Moistening Processes Before the Convective Initiation of Madden-Julian Oscillation Events During the CINDY2011/DYNAMO Period

Moistening processes prior to the convection initiation of the Madden-Julian Oscillation (MJO) events that occurred during the Cooperative Indian Ocean experiment on intraseasonal variability in the year 2011 (CINDY2011)/Dynamics of the Madden-Julian Oscillation (DYNAMO) Intensive Observation Period (IOP) were investigated. MJO events that initiated in mid-October and mid-November 2011 (hereafter, MJO1 and MJO2, respectively) were analyzed using ECMWF Reanalysis and satellite data. Convection initiation was defined by the beginning of continuous intraseasonal (20–80-day period) negative outgoing long wave radiation over the equatorial western Indian Ocean. Moisture budgets at intraseasonal time scale were diagnosed. The MJO convection initiation in both cases had been preceded by activation of convection to the south of the Equator and enhancement of equatorial easterly anomalies in the lower to middle troposphere. Moisture advection by intraseasonal easterly anomalies was comparable to or larger than vertical advection during 5–10 days before the convection initiation date. In MJO1, advection of intraseasonal moisture anomalies by basic zonal wind in the middle troposphere and advection by equatorward intraseasonal winds in the lower troposphere also contributed to moistening. In MJO2, acceleration of deep easterly anomalies led rapid moisture build up within several days. The easterly anomalies in both cases formed a part of wave train crossing over the southern Indian Ocean. These results suggest that extratropical forcing sometimes work through moistening in the lower to middle troposphere by enhancement of easterlies at the preconditioning stage of MJO convection initiation over the western Indian Ocean.