The role of interactions between multiscale circulations on the observed zonally averaged zonal wind variability associated with the Madden-Julian oscillation

Abstract

The mechanisms driving the upper-tropospheric zonal mean intraseasonal zonal wind associated with the Madden-Julian oscillation are examined through budget analysis during boreal winter. To diagnose the role of nonlinear and cross-scale interactions, the wind fields are decomposed into three temporal bands, including the intraseasonal time scale (30-100 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-timescale zonally symmetric and asymmetric circulation helps 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 that then helps to accelerate further the intraseasonal zonal wind in the extratropics. Results also suggest that feedbacks between the anomalous intraseasonal circulation and transient eddies have some sensitivity to event-to-event variability of the MJO.