The Madden-Julian Oscillation (MJO), the dominant mode of intraseasonal variability of tropical convection over the Indo-Pacific warm pool, affects many types of weather and climate phenomena globally via its teleconnections. Despite the progress made in the past decades on understanding the MJO, many fundamental questions about its observed characteristics remain unanswered. One of the remaining questions is why the major convective envelope of the MJO detours the Maritime Continent (MC) southward only during boreal winter (December-March; DJFM), which the current study aims to address.

To examine processes affecting the MJO detouring, the MJO-related variance of outgoing longwave radiation (OLR) and column-integrated moisture anomalies in DJFM are compared with those in the seasons before (October-November; ON) and after (April-May; AM). The moisture-precipitation relationship and wind-evaporation feedback as well as the mean state variables that are crucial to these processes are also investigated for each season.

While intraseasonal variance of OLR is much larger in the southern MC (SMC) during DJFM, the intraseasonal moisture variance is comparable among the seasons, implying that the seasonal locking of the MJO’s southward detouring cannot be explained by the magnitude of moisture anomalies alone. Meanwhile, moisture sensitivity of precipitation is higher in DJFM than in other seasons, resulting in a more efficient conversion of anomalous moisture to anomalous precipitation. The increase in moisture sensitivity of precipitation is mainly due to the greater mean precipitation. In addition, with the westerly mean wind in the lower troposphere, DJFM is distinguished from other seasons by a stronger wind-evaporation feedback on intraseasonal convection anomalies; surface latent heat flux is positively correlated with convection. It is found that the seasonal cycle of moisture-precipitation coupling, wind-evaporation feedback, and the mean state factors important for them closely follows that of Australian monsoon, which is active exclusively in DJFM.

Our results suggest that the MJO’s southward detouring of Indonesia is seasonally locked because it occurs preferentially when the Australian monsoon system brings the background states that are favorable for MJO development in the SMC.