Initiation and Multi-scale Interaction of the Madden-Julian Oscillation

The characteristic features of the convection initiation associated with the Madden-Julian Oscillation (MJO) in boreal winter were investigated through the diagnosis of the ERA-40, CFSR and other reanalysis products. It was found that the western equatorial Indian Ocean is a key region for the MJO initiation. A marked increase of the atmospheric convective instability, represented by the difference of equivalent potential temperature between the planetary boundary layer (PBL) and upper troposphere, happens prior to the onset of the MJO convection over the initiation region. The cause of such an increase is primarily attributed to the increase of specific humidity and temperature in PBL. A further diagnosis of the PBL moisture budget shows that the moisture increase prior to the MJO initiation is caused by both the horizontal advection and the apparent moist source, while vertical advection (or PBL divergence) is not important. The horizontal advection is primarily attributed to the advection of the mean specific humidity by the MJO flow and the advection of the intraseasonal specific humidity anomaly by the mean flow. The PBL temperature increase prior to MJO initiation is caused by horizontal advection and the adiabatic warming of anomalous descending motion associated with downstream forcing of previous suppressed-phase MJO.

The multi-scale interactions among synoptic-scale (3-10-day) disturbances, MJO and lower-frequency (greater than 90 days) background state were examined based on the diagnoses of eddy kinetic energy, the nonlinear rectification of surface latent heat fluxes, and eddy momentum transport. The nonlinearly rectified latent heat flux (LHF) due to eddy-mean flow interaction accounts for 30-40% of the total intraseasonal LHF over the tropical Indian and western Pacific Ocean. The MJO momentum budget analysis shows that the eddy momentum flux convergence favors the eastward propagation of the MJO by inducing a positive zonal wind tendency in the lower troposphere to the east of the MJO westerly center. Depending on longitudinal location, the eddy momentum transport accounts for 10-30% of the total zonal wind tendency. It was noted that the vertical eddy momentum transport is negligible, which contradicts some previous theories.