Distinctive features of rainfall over the Indian homogeneous rainfall regions between strong and weak Indian summer monsoons

Studies of Indian summer monsoon rainfall (ISMR: June-September) on regional scales are critically important for various applications related to agriculture and water management in India. Based on the coherent rainfall over regional scales, the India Meteorological Department defined four so-called homogeneous regions: northwest India (NWI), northeast India (NEI), central India (CI), and south peninsula India (SPIN). Here we present the salient features of daily rainfall behavior over these four regions and their association with the strength of ISMRs between strong and weak monsoons. Our results reveal that rainfall rates are primarily stronger (weaker) over NWI, CI, and SPIN during strong (weak) monsoons, while rainfall rates over NEI show no remarkable changes between strong and weak monsoons. Specifically, stronger rainfalls over NWI, CI, and SPIN during strong monsoons are generally caused by prolonged and strong wet spells followed by short and weak dry spells, primarily during the mature phase and the withdrawal phase. In contrast, weaker rainfalls over NWI, CI, and SPIN during weak monsoons are mainly caused by the prolonged and strong dry spells, which mostly occur in July-August, followed by short wet spells. The distinctive rainfall features over each region are closely associated with the characteristics of regional convective activity over each region. The rainfall rates over NEI appear insensitive to the strength of ISMRs. Finally, a probability density function analysis indicates that the rainfall rates over the four homogeneous regions between strong and weak monsoons can be characterized by the likelihood of occurrence of different rainfall ranges.