Abstract: Based on basic physical element fields from data of .FGGE IB such as u-v components, temperature, moisture, divergence,vorticity and vertical motion etc.,a detailed analysis on the development and maintainance of a strong monsoon trough-ITCZ in South Asia was made in this paper. The effect of the upper level easterly jet stream on the depression formation and the interaction between the weather systems of two monsoon regimes in South Asia and Southeast Asia were also discussed.