Mechanistic Analysis of the Suppressed Convective Anomaly Precursor Associated with the Initiation of Primary MJO Events over the Tropical Indian Ocean

A primary Madden–Julian Oscillation (MJO) event is one with no immediately preceding MJO event. Most primary MJO events initiating over the tropical Indian Ocean are preceded by a local suppressed convective anomaly (SCA). Based on daily outgoing longwave radiation and atmospheric circulation reanalysis data, composite analyses are performed to reveal the dynamical and thermodynamical mechanisms responsible for the generation of the precursor SCA associated with anomalous descending motion. During the developing stage, before the maximum SCA, the anomalous descending motion in the upper troposphere is dynamically forced by anomalous convergence over the eastern equatorial Indian Ocean. The anomalous convergent winds result from changes in subtropical circulation structures forced by extratropical disturbances in association with equatorward advection of positive potential vorticity. In the lower troposphere, the anomalous stable boundary layer with a strong thermal inversion is the dominant factor leading to descending motion. The sea surface temperature (SST) perturbations in the tropical Indian Ocean are an external forcing that generates the SCA through air-sea interactions. The negative SST anomalies in the northwestern portion of the tropical Indian Ocean act as a heat sink to cool the lower-tropospheric atmosphere, forcing strong descending motion and forming an anomalous anticyclone north of the equator. Meanwhile, the positive SST anomalies in the southeastern portion act as a heat source that forces anomalous divergent westerlies and northwesterlies toward the warmer SST area. Thus, anomalous descending motion is induced around the equator.