Abstract
The initiation of the Madden–Julian oscillation over the Indian Ocean is examined through the use of a moisture budget that applies a version of the weak temperature gradient (WTG) approximation that does not neglect dry adiabatic vertical motions. Examination of this budget in ERA-Interim reveals that horizontal moisture advection and vertical advection by dry adiabatic lifting govern the moistening of the troposphere for both primary and successive MJO initiation events. For both types of initiation events, horizontal moisture advection peaks prior to the maximum moisture tendency, while dry adiabatic lifting peaks after the maximum moisture tendency. Once convection initiates, moisture is maintained by anomalous radiative and adiabatic lifting. The dry adiabatic lifting during successive MJO initiation is attributed to the return of the circumnavigating circulation from a previous MJO event, while in primary events the planetary-scale circulation appears to originate over South America. Examination of the same budget with data from the DYNAMO northern sounding array shows that adiabatic lifting contributes significantly to MJO maintenance, with a contribution that is comparable to that of surface heat fluxes. However, results from the DYNAMO data disagree with those from ERA-Interim over the importance of adiabatic lifting to the moistening of the troposphere prior to the onset of convection. In spite of these differences, the results from the two datasets show that small departures from WTG balance in the form of dry adiabatic motions cannot be neglected when considering MJO convective onset.
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