Both the CESM-simulated NNU-2K dataset and proxy reconstructions of Indian Summer Monsoon (ISM) precipitation over the past two millennia reveal a significant centennial-scale period, including periodicities of 105, 150, and 200 years. The 105- and 200-year cycles identified in the NNU-2K all-forcing (AF) experiment closely match those found in the volcanic single-forcing (Vol) experiment, suggesting that volcanic activity is a major driver of these variations. Volcanic forcing induces global cooling, which reduces the land–sea thermal contrast and weakens the monsoon circulation. Furthermore, stronger cooling in the Northern Hemisphere decreases the interhemispheric temperature gradient and weakens the trans-equatorial pressure gradient. This, in turn, suppresses cross-equatorial low-level flow from the Southern Hemisphere, further reducing ISM precipitation. The 105- and 150-year periodicities are also consistent with those in the total solar irradiance (TSI) single-forcing experiment, indicating a substantial response to solar variability. Increased solar irradiance enhances Northern Hemisphere warming, strengthening both the interhemispheric temperature gradient and the cross-equatorial pressure gradient. These changes facilitate stronger northward cross-equatorial flow in the lower troposphere, intensifying the ISM and increasing precipitation. Concurrently, solar forcing amplifies the thermal contrast between the Eurasian continent and the Indian Ocean, further reinforcing monsoon circulation. The 150-year cycle is also evident in the control (Ctrl) experiment, implicating internal climate variability as an additional mechanism. Analysis reveals a quasi-decadal Pacific Decadal Oscillation (PDO)-like sea surface temperature anomaly in the North Pacific. Its negative phase is linked to reduced sea-level pressure over the ISM region, enhanced low-level convergence, and increased precipitation. It also strengthens the Mascarene High over the Indian Ocean, intensifying the Somali Jet and southwesterly monsoon winds, which promote greater moisture transport into the ISM domain.

The Indian monsoon is an important component of the tropical climate system. Studies suggest that both precipitation from the NNU-2K AF experiments and Little Ice Age (LIA) proxy data reveal a significant 56-year period in the Indian summer monsoon (ISM). Volcanic activity influences the multi-year and decadal characteristics of Indian monsoon precipitation. A comparative analysis of multi-decadal variations in ISM precipitation from the AF experiments and the single-factor sensitivity experiments shows a correlation of 0.45 (p < 0.05), indicating dependence on volcanic activity. The 56-year period of ISM precipitation in the AF experiments is consistent with both the single-factor sensitivity experiments and precipitation influenced by volcanic activity. Further analysis reveals that cooling over Eurasia and warming of the Indian Ocean weaken the thermal contrast between land and sea, thereby reducing ISM intensity and decreasing ISM precipitation. Conversely, enhanced volcanic activity induces widespread cooling across the Northern Hemisphere, which shifts the Intertropical Convergence Zone (ITCZ) southward. This reduction in evaporative capacity and moisture content within the monsoon region ultimately decreases precipitation across the monsoon belt.