Maritime continent aerosols reinforce the MJO propagation barrier under El Niño conditions

Abstract The Maritime Continent (MC), a critical region for Madden–Julian oscillation (MJO) propagation, is also a global hotspot for biomass burning aerosol emissions. While the barrier effect on MJO propagation over the MC is well documented, the role of aerosols in this process and its underlying mechanisms remain unrevealed. Combining observations, reanalysis datasets and moisture budget diagnostics, this study demonstrates that absorbing aerosols from El Niño-enhanced wildfires significantly modulate MJO eastward propagation. During the boreal autumn (August–November), the peak season for aerosol loading, El Niño-induced subsidence over the MC exacerbates regional dryness and promotes widespread wildfires, further elevating aerosol concentrations. To isolate the aerosol impact from El Niño effects, we classified the MJO events under high- and low-aerosol conditions while controlling for similar Niño 3.4 sea surface temperature anomalies. Composite analyses reveal that the high aerosol loads disrupt the propagation of MJO convective envelopes, whereas MJO sustains eastward propagation under low-aerosol conditions. Mechanistically, enhanced shortwave absorption by elevated aerosols induces a boundary layer cold anomaly that suppresses low-level convergence, while simultaneous heating of the lower-to-mid troposphere enhances atmospheric stability. This combined thermal and thermodynamic effect weakens boundary layer moisture anomalies, inhibiting the development of shallow convection east of the MJO deep convection that is essential for sustained propagation. The findings highlight the overlooked role of MC aerosols in the MJO barrier effect via aerosol-radiation-convection interactions, suggesting that improved representation of aerosols in climate models may enhance MJO predictability.