Seasonality and meridional propagation of the MJO

The Madden-Julian oscillation (MJO) is known to have a substantial impact on the variability of the Asian-Australian summer monsoons. An important, but not well understood, aspect of the MJO-monsoon connection is the meridional propagation of bands of enhanced or reduced precipitation that are especially pronounced during the northern Summer. In this study, the nature of the seasonality of the MJO is examined, with a focus on the meridional propagation, using both observations and simulations with an atmospheric general circulation model (AGCM). A key result is that the AGCM, when forced with idealized eastward propagating equatorial dipole heating anomalies, reproduces the salient features of the observed seasonality in the precipitation and wind fields associated with the MJO, including meridional propagation into the Indian and Australian summer monsoon regions. An analysis of the simulations and observations shows that the off-equatorial precipitation anomalies are initiated by surface frictional convergence/divergence associated with the Rossby wave response to the leading pole of the equatorial heating dipole. The off-equatorial precipitation anomalies develop further by interacting with the trailing pole of the equatorial dipole heating to produce a northwest-southeast (or southwest-northeast) oriented line of surface convergence/divergence that propagates to the east. Since the prescribed heating does not vary by season. the seasonal asymmetry in the response must be the result of the seasonal changes in the background state. In particular, the results suggest that seasonal changes in both the vertical wind shear and static stability play a role.