An Observational Study of the Intraseasonal Poleward Propagation of Zonal Mean Flow Anomalies

The poleward propagation of zonal-mean relative angular momentum (MR) anomalies is examined using NCEP– NCAR Reanalysis data for both the winter and summer seasons of the Northern and Southern Hemisphere. This analysis is performed with a regression analysis using base latitudes in the subtropics, midlatitudes, and high latitudes. It is found that the poleward MR anomaly propagation occurs at all latitudes, with the propagation speed being greater in the subtropics and high latitudes, compared to midlatitudes. Other fields, such as eddy angular momentum flux convergence, eddy heat flux, friction torque, and 300-mb streamfunction, are regressed for the Northern Hemisphere winter and the Southern Hemisphere summer. The main finding is that in the subtropics and midlatitudes, the poleward MR anomaly propagation is primarily due to high-frequency (,10 day) transient eddy angular momentum flux convergence and in high latitudes the propagation is mostly due to the summation of cross-frequency and low-frequency (.10 day) eddy angular momentum flux convergence. For the Northern Hemisphere winter, the anomalous eddy angular momentum flux convergence due to the interaction between stationary and transient eddies also contributes to the poleward MR anomaly propagation. The regression analysis suggests that a high-frequency transient eddy feedback is taking place that influences the poleward propagation of the MR anomalies. However, the effectiveness of this feedback is limited by the summation of the cross-frequency and low-frequency eddy angular momentum flux convergence, as once the MR anomaly reaches its largest amplitude, this summation of terms dominates the eddy angular momentum flux convergence and, together with the friction torque, contributes to the decay of the MR anomaly.