Processes responsible for extended monsoon breaks over India

During boreal summer, an inherent property of the south Asian monsoon is the existence of active-break cycles. We define “break monsoon epoch” as: three consecutive days when the area-averaged rainfall (or OLR) over central India is less than one standard deviation from its long-term mean. A diagnosis of observed daily rainfall shows that short breaks (breaks extending to 3 days) and extended breaks (breaks persisting for 7 days or more) are more frequent over central India. In the present study, processes responsible for extended breaks are examined in both observations, and in long-integrations of a coupled model. This particular model has shown demonstrative skill in capturing the active/break cycles of the monsoon. Diagnostics from both observations and model runs indicate that well-before the initiation of extended breaks over central India, enhanced rainfall and associated ascent are noticeable over the tropical western Pacific, but during the break persistence, rainfall increases over the equatorial Indian Ocean. The hypothesis that mutual interaction among the regional heat sources together with the role of boundary forcing may be responsible for extended breaks is tested. A series of sensitivity experiments with an atmospheric general circulation model are performed to confirm our hypothesis.