Dry Air Intrusion and Synoptic Variability in the ECMWF Stochastic Ensemble Forecasts of the MJO Over the Equatorial Indian Ocean

The Madden Julian Oscillation (MJO) convective initiation and eastward propagation across the equatorial Indian Ocean and the Maritime Continent are not well understood and difficult to predict. Observations from the DYNAMO field campaign show multiscale interaction among deep convection, synoptic and planetary scale circulation during the MJO convective initiation. A recent study by Kerns and Chen [2014, JGR-Atmospheres] suggested that dry air intrusions may play an important role in the eastward propagation of the MJO large-scale convection. The dry air intrusion was especially pronounced when there were synoptic Rossby gyre enhanced the equatorial low level westerlies. It would diminish the deep convection on the west side favoring eastward propagation. This study further investigate how synoptic variability and dry air intrusion affect model prediction of the eastward propagation of MJO convection using the ECMWF stochastic ensemble forecast. A dry air intrusion index based on horizontal moisture advection was developed, and strong eastward propagation during DYNAMO and other years was associated with large negative values (i.e., an advective drying trend) of the dry air intrusion index. This metric is applied to a 60 member ECMWF global ensemble for the DYNAMO November 2011 MJO event. In the ensemble, 15 members use only initial condition perturbations (IC); 15 use IC and Stochastically Perturbed Parameterization Tendencies (SPPT); 15 use IC and the Stochastic Kinetic Energy Backscatter Scheme (SKEBS); and 15 members use IC, SPPT, and SKEBS together. The recently developed Large-scale Precipitation Tracking (LPT) algorithm is used to determine the west side of the convective envelope as opposed to using a fixed longitude. There is a robust dry air intrusion signal for ensemble members with strong propagation across the maritime continent compared to those not crossing the maritime continent. Vorticity tracking and spatial filtering are used to quantify the contributions of the synoptic gyre disturbances and the MJO scale westerlies to the dry air intrusion index.