This study investigates the relationship between the Madden-Julian Oscillation (MJO) variability and the mean state in the Indo-Pacific warm pool during austral summer, with a particular focus being on the role of the background horizontal moisture gradient. The MJO events that start off the Indian Ocean are examined using lag-regression analysis and also by tracking them using the OLR-based MJO index (OMI).

It is found that the mean zonal and meridional moisture gradient around the Indo-Pacific warm pool varies systematically with a dipole of sea surface temperature (SST) anomalies in the Southeastern Indian Ocean (SEIO), which is correlated with but not entirely dependent on the El Nino-Southern oscillation. The interannual variation of the mean state moisture gradient associated with the SEIO SST dipole causes significant changes in the MJO amplitude and propagation speed in the Maritime Continent (MC). When the mean state meridional moisture gradient is anomalously steep in the equatorial MC in relation to the SEIO SST dipole, the rate of moistening to the east of enhanced MJO convection becomes stronger, leading to a faster and stronger MJO over the MC. The moist static energy budget analysis based on the ‘moisture mode’ framework supports the notion that the changes in the mean state moisture gradient is what causes the observed modulation of MJO characteristics in the MC.