Propagation and initiation mechanisms of the Madden‐Julian oscillation

Abstract

An observational study of the propagation and onset mechanisms of the Madden‐Julian oscillation (MJO) is performed using cyclostationary empirical orthogonal function analysis and a Kelvin‐Rossby wave decomposition method for the NOAA outgoing longwave radiation and the NCEP/NCAR Reanalysis data. In these analyses, two different regions of surface convergence are observed ahead of MJO convection by different wind components as previously found in a SST‐coupled GCM simulation. At the leading edge of enhanced convection, the friction‐induced meridional convergence appears during the developing phase when the enhanced convection is located over the Indian Ocean and the Maritime Continents. Another region of surface convergence is identified just east of an enhanced convection core, and this tends to pull the convective core to the east. The surface convergence is formed by zonal wind convergence, and Kelvin and Rossby waves both play a comparable role in the formation of the convergence zone. Therefore the frictional Kelvin‐Rossby wave‐CISK (conditional instability of the second kind) is regarded as a primary factor for the eastward propagation of the MJO. Over the western Indian Ocean, moistening in the boundary layer occurs ∼2 weeks earlier than the beginning of a new MJO cycle. This moistening accompanies low‐level convergence by encircling Kelvin waves which propagated from the region of enhanced convection during the previous cycle, as well as by Rossby waves generated from the region of reduced convection over the Indian Ocean. This earlier formation of the boundary‐layer moisture convergence provides a favorable environment for triggering new convection. This interaction of Kelvin and Rossby waves also accounts for the suppression of convection from the western side of the convection core. For example, over the Indian Ocean, anomalous surface divergence appears at the western edge of the enhanced convection due to both circumnavigating Kelvin waves from the region of reduced convection of the previous cycle and Rossby waves from the region of the enhanced convection itself. Thus the MJO is a self‐maintaining and self‐generating form of tropical variability through the interaction between convection and large‐scale circulation in the presence of boundary‐layer dynamics.