Propagation mechanisms for the Madden‐Julian Oscillation

Abstract

The Madden‐Julian Oscillation (MJO) is examined using 20‐years of outgoing long‐wave radiation and National Centers for Environmental Prediction/National Center for Atmospheric Research re‐analysis data. Two mechanisms for the eastward propagation and regeneration of the convective anomalies are suggested. The first is a local mechanism operating over the warm‐pool region. At the phase of the MJO with a dipole structure to the convection anomalies, there is enhanced tropical convection over the eastern Indian Ocean and reduced convection over the western Pacific. Over the equatorial western Indian Ocean, the equatorial Rossby wave response to the west of the enhanced convection includes a region of anomalous surface divergence associated with the anomalous surface westerlies and pressure ridge. This lends to suppress ascent in the boundary layer and shuts off the deep convection, eventually leading to a convective anomaly of the opposite sign. Over the Indonesian sector, the equatorial Kelvin wave response to the east of the enhanced convection includes a region of anomalous surface convergence into the anomalous equatorial surface easterlies and pressure trough, which will tend to favour convection in this region. The Indonesian sector is also influenced by an equatorial Rossby wave response (of opposite sign) to the west of die reduced convection over the western Pacific, which also has a region of anomalous surface convergence associated with its anomalous equatorial surface easterlies and pressure trough. Hence, convective anomalies of either sign tend to erode themselves from the west and initiate a convective anomaly of opposite sign via their equatorial Rossby wave response, and expand to the east via their equatorial Kelvin wave response. The second mechanism is global, involving an anomaly completing a circuit of the equator. Enhanced convection over the tropical western Pacific excites a negative mean‐sea‐level pressure (m.s.l.p.) anomaly which radiates rapidly eastward as a dry equatorial Kelvin wave at approximately 35 m s−1 over the eastern Pacific. It is blocked by the orographic barrier of the Andes and Central America for several days before propagating through the gap at Panama. After rapidly propagating as a dry equatorial Kelvin wave over the Atlantic, the m.s.l.p. anomaly is delayed further by the East African Highlands before it reaches the Indian Ocean and coincides with the development of enhanced convection at the start of the next MJO cycle.