Generated using version 3.0 of the official AMS LATEX template Dynamical feedbacks and the persistence of the NAO

• Main Authors: Elizabeth A. Barnes, Dennis L. Hartmann
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: North Atlantic; North Atlantic oscillation
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.533.448; http://www.atmos.washington.edu/~dennis/Barnes_Hartmann_2009.pdf

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 persis-tence of the NAO vorticity anomaly. The adiabatic cooling associated with the rising motion in the vorticity maximum sustains the thermal structure of the NAO anomaly, enhancing the baroclinicity and thus eddy generation. This constitutes a positive eddy feedback that helps maintain the NAO. The positive eddy feedback occurs only in the midlatitude 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. 1 1.