全球变暖的背景下，全球陆地季风降水年代际变化在近50年表现出显著减少的趋势，并在其上叠加了准20年周期的年代际振荡。纵观过去，对驱动全球陆地季风降水年代际变化的原因已有不少研究，但并未得出一致的结论，因此这仍是一个值得进一步探讨的问题。本文基于观测数据和CMIP5多模式的全强迫试验、单独外强迫试验以及工业革命前控制试验结果，试图寻找驱动全球陆地季风降水年代际变化的原因，并量化它们的贡献。主要研究成果如下： (1) 观测与历史模拟试验的多模式集合平均得到的全球陆地季风降水指数（GLMPI）的气候态逐月降水变化与空间分布显示，采用的5个CMIP5模式能够很好地再现全球陆地季风降水的气候态变化，模拟结果具有一定的可靠性； (2) 观测的全球陆地季风降水在1951–2004年具有显著减少的趋势，为?0.30 mm/day/100a ，能够解释全球陆地季风降水年代际变化总方差的30%，主要归因于人为气溶胶外强迫。进一步分析发现，全球陆地季风降水的下降趋势主要由北半球陆地季风降水显著减少导致，南半球陆地季风降水并未表现出明显的变化趋势。北半球陆地季风降水的减少趋势主要归因于人为气溶胶外强迫。过去50年北半球气溶胶的含量增加速度较南半球快，气溶胶的冷却效应削弱了温室气体增暖，使得北半球中低纬度地区较南半球冷，南北半球温差减少，导致北半球季风环流减弱，进一步抑制北半球陆地季风降水。 (3) 除了向下趋势外，观测中全球陆地季风降水年代际变化在近50年还表现出准20年年代际振荡，解释了全球陆地季风降水年代际变化总方差的70%，这部分变化主要受气候系统内部变率影响，其中中太平洋海表温度异常和北极涛动可以解释其55%的变化。

Global land monsoon precipitation (GLMP) has shown quasi-20-year oscillations overlying a significant downward trend over the past 50 years (1951–2004). However, what drives this decadal variation remains an open question. Based on observed datasets and CMIP5 models, in this study we explored the possible drivers for the decadal variation of GLMP and quantified their contributions. The main results is as follows: (1) The multi-model ensemble mean (MME) of the five CMIP5 models historical simulations provides a suitable representation of GLMP in terms of its climatological annual cycle and spatial distribution. Therefore, using the five models to conduct the model-based attribution of changes in GLMP is reliable. (2) A downward trend of ?0.30 mm/day/100a is observed in GLMP over 1951–2004 that explains 30% of total decadal variance, and it is primarily attributed to anthropogenic aerosols (AAs) external forcing. Further analyses showed that the downward trend in GLMP mainly results from the widespread drying over Northern Hemisphere (NH) monsoon domain, there have no significant trends in Southern Hemisphere (SH) land monsoon precipitation. Faster growth of AAs amount in NH than SH over the past 50 years results in a reduced hemispheric thermal contrast (cooler NH–warmer SH) due to more AA-forced cooling in NH. Reduced hemispheric thermal contrast facilitates a weakened NH summer monsoon circulation and finally suppresses monsoon precipitation. (3) Besides the downward trend, the remaining variation of GLMP exhibited quasi-20-year oscillations that explain 70% of total decadal variance. The decadal oscillations are primarily attributed to internal variability, in which the central tropical Pacific sea surface temperature anomaly and Arctic Oscillation together can explain 55% of its variation.