The onset and life span of the Madden–Julian oscillation

Abstract

While the canonical life cycle of the Madden–Julian oscillation (MJO) is relatively well studied, little attention has been given to the determination of its periodicity and the detailed low-level dynamic structure during the onset phase. Here we present observational evidence of significant dynamical linkage between MJO convection and life span of MJO events. It is revealed that over the Indian Ocean, the low-level moisture convergence (LLMC) induced by easterly zonal wind anomaly and equatorward converging meridional wind anomaly preconditions about 3–5 days prior to the development of onset convection. Further, it is explicitly shown that the stronger LLMC tends to induce more intense MJO convection in the Indian Ocean, indicating that the LLMC paradigm is effectively operative in the initiation of MJO convection. The results show that the life span of the MJO is determined by the strength of the convective coupling with the large-scale circulation. That is, stronger MJO convection exhibits a longer life span due to stronger coupling between the tropical convection and circulation, and hence the slower eastward propagation of low-level circulation. In contrast, the weak MJO events favor a short period of the cycle and bear a similarity to the convectively coupled equatorial Kelvin waves. Thus, the greater LLMC tends to induce the greater MJO convection, which in turn gives rise to the strong coupling with lower-level circulation and propagating slowly to the east. The linear relationship between the convection strength and life span is shown to be greater in boreal winter.