Mechanisms shaping active‐break spell variability in flood and drought years of the Indian Summer Monsoon

Abstract This study investigates the contrasting characteristics of intraseasonal variability in flood and drought years during the Indian Summer Monsoon, focusing on active (wet) and break (dry) spells over Central India (21°–N, 72°–E). Using daily gridded rainfall data from the India Meteorological Department and ERA5 reanalysis (1979–2020), we show that flood years feature more frequent active spells (4.6 per season), whereas drought years exhibit longer and more frequent break spells (6–7 days, 3.9 per season). Lagged composites of mean sea‐level pressure (MSLP) reveal distinct evolution of intraseasonal anomalies: in flood years, positive MSLP anomalies propagate northwestward from the Bay of Bengal, while in drought years they stagnate over Central India. Multichannel singular spectrum analysis identifies dominant intraseasonal oscillations (ISOs): high‐frequency (10–20 day) northwestward‐propagating ISOs dominate in flood years, whereas low‐frequency (20–60 day) poleward‐drifting modes prevail in drought years. Composites of total column water indicate stronger midlatitude dry‐air intrusions that intensify break phases in drought years. To diagnose the underlying drivers, we decompose the seasonal moisture budget that isolates interannual variations in advective processes. Advection of mean moisture gradients by mean winds emerges as the leading term differentiating flood and drought years—positive in flood years and negative in drought years. Finally, ‐means clustering based on rainfall variance explained by ISO modes identifies four distinct regimes of intraseasonal variability. The regime with dominant low‐frequency intraseasonal oscillation (LF‐ISO) activity and suppressed high‐frequency intraseasonal oscillation (HF‐ISO) contributions corresponds to the lowest rainfall, whereas in contrast, the opposite regime strong HF‐ISO activity corresponds to excess rainfall. These results highlight how persistent contrasts in ISO activity, moisture advection, and midlatitude interactions shape the organization of active and break spells and provide new dynamical insights into the interplay of intraseasonal processes and interannual extremes in the South Asian monsoon.