The role of water vapor and cloud feedback on the evolution of the Indian summer monsoon over the last 22,000 years

In the paleo literature, the emphasis has been on the role of insolation in driving monsoons on orbital timescales, but not on the role of feedbacks internal to the climate system. Here, using the energetics framework, we have underscored the effect of water vapor on the Indian summer monsoon over the last 22,000 years in transient climate simulation, called the TraCE-21K. We show that water vapor amplifies the impact of variations in insolation during cold climates like the Last Glacial Maximum. Insolation affects water vapor through its impact on sea surface temperature. During warmer periods like the Holocene, insolation drives monsoon through its influence on the net energy at the top of the atmosphere. Cloud radiative feedbacks are prominent during these periods. Thus, there are two pathways through which insolation drives monsoons. These pathways can be delineated quantitatively using the energetics. We show further that simultaneous variations in greenhouse gases and ice sheets enhance the effect of water vapor on monsoons. Hence, the sensitivity of monsoon to local summer insolation is different during different periods. Our results suggest that feedbacks play a crucial role in the evolution of Indian monsoon on orbital timescales.