Abstract

Motivated by the previous case study, this work shows that dynamical variations of mixed Rossby–gravity waves with tropical depression-type circulations (MRGTDs) are one possible drivers of convective initiation and propagation of the Madden–Julian oscillation (MJO) by performing statistical analysis. MJO events initiated in the Indian Ocean (IO) in boreal winter are objectively identified solely using outgoing longwave radiation data. The lagged-composite analysis of detected MJO events demonstrates that MJO convection is initiated in the southwestern IO (SWIO), where strong MRGTD–convection coupling is statistically found. Further classification of MJO cases in terms of intraseasonal convection and MRGTD activities in the SWIO suggests that 26 of 47 cases are related to more enhanced MRGTDs, although they can also be secondarily affected by Kelvin waves. For those MRGTD-enhanced MJO events, MJO convective initiation is primarily triggered by low-level MRGTD circulations which develop via the enhancement of downward energy dispersion in accordance with upper-tropospheric baroclinic conversion. This is supported by the modulation of MRGTD structure associated with zonal wave contraction due to upper-tropospheric zonal convergence, and plentiful moisture advected into the western IO. Following this MRGTD-induced MJO triggering and mid-tropospheric pre-moistening in the IO contributed by MRGTD shallow circulations as well as intraseasonal winds during the MJO-suppressed phase, low-level MRGTD winds with eastward group velocity successively trigger convection to the east, which helps MJO convective propagation over the IO. The interannual atmospheric variability may affect whether the presented MRGTD-related processes are effective or not.

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