Time scales of shallow-to-deep convective transition associated with the onset of Madden-Julian Oscillations

This study statistically investigates the timing and underlying processes of the shallow-to-deep convective transition (SDT) associated with Madden-Julian Oscillations (MJO) initiation over the Indian Ocean. Results show that SDT periods have a median value of 8–10 days with a wide spectrum of 2–20 days. SDTs lasting 10–20 days occurred nearly 50% of the time, consistent with gradual tropospheric moistening and destabilization leading to MJO initiation as described by the discharge-recharge theory. Rapid SDTs (<7 days) took place in 25% of the MJOs studied, including two MJOs observed during Dynamics of the MJO in which previous studies suggested large-scale moistening dominates the local convective moistening during SDT. SDT length is only weakly correlated to MJO amplitude and duration defined by the Real-Time Multivariate MJO index. SDT length is negatively correlated with the duration of the shallow convective period preceding the SDT but weakly correlated with individual parameters associated with preonset environments.