Triggering Mechanism of Precursor Suppressed Convective Anomaly associated with Primary MJO Initiation over Indian Ocean

There is a precursor, local and suppressed convective anomaly before the start of the primary MJO, a composite analysis based on the OLR, NCEP/NCAR and OAFlux reanalysis datasets to reveal ultimate cause of the precursor suppressed convective anomaly generation. Two descent centers in the upper- and lower-troposphere over Indian Ocean were observed to form during pre-generation stages. The 200hPa descent is caused by 150hPa convergence associated with positive PV inflowing into tropics. And the change of PV can ultimate link to the change of extratropical wave activity flux indicates the importance of extratropical forcing. Meanwhile, the non-clear sky and strong sea surface westerlies leads a high heat loss of eastern Indian Ocean by weakening solar radiation and increased sea surface latent release. The effect of cold underlying sea combined with stronger convective inhabit energy and stable inversion layer is in favor to produce descent in the lower-troposphere. The upper- and lower-descent strengthen and coupled, eventually lead to suppressed convective anomaly generation. Our analysis suggests that the energy propagation of extratropical Rossby waves lead to PV inflowing into tropics might be one of possible triggering mechanisms for primary MJO initiation. This supports a growing number of studies linking extratropical dynamics with the modulation of tropical convective anomalies and highlights the necessary for such processes to be cooperated with lower thermal condition in suppressed convective anomaly generation processes.