Upscale Feedback of High-frequency Tropical Disturbance to Latent Heat Flux of MJO

The effect of high-frequency (HF) variability on latent heat flux (LHF) of the Madden-Julian Oscillation (MJO) during the boreal winter was investigated through diagnosis using two reanalysis datasets and model experiments of an atmospheric general circulation model (AGCM). The diagnostic results showed that the activity of HF disturbances exert an upscale feedback to the MJO via nonlinearly rectified LHF at intraseasonal timescale. The contribution of intraseasonal LHF via nonlinear rectification accounts for >20% of the total intraseasonal LHF over the active MJO regions. The nonlinearly rectified LHF calculated based on HF-background flow interaction is in phase with MJO convection, while the total intraseasonal LHF appears to the west of the MJO convective center. This suggests that the nonlinearly rectified LHF from HF disturbances is conducive to the maintenance and propagation of MJO convection. To confirm the role of HF to MJO convection activity, we carried out a series of experiments using the AGCM of ECHAM4, which captures well the general features of MJO. We selected a number of MJO cases with enhanced convective signals and significant eastward propagation from a 30-year climatological simulation. Once the HF components of surface wind and moisture were excluded in model integration for each MJO case, most of the simulated MJO convections showed weakened activity and died out more rapidly as compared to the simulations containing all time-scale components. The output of sensitivity experiments supports the diagnostic results that HF disturbances would contribute to the eastward propagation of MJO convection through nonlinearly rectified LHF.