Kelvin wave time scale propagation features of the Madden-Julian Oscillation (MJO) as measured by the Chen-MJO index

Abstract

[1] Kelvin wave time scale propagation features in an indexed measure of the strength of the Madden-Julian Oscillation (MJO) are investigated. Examination of reanalysis data and Chen-MJO index data for three 14 May to 21 September periods shows that recurring events near Sumatra frequently associated with convectively coupled (CC) Kelvin waves tend to correspond in a time-lagged manner with increases in indexed MJO strength at various longitudes. The events, characterized by low-level divergence within a cyclonic circulation off the west coast of Sumatra, combined with northward advances in the dynamic equator, tend to lag increases in indexed MJO strength by several days in the Indian Ocean and Maritime Continent and to lead such increases farther to the east. Maximum time-lagged correlation coefficients between increases in indexed MJO strength and a measure of the strength of the divergence events near Sumatra are consistent with a disturbance propagating eastward at Kelvin wave phase speeds. A similar signal is demonstrated in a 32 year data set of the Chen-MJO index by examining time-lagged correlations between increases in indexed MJO strength at various longitudes and corresponding increases for selected events at 80°E and 160°E. The results suggest an enhanced rate of increase in MJO strength as the apparent Kelvin wave signal passes, roughly marking the beginning of an extended period of MJO growth. Potential limitations of the current analysis are discussed. A look at a particular MJO episode provides evidence consistent with the statistical results, while supporting a role of sequential CC Kelvin wave passages in the propagation of the MJO.