Role of stratiform rainfall in modifying the northward propagation of monsoon intraseasonal oscillation

The stratiform and convective rain rate measurement from the Tropical Rainfall Measuring Mission (TRMM) satellite data can give a realistic idea of the latent heating distribution in the tropics. The vertical profile of diabatic heating related to the convective and the stratiform rain is known to modulate the dynamical response of atmosphere and also the vertical structure of Madden-Julian Oscillation. In this study, the contribution of the stratiform and convective rain rate to the total rain rate during different phases of the northward propagating boreal summer monsoon intraseasonal oscillation (ISO) is examined using the TRMM data. Two new insights have emerged from this analysis. First, unlike conventional wisdom, the convective component shows weak northward propagation and grows and decays in situ during the evolution of active and break phases, while the northward propagation of the monsoon ISO is largely achieved by organized movement of the stratiform component. Second, the trademark meridional dipole pattern of total rainfall between the monsoon trough zone and the southern equatorial Indian Ocean also arises largely from the contribution of stratiform anomalies. As the northward propagation of the monsoon ISO is known to be due to the anomalous response of the atmosphere to heating in the presence of mean easterly vertical shear, modification of the vertical profile of heating due to a contribution from stratiform rain could influence the northward propagation of the monsoon ISO. This hypothesis is tested using a simple global atmospheric circulation model to study the response of the convective and stratiform heating profiles on the modification of the mean condition. Modification in the large-scale response of the atmosphere as a result of proper specification of convective and stratiform heating anomalies indicates that the presence of stratiform heating favors the northward propagation of the heat source thereby facilitating the positive feedback leading to northward propagation of the monsoon ISO. These results underline the importance of simulating the partitioning of convective and stratiform rain by cumulus parameterization in climate models if they have to get the space-time structure of the summer ISOs correct.