Relationship between the Madden-Julian Oscillation and the Convectively Coupled Equatorial Wave Activity

The Madden-Julian Oscillation (MJO) and the convectively coupled equatorial waves (CCEWs), such as the equatorial Kelvin, Rossby, and mixed Rossby-gravity waves, are fundamental components of the tropical atmosphere, contributing to a large part of tropical variability on both weather and climate scales. The interactions between the MJO and the CCEWs have long been recognized. While the MJO is found to exert pronounced modulation on the CCEW activity, important roles of equatorial Kelvin and Rossby waves for the initiation of the MJO have also been suggested. In this study, new observational evidences of modulation of CCEWs by the MJO as well as possible roles of the CCEWs in initiating the MJO convection over the western Indian Ocean are presented. It is found that the magnitude of the CCEW variations associated with the MJO can be as large as 20% of its climatology. While enhanced (reduced) CCEW activity generally coincides with and follows the envelope of active (suppressed) MJO convection, upstream or downstream impacts on CCEW activity by the MJO are also noted. One region of particular interest is the South America where the Kelvin wave activity is found to be substantially enhanced during MJO Phase 1 as defined by the Wheeler-Hendon Index, when the enhanced MJO convection is present over the Africa. Further analysis illustrates that the enhancement of the Kelvin wave activity in this region is largely attributed to increased occurrence of stronger individual Kelvin waves, while the characteristic wavelength of Kelvin waves does not show much variation. Moreover, in agreement with several previous studies, Kelvin wave activity over the South America tends to be positively correlated with the anomalous easterly vertical wind shear associated with the MJO. Our preliminary analyses on the roles of the Kelvin wave for the initiation of the MJO over the western equatorial Indian Ocean suggest that increased Kelvin wave activity emerges as early as six days prior to the initiation of the MJO and as far upstream as in the South America. Roles of the equatorial Rossby and mixed Rossby-gravity waves for the initiation of the MJO are also under investigation.