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 scientific journal PNAS, 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 vapour," 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 vapour 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 vapour 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 behaviour is what we call bistability," says Katzenberger. "At the same level of solar radiation, the atmosphere can either be dry or rainy, depending on the preceding state."
"We've long known that systems like the ocean or the massive ice sheets have some sort of memory. But the atmosphere? That was thought impossible," adds study co-author Anders Levermann, who leads the department of Complexity Science at PIK. "This memory effect leads to a switch-like behaviour in monsoon rainfall, a seasonal flip from 'off' to 'on' and back again. And crucially, it doesn't happen gradually -- it's abrupt, sudden." Such abrupt shifts are characteristic of other tipping elements in the climate system, but the monsoon is special, Levermann says: "What's particularly notable is that the monsoon crosses its tipping point every year and then returns. This could enable us in the future to actually identify the tipping point with observational data and develop an early warning system."
A combination of observation, theory and simulation
To unravel the mechanism behind this bistable behaviour, the team used both real-world data and simulations with a high-resolution atmospheric general circulation model developed at Princeton University. In an idealised "Monsoon Planet" setup, they isolated the atmosphere from slower Earth system components like the oceans. The simulations showed that monsoon rainfall can flip between a dry and wet state without the thermal inertia of the ocean. Key to this behaviour is the formation of a robust column of atmospheric moisture that stabilises rainfall over weeks. The central tipping point in this system can clearly be identified as a threshold, explains Katzenberger: "When atmospheric water vapour exceeds around 35 kilograms per square metre, the monsoon switches on. If it falls below that, it switches off. This abrupt, threshold-based response defines the bistability."
If this dynamic were disrupted, for example through pollution or global warming, we could face major challenges, concludes Levermann: "This would have dramatic consequences for billions of people in regions like India, Indonesia, Brazil and China who depend with their livelihood on monsoon rainfall -- It would disrupt not only our climate system, but our societies worldwide."
