Synoptic –Mesoscale Variability and Its Impact on Prediction of MJO Initiation in the Tropical Indian Ocean: Observations and Model Forecasts in DYNAMO

The Madden Julian Oscillation (MJO) is the dominant intraseasonal (30-80 day) mode of variability in the tropics. It has significant impacts on global weather systems including heat waves, drought/floods, winter storm tracks, and tropical cyclones. Nevertheless, medium-range weather forecast models have difficulty in predicting the MJO, in particular, its initiation in the Indian Ocean. One reason why predicting MJO initiation is difficult is that the physical processes controlling moist convection and their multi-scale interaction are not fully understood. Observations from the Dynamic of MJO (DYNAMO) field campaign show that the moist convection seem to organize on various time and spatial scales. In addition to the equatorial wave-like organizations, it is found that although widespread convection does occur in the equatorial region during the suppressed phase of MJO, it tends to be focused off the Equator, often in a double-ITCZ configuration. During MJO onset, the convection becomes more concentrated along the Equator. The question is what is the trigger for the initial MJO equatorial convection over the Indian Ocean?

One of possible mechanisms is that dir air intrusions into the near-equatorial Indian Ocean may affect the convection in the ITCZ, which contribute to the shift of convection toward the equator during the onset of MJO. The NOAA P3 aircraft dropsonde data from DYNAMO show a sharp moisture gradient near 5oS on 22 November 2011, which is confirmed by satellite retrievals over a large region. Soundings taken at Diego Garcia showed an abrupt transition between moist and dry conditions in 3-6 hours when the dry air moved in from the south. Similar equator-ward push of dry air seems to exist a few days prior to the initiation of the MJO convection in the October and November MJO initiation events in DYNAMO. This synoptic evolution forced convection to preferentially develop on the Equator. Equatorial convection associated with a third MJO event in late February 2012 was also preceded by a push of dry air, this time from the northern Hemisphere.

This study examines the global model predictions of the moist convection and associated synoptic systems during DYNAMO. The initiation of convection in the Indian Ocean and eastward propagation of the MJO active convection envelope depend critically on stochastic, synoptic/mesoscale events, which the models often predict poorly. We use the satellite IR-based hourly cloud clusters tracking over the entire tropical Indian, the NOAA aircraft dropsonde and sounding data from the DYNAMO sites, and satellite TPW and AIRS data to evaluate the ECMWF and GFS model forecasts. At this initial phase, the verification is based on feature identification for these cases. The question is whether the GFS and ECMWF forecasts could predict these observed features for the MJO events observed during DYNAMO (October 2011 – March 2012). Also, the model forecast statistics of convective activity and vertical moisture profile are compared with cloud cluster tracks and DYNAMO radiosonde data.