Contributions of Equatorial Rossby and Kelvin Waves to the Initiation and Propagation of Different MJO Types

MJO-only time series are first constructed from the leading ten combined EOF (CEOF) modes of Wheeler and Hendon where the equatorial Rossby (ER) and Kelvin waves are removed. The Hovmoller diagrams of the series in OLR are then examined to classify the MJO events during 1979-2013 into four types: Indian Ocean only (I), Pacific only (II), Indian Ocean and Pacific with/without a Maritime-Continent barrier (III/IV). Among the 206 MJO events, 126 belong to type IV, 37 as III, 24 as I, and 19 for II. Full ER and Kelvin waves from raw daily anomalous OLR data are finally superimposed on the MJO-only time series to investigate their contributions to the different types of MJO. These waves substantially affect the majority of MJO events. Both moist and dry waves can change the MJO from one type to another. In-phase moist waves tend to enhance the MJO initiation, amplitude and eastward propagation, while dry waves tend to suppress the MJO development. Some dry packets can even produce a barrier for MJO, which may explain why MJO-like signals do not move across the Maritime Continent in some climate and operational models. Three MJO events during DYNAMO are analyzed for case studies.