Role of Interactions between Aerosol Radiative Effect, Dynamics, and Cloud Microphysics on Transitions of Monsoon Intraseasonal Oscillations

Extended-range prediction of monsoon intraseasonal oscillations (MISOs), crucial for agriculture and water management, is limited by their event-to-event variability. Here, the authors propose a hypothesis supported by a number of model simulations involving detailed cloud microphysical processes indicating that aerosols contribute significantly to the transitions from break to active phases of MISO. The role of aerosol indirect effect in the process of invigoration of precipitation is demonstrated with a high-resolution regional model for Indian summer monsoon breaks that are followed by an active condition (BFA) and contrasted with breaks that are not followed by an active condition (BNFA). The BFA are characterized by higher concentrations of absorbing aerosols that lead to a stronger north-south low-level temperature gradient and strong moisture convergence. Forced uplift beyond the freezing level initiates the cold-rain process involving mixed-phase microphysics and latent heat release at higher levels, thereby invigorating convection, enhancing precipitation, and resulting in an active condition. While more aerosols tend to reduce the cloud drop size and delay the warm rain, it is overcome by the higher moisture convergence during BFA and invigoration by cold-rain processes. The net production of rainfall is sensitive to cloud structure as it depends on the relative strength of the warm- and cold-rain initiation processes. The results indicate the importance of aerosols on transitions of MISO and a pathway by which they influence the transitions involving complex interactions between direct radiative forcing, large-scale dynamics, and cloud microphysics. Broader implications of these results in event-to-event variability of MISO and its predictability are also highlighted.