated with anomalous northward eddy propagation away from the

jet.