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 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 inter-hemispheric 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 inter-hemispheric 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.