Precipitation diurnal cycle over the Maritime Continent modulated by the MJO

The modulation of diurnal cycle of precipitation over the Maritime Continent (MC) by the Madden-Julian Oscillation (MJO) during boreal winter was studied for the period of 1998-2014 using TRMM rainfall and MERRA reanalysis data. The RMM index by Wheeler and Hendon is used to define MJO phases. It is found that there are statistically significant differences in the diurnal cycle amplitude between different MJO phases. The amplitude of precipitation diurnal cycle is largest (smallest) during phases 2-3 (6-7), especially for the diurnal cycle over the western MC. Analysis of the local precipitation budget indicates that vertically integrated horizontal moisture convergence plays an important role in causing the amplitude difference. A further diagnosis indicates that the difference is primarily attributed to the convergence of the MJO-scale moisture by diurnal wind. The enhanced diurnal wind activity during phases 2-3 results from enhanced condensational heating on the diurnal scale, owing to the increase of surface moisture caused by MJO induced boundary layer convergence in front of convection. The increase of the background low-level moisture associated with MJO further promotes a positive circulation-convection feedback to enhance the diurnal cycle activity. Opposite processes operate at phases 6-7.