The atmospheric memory that feeds billions of people: Monsoon rainfall mechanism

Summary

Meridional monsoon circulation on the Monsoon Planet. Credit: Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2418093122 Across the globe, monsoon rainfall switches on in spring and off in autumn. Until now, this seasonal pattern was primarily understood as an immediate response to changes in solar radiation. A new study by the Potsdam Institute for Climate Impact Research (PIK), published in the journal Proceedings of the National Academy of Sciences, shows for the first time that the atmosphere can store moisture over extended periods, creating a physical memory effect. It allows monsoon systems to flip between two stable states. Disrupting this delicate balance would have severe consequences for billions of people in India, Indonesia, Brazil and China. "The atmosphere can 'remember' its previous state by storing physical information in the form of water vapor," explains Anja Katzenberger, PIK researcher and author of the study. "In practical terms, this means that even though solar radiation increases or decreases with the seasons, the atmosphere doesn't always respond immediately. During spring, water vapor accumulates over days and weeks. This reservoir determines the onset of monsoon rainfall in early summer and maintains it even as solar influx starts to decline in autumn." Path dependence in the atmosphere: How the monsoon 'remembers' Combining observational data from India, China and other monsoon regions with atmospheric simulations, the research team shows that the state of the atmosphere depends on its seasonal history: If it's already raining, the rain persists. But if it has been dry, it is hard to initiate rainfall. In spring, the atmosphere is typically dry and needs to "fill up" with water vapor before the monsoon can start. In contrast, the post-monsoon atmosphere in autumn remains moist and continues to support rainfall even as solar radiation weakens. "This behavior is what we call bistability," says Katzenberger. "At th...