Large Scale Precipitation Tracking and the MJO Over the Indo-Pacific Warm Pool

While various methods have been developed for tracking the global dynamical signal of the MJO, tracking the convection and precipitation associated with the MJO over the Indo-Pacific warm pool remains a challenge. In this study, a large-scale precipitation tracking (LPT) method is developed using the TRMM 3B42 rainfall data. LPT uses spatially smoothed 3-day rainfall accumulation to identify and track precipitation features in time with a minimum feature size of 300000 km2 (e.g., 500 km X 600 km) and time continuity at least 10 days. We conducted LPT analysis over the Indo-Pacific warm pool for October-March for 17 years (1998-2015). Not all large-scape precipitation tracks (LPTs) are attributable to the MJO. Among the 199 LPTs, there were 54 LPTs that had a mean net eastward propagation of at least 2 m/s. Only the eastward propagating LPTs are considered to be MJO/MJO-like convective initiation events. These LPTs capture the diversity of the MJO convection, including complex behaviors like stalling, retrograding, and discrete propagation, which are not well depicted by the Realtime Multivariate MJO (RMM) index or the OLR MJO Index (OMI). The longitude and size information from LPT are used to construct a phase diagram for direct comparison with the RMM and OMI. During the 17 years, there were 17 instances with a MJO signature in the RMM without eastward propagating LPTs. Among the 54 eastward propagating LPTs, 27 propagated through the Maritime Continent (MC), while the other 27 did not, which confirms the MC barrier effect. For the cases crossing from the Indian to W. Pacific cases (MC-crossing), 18 (67%) of them had a significant MJO signature from the IO to the west Pacific in the RMM index and one case with a RMM signal in the west Pacific but not originating in the IO. In contrast, only 6 (22%) of the non-MC-crossing cases occurred with an RMM MJO signal, although 2 other cases had a RMM signal in the west Pacific but not originating in the IO. The most MC-crossing LPTs occurred in December (9 out of the 27 cases). More MC-crossing events were observed during La Nina than El Nino conditions, which is consistent with observations of stronger and more frequent MJO events identified by RMM during La Nina years.6 on 11-27-2015-->