Distinct Characteristics of Active and Break Spells in Flood and Drought Years of the Indian Summer Monsoon

The Indian summer monsoon displays intraseasonal variability with alternating "active" (intense rainfall) and "break" (deficient rainfall) phases. Analysis of Indian Meteorological Department (IMD) daily rainfall data (1979–2020, June–September) over Central India (CI) shows that active spells are more frequent during flood years (4.6 events/year) than drought years (2.3 events/year), with similar durations (3–4 days). In contrast, break spells are more frequent and prolonged in drought years (3.9 events/year, 6–7 days, occasionally exceeding 10 days) compared to flood years (1.2 events/year, 3–4 days).  

 Composites of mean sea level pressure reveal distinct intraseasonal dynamics between flood and drought years. During flood years, positive pressure anomalies propagate northwestward from the Bay of Bengal, while in drought years, they propagate poleward and stagnate over Central India, reducing active spell frequency. Similarly, break spells exhibit westward-moving anomalies in flood years, whereas drought years are characterized by stationary anomalies and poleward propagation.  

 Intraseasonal oscillations (ISOs) derived from IMD rainfall data strongly influence active and break spells. Flood years are characterized by high-frequency ISOs (HF-ISOs) with westward propagation, enhancing active spells, while drought years are dominated by low-frequency ISOs (LF-ISOs) with poleward movement, leading to prolonged breaks. Over 90% of these spells align with HF-ISO in flood years and LF-ISO in drought years. Total column water composites reveal frequent midlatitude dry air intrusions during drought years, contributing to extended break spells. Moisture budget analysis indicates that differences in mean moisture advection by mean winds drive anomalies, with positive values during flood years and negative values during drought years. 

 K-means clustering reveals the relationship between ISO variability and seasonal rainfall through four clusters based on variance explained by LF-ISO and HF-ISO. The cluster with the strongest LF-ISO and weakest HF-ISO records the lowest rainfall (92% of the long-term mean), while the opposite cluster experiences the highest rainfall (112% of the long-term mean). These findings align with observed HF and LF ISO intensities during flood and drought years. Strong HF-ISO activity is associated with enhanced formation and northwestward propagation of low-pressure systems from the Bay of Bengal to Central India, contributing to above-normal rainfall. Additionally, the strong HF-ISO cluster features strong low-level westerlies supported by upper-level easterlies, alongside tropospheric conditions limiting dry air intrusion from midlatitudes. In contrast, low rainfall in the cluster with large LF-ISO variance coincides with low-level easterly anomaly, and concomitantly weaker moisture transport from the Arabian Sea (AS).  Clusters with maximum LF-ISO intensity feature mid-tropospheric high pressure over CI, reflecting downdrafts of dry, cold upper-level air that suppress convection and cause seasonal rainfall deficits. Midlatitude intrusions are observed in clusters with moderately strong LF-ISO intensity, accompanied by southeasterly winds northwest of CI. These intrusions are weaker, maintaining rainfall near the long-term mean.  

This study underscores the contrasting active and break spells during flood and drought years, highlighting the role of ISOs, atmospheric dynamics, and thermodynamic processes.