Evolution of Potential Vorticity as a Sign of MJO Initiation: A Case Study

Initiation is one of the aspects of the Madden-Julian Oscillation (MJO) that remain poorly understood. This study focuses on structures of precipitation, diabatic heating, and potential vorticity (PV) that can be used to tell whether an MJO event may be initiated once active convection occurs in a large area over the tropical Indian Ocean. Three scenarios are considered: active convection leads to initiation of the MJO, equatorial Rossby wave, and neither. Each scenario is represented by a specific case. This case study, in combination with an earlier composite study, provides several hints to distinguish these three scenarios. The only sign for MJO initiation before convection becomes active in a large area is slow eastward movement of negative anomalies in precipitation and diabatic heating under the influence of the large-scale circulation associated with a preexisting MJO event. Without such an influence, the essential signs for MJO initiation after convection becomes active are a persistent vertical dipole of cyclonic PV generation immediately below the level of maximum diabatic heating (400 - 500 hPa) and anticyclonic PV generation immediately above, and a persistent cyclonic PV pair straddling the equator in the lower-mid troposphere (500 - 700 hPa). An eastward motion of these signs would distinguish initiation of the MJO from that of the equatorial Rossby wave. A lack of these signs would indicate no initiation of either the MJO or Rossby wave. In addition, an eastward moving anticyclonic PV pair straddling the equator in the lower-mid troposphere prior to active convection, and a swallowtail pattern in positive precipitation anomalies are also good signs of MJO initiation, but they are not always discernable. The results from this study suggest that quantitative indices can be developed to help forecast MJO initiation and this will be validated during the DYNAMO filed campaign.