The Variability of Spatiotemporal Characteristics of Anomalous Convective Envelopes Associated with the Madden-Julian Oscillation

The space and time scales of anomalous convective envelopes associated with the Madden-Julian Oscillation are examined to better understand its regional and event-to-event variability. Popular use of leading EOFs to represent the MJO in past studies have led to less appreciation of the wide variability in MJO space and time characteristics. Detailed documentation of how much variability the MJO has in its space and time scales has not been done previously. However, close examination of its observed variability is still important for understanding its characteristics and dynamics as well as for verification of model skill in simulating the MJO.

In this study, the space and time scales of individual MJO convective events are estimated by directly measuring the temporal and spatial extent of outgoing longwave radiation anomalies in data filtered for the MJO band. The result shows that MJO convective events vary widely in space, time, and propagation speed. Various space and time scales are also embedded within each event. MJO convective envelopes tend to have faster propagating subscales at their leading edges, followed by subscales that are propagating more slowly. The regional variability in the characteristics of the enhanced and suppressed envelopes of convection differs slightly as well. The propagation speed of overall convective envelope tends to increase with the strength of convection. This relationship is contradictory to previously suggested mechanism of slowing down of the propagation speed due to the diabatic heating associated with convection that has been demonstrated for zonally narrow slow convective elements. The strength of convection also tends to increase as the zonal extent of convection increases over the Maritime Continent and the western Pacific Ocean, which is consistent with speeding up of phase speed according to the dry wave dispersion relationship. These results suggest that dry dynamical processes that determine the characteristics of circulation also play a role in determining the scales of MJO convection.