Comparison of the structure and evolution of intraseasonal oscillations before and after onset of the Asian summer monsoon

High-resolution satellite-derived data and NCEP-NCAR reanalysis data are used to investigate intraseasonal oscillations (ISO) over the tropical Indian Ocean. A composite evolution of the ISO life cycle is constructed, including the initiation, development, and propagation of rainfall anomalies over the tropical Indian Ocean. The characteristics of ISO over the tropical Indian Ocean are profoundly different before and after the onset of the Indian summer monsoon. Positive precipitation anomalies before monsoon onset appear one phase earlier than those after monsoon onset. Before monsoon onset, precipitation anomalies associated with ISO first initiate in the western tropical Indian Ocean and then propagate eastward along the equator. After monsoon onset, convective anomalies propagate northward over the Indian summer monsoon region after an initial eastward propagation over the equatorial Indian Ocean. Surface wind convergence and air-sea interaction play critical roles in initiating each new cycle of ISO convection.