An objective definition of the Indian summer monsoon season and a new perspective on the ENSO monsoon relationship
The concept of an interannually varying Indian summer monsoon season is introduced here, considering that the duration of the primary driving of the Indian monsoon - the large-scale meridional gradient of the deep tropospheric heat source - may vary from one year to another. Onset (withdrawal) is defined as the day when the tropospheric heat source shifts from south to north (north to south). This physical principle leads to a new thermodynamic index of the seasonal mean monsoon. While the traditional measure of seasonal rainfall, averaged from 1 June to 30 September, indicates a breakdown of the ENSO-monsoon relationship in recent decades, it is argued that this breakdown is partly due to the inappropriate definition of a fixed monsoon season. With a new physically based definition of the seasonal mean, the ENSO-monsoon relationship has remained steady over the decades. El Ni