The Role of Interactions between Multi-Scale Circulations on the Observed Zonally Averaged Zonal Wind Variability Associated with the Madden-Julian Oscillation

The mechanisms driving the upper tropospheric zonal mean intraseasonal zonal wind associated with the Madden-Julian Oscillation are examined through budget analysis during the boreal winter. In order to diagnose the role of nonlinear and cross-scale interaction forcing, the wind fields are decomposed into three temporal bands, including the intraseasonal time scale (20-100 days), and periods shorter and longer than the intraseasonal time scales. The intraseasonal zonal mean circulation and its driving mechanisms are first examined based on the leading EOFs of the intraseasonal zonal wind. Consistent with previous studies of intraseasonal atmospheric angular momentum, the upper troposphere zonal mean intraseasonal zonal wind anomaly begins in the tropics and propagates poleward. Results show that interaction between the background state and intraseasonal time scale zonally symmetric and asymmetric circulation help drive changes in the tropical intraseasonal zonal wind and its poleward propagation. The intraseasonal anomalous circulation also modulates the characteristics of the transient eddies that induce anomalous momentum flux convergence and helps to accelerate further intraseasonal zonal wind in the extratropics. However, results suggest that the feedbacks between the anomalous intraseasonal circulation and the transient eddies have some sensitivity to the event-to-event variability of the MJO.