Large-Scale and Convective Characteristics of the ITCZ and MJO Initiation Over the Indian Ocean

The MJO and the ITCZ are both distinctive forms of large-scale convective organization over the Indian Ocean.  One of the surprising outcomes of the DYNAMO field campaign was that an interaction between the MJO and ITCZ was observed for the first time. In this study, this interaction is investigated using TRMM 3B42 rainfall data and a global reanalysis product. There are three scenarios of MJO initiation: An existing ITCZ grows stronger and broader as its convection moves toward the equator and transforms into an MJO event; An existing ITCZ becomes weaker before convective initiation of an MJO event; No ITCZ preceding MJO initiation.  Cloud and rainfall statistics are calculated based on the TRMM precipitation features (PFs). During the transition from the ITCZ to the MJO the convective part of PFs becomes larger and stronger, and the stratiform parts increases in size. These changes in convection from the ITCZ to MJO are accompanied by a systematic increase in large-scale moisture and a deepening of the large-scale wind structure. MJO initiation associated with a preceding ITCZ has a moister environment than that without a preceding ITCZ. These results suggest that the transition between the ITCZ and MJO is not only a relocation of the maximum precipitation from a narrow band south of the equator to a broader tropical area but also a shift in the convective regime.