Is the Atmospheric Zonal Index Driven by an Eddy Feedback?

The authors address the question of whether or not eddy feedback plays an important role in driving the anomalous relative angular momentum associated with the zonal index (ZI) in the atmosphere. For this purpose, composites of anomalous relative angular momentum and anomalous eddy angular momentum flux convergence (eddy forcing) are examined with National Centers for Environmental Prediction–National Center for Atmospheric Research Reanalysis data. By using an empirical orthogonal function analysis, it is found that ZI behavior dominates the summer season of both hemispheres and also the winter season of the Southern Hemisphere. For the summer season, the ZI is characterized by meridional displacements of the midlatitude eddy-driven jet, and for the Southern Hemisphere winter it is characterized by a simultaneous movement of the subtropical and eddy-driven jets in the opposite direction. For the ZI of each of the above seasons, unfiltered eddy forcing did not exhibit a prominent eddy feedback. However, suggestive evidence for a feedback by high-frequency eddies (period less than 10 days) was found. These eddies act to prolong the lifetime of the ZI anomalies against the dissipative influences of both lowfrequency (period greater than 10 days) and cross-frequency (eddy fluxes that involves the product of highand low-frequency disturbances) eddy forcing and the friction torque.