A recent study found that the atmospheric memory, not just sunlight, drives the onset and retreat of monsoons. 

Key Findings

• Atmosphere stores physical information in the form of water vapor, thereby controlling the onset and withdrawal of the monsoon.
  • Earlier assumption was that this seasonal pattern of monsoon was primarily as an immediate response to changes in solar radiation.
• The state of 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. This behaviour is called bistability. 
• At the same level of solar radiation, the atmosphere can either be dry or rainy, depending on the preceding state.