Decadal Responses of Global Land Monsoon to Two Surface Thermal Modes in the Last Millennium

Abstract This study investigates the influence of natural external forcing over the last millennium (950–1850) on global land monsoon variability at a decadal scale. Two major players operate through changes in global mean surface temperature (GMST) and in the tropical Pacific temperature gradient (TPTG). They modulate precipitation, either individually or jointly in the form of cool-GMST and weak-TPTG episodes. A diagnostic with moisture budget decomposition is performed to separate dynamic and thermodynamic contributions leading to monsoon precipitation. Cool GMST primarily suppresses the summer precipitation via thermodynamic contributions, with a complex winter response featuring competing thermodynamic gains and evaporation losses. In contrast, weak TPTG involves significant dynamic adjustments, driving an interhemispheric dipole for the summer land monsoon precipitation: decreases in the Northern Hemisphere via anomalous subsidence, and increases in the Southern Hemisphere via the thermodynamic contributions and evaporation. The enhanced winter monsoon precipitation (especially in the Southern Hemisphere) in the case of weak TPTG is largely driven by the dynamics and evaporation. Under compound conditions, the overall monsoon intensity response can be largely interpreted as a linear superposition of the individual drivers. Summer precipitation is substantially reduced, primarily due to cooling-induced thermodynamic suppression, with additional dynamic contributions. The winter response shows a distinct interhemispheric pattern, with precipitation increasing in the Southern Hemisphere and slightly decreasing in the Northern Hemisphere, reflecting different controls by cool GMST and weak TPTG. This study provides a valuable framework for interpreting the past monsoon variability and projecting its future variation. Significance Statement This study investigates how two key atmospheric drivers, the global mean surface temperature (GMST) and the tropical Pacific temperature gradient (TPTG), modulated the global land monsoon (GLM) over the last millennium. We demonstrate that these drivers operate through distinct physical pathways: Cool-GMST conditions primarily impact GLM through thermodynamic mechanisms, suppressing summer precipitation and inducing complex winter responses with competing effects. In contrast, weak-TPTG (El Niño–like) conditions often induce strong dynamic adjustments, driving a summer precipitation dipole and significantly influencing winter rainfall, particularly in the Southern Hemisphere. When these drivers act concurrently, we find that the overall GLM intensity response, measured by the annual precipitation range (APR), can be largely understood as a linear superposition of their individual effects, explaining a significant portion of the APR anomaly. Our moisture budget analysis further reveals how this macroscale linearity emerges: Amplified summer drying is dominated by cooling-related thermodynamics, while the winter response features a distinct hemispheric contrast [Southern Hemisphere (SH) wetting, Northern Hemisphere (NH) slight drying] driven by specific thermodynamic and evaporative controls, highlighting the season-dependent interplay. These findings provide a simplified yet mechanistically grounded framework for interpreting complex climate events like volcanic eruptions, improving our fundamental understanding of monsoon dynamics and natural climate variability.