Accurate prediction of global land monsoon rainfall on a sub-seasonal (2–8 weeks) time scale has become a worldwide demand. Current forecasts of weekly-mean rainfall in most monsoon regions, however, have limited skills beyond two weeks, calling for a more profound understanding of monsoon intraseasonal variability (ISV). We show that the high-frequency (HF; 8–20 days) ISV, crucial for the Week 2 and Week 3 predictions, accounts for about 53–70% of the total (8–70 days) ISV, generally dominating the sub-seasonal predictability of various land monsoons, while the low-frequency (LF; 20–70 days)’s contribution is comparable to HF only over Australia (AU; 47%), South Asia (SA; 43%), and South America (SAM; 40%). The leading modes of HFISVs in Northern Hemisphere (NH) monsoons primarily originate from different convectively coupled equatorial waves, while from mid-latitude wave trains for Southern Hemisphere (SH) monsoons and East Asian (EA) monsoon. The Madden-Julian Oscillation (MJO) directly regulates LFISVs in Asian-Australian monsoon and affects American and African monsoons by exciting Kelvin waves and mid-latitude teleconnections. During the past four decades, the HF (LF) ISVs have considerably intensified over Asian (Asian-Australian) monsoon but weakened over American (SAM) monsoon. Sub-seasonal to seasonal (S2S) prediction models exhibit higher sub-seasonal prediction skills over AU, SA, and SAM monsoons that have larger LFISV contributions than other monsoons. These results suggest an urgent need to improve the simulation of convectively coupled equatorial waves and two-way interactions between regional monsoon ISVs and mid-latitude processes and between MJO and regional monsoons, especially under the global warming scenarios.

在次季节（2-8 周）时间尺度上准确预测全球陆地季风降雨已成为全球需求。然而，目前对大多数季风地区每周平均降雨量的预测在两周以上的技能有限，需要对季风季节内变率 (ISV) 有更深入的了解。我们表明，对于第 2 周和第 3 周的预测至关重要的高频（HF；8-20 天）ISV 约占总（8-70 天）ISV 的 53-70%，通常占主导地位。各种陆地季风​​的季节性可预测性，而低频（LF；20-70 天）的贡献仅在澳大利亚（AU；47%）、南亚（SA；43%）和南美洲（山姆；40%）。北半球 (NH) 季风中 HFISV 的主导模式主要来自不同的对流耦合赤道波，而从中纬度的波列则用于南半球（SH）季风和东亚（EA）季风。Madden-Julian 振荡 (MJO) 直接调节亚澳季风中的 LFISV，并通过激发开尔文波和中纬度遥相关影响美洲和非洲季风。在过去的 40 年中，HF (LF) ISV 在亚洲（亚澳）季风中显着增强，但在美洲（SAM）季风中减弱。次季节到季节 (S2S) 预测模型对 AU、SA 和 SAM 季风表现出更高的次季节预测技能，这些季风的 LFISV 贡献大于其他季风。这些结果表明迫切需要改进对流耦合赤道波的模拟以及区域季风 ISV 与中纬度过程之间以及 MJO 与区域季风之间的双向相互作用，