Rainfall regimes, their transitions, and long-term changes during Indian summer monsoon

Abstract. We present a diagnostic framework and accompanying dataset of daily rainfall regimes during the Indian Summer Monsoon (ISM) for June–September 1961–2018. Using high-resolution (0.25°) daily rainfall and unsupervised k-means clustering, eleven objectively defined spatial rainfall patterns were identified and linked with characteristic low-level winds, sea-level pressure and moisture fields, separating different modes of active and break phases. The dataset provides (i) centroid rainfall patterns of each regime and (ii) daily cluster IDs, enabling reconstruction of the full temporal sequence of rainfall regimes and calculation of transition probabilities between states. Transition analysis confirms that break phases are the most persistent while monsoon depressions are more transient, mirroring observed synoptic life cycles. A decomposition of rainfall change between 1961–1989 and 1990–2018 shows that drying in Northeast India (∼9 %) is driven by fewer break periods with northeast-focused rainfall, whereas Gangetic Plain drying (∼7 %) is linked to both intensity and frequency changes. This regime-based approach provides a powerful diagnostic tool to examine synoptic drivers, long-term changes in rainfall intensity and frequency, regime transition dynamics, and to evaluate model representation of ISM variability, teleconnections and trend attribution.