Evolution of Land Monsoon Precipitation During the Holocene: Difference and Cause

The Holocene, an interglacial period analogous to the present, provides a critical reference for understanding monsoon evolution and hydroclimate variability. Based on TraCE-21ka simulation, this study investigates regional and hemispheric-scale land monsoon precipitation evolution throughout the Holocene. During boreal summer (June-August, JJA), the Asian Monsoon (AM) and North African Monsoon (NAFM) exhibit decreasing precipitation trends, whereas the North American Monsoon (NAM) shows an increasing trend. Meanwhile, the Australian (AUSM), South African (SAFM), and South American (SAM) monsoons experience weakened precipitation. During boreal winter (December-February, DJF), five of the six regional monsoon systems exhibit enhanced precipitation. These contrasting regional responses collectively result in pronounced hemispheric difference in land monsoon evolution. Northern Hemisphere Land Monsoon (NHLM) precipitation reduction is approximately twice that of Southern Hemisphere Land Monsoon (SHLM) during JJA, whereas SHLM precipitation increase is approximately five times greater than NHLM during DJF. Orbital forcing, through its modulation of precessional insolation and hemispheric land-sea thermal contrasts, plays a dominant role in regulating long-term Holocene monsoon precipitation evolution. Moisture Budget Decomposition reveals that JJA NHLM precipitation decline results from combined thermodynamic effect, whereas SHLM precipitation reduction is primarily driven by dynamic processes. During DJF, NHLM precipitation enhancement is mainly driven by dynamic processes, while SHLM precipitation increase reflects the combined influence of dynamic and thermodynamic processes. These findings provide new insights into the mechanisms governing hemispheric monsoon variability and long-term hydroclimate responses to external forcing.