While anomalous convection associated with the Madden-Julian oscillation (MJO) tends to ‘detour’ toward the southern MC (SMC) when it traverses the MC during boreal cold seasons, the amplitude and phase speed of MJO convection exhibit stark differences between DJF and MAM. MJO precipitation anomalies in the SMC are about 70% larger in DJF than in MAM and DJF MJO events propagate through the SMC region with a phase speed that is about 40% lower than that of the MAM events. To understand the role of the mean state on the MJO characteristics between the two seasons, the column-integrated moisture budget of the MJO and moisture-convection coupling over SMC are analyzed and compared.
For the maintenance of MJO precipitation anomalies, the background column relative humidity (CRH) plays a key role. In DJF, the upward branch of the local Hadley circulation is located in SMC, accompanied by a higher background CRH. With the higher background CRH, precipitation anomalies showed a greater sensitivity to moisture anomalies (i.e., a shorter convective moisture adjustment timescale), leading to the greater MJO precipitation anomalies in the SMC. In MAM, the upward branch of the local Hadley circulation moves to the north, lowering the background CRH, leading to a weaker precipitation anomalies.
The background low-level wind is found to be important in causing the difference in the phase speed of MJO convection. In DJF, the superposition of the westerly mean wind and MJO-induced westerly wind anomalies enhances evaporation to the west of MJO convection, leading to the slower eastward propagation. This wind-evaporation feedback also appears to contribute to the greater MJO amplitude in DJF. In MAM, with the background low-level easterlies in SMC, the sign of the wind-evaporation feedback is reversed. As a result, enhanced surface latent heat flux anomalies are located to the east of MJO convection, accelerating the eastward propagation of MJO convection.