Tropical rainfall exhibits a prominent annual cycle, with characteristic amplitude and phase representing the range between wet and dry seasons and their onset timing, respectively. Previous studies note enhanced amplitude over ocean and delayed phase over land in model projections of global warming, underpinned by first-order physical principles. However, it is unclear whether these changes have emerged in observations. Here we use gridded precipitation datasets to report a seasonal delay of 4.1 ± 1.1 and 4.2 ± 0.9 days (P < 0.05) during 1979–2019 over the northern tropical land and Sahel, respectively. Most of the delay is driven by external forcings, dominated by greenhouse gases (GHG) and anthropogenic aerosols (AER). Increasing GHG and decreasing AER in the recent decades delay rainfall by producing a moister atmosphere, thus increasing its lag in response to seasonal solar forcing. As GHG increase and AER decrease, these seasonal delays are projected to further amplify in the future.

热带降雨表现出显着的年周期，其特征幅度和相位分别代表雨季和旱季之间的范围及其开始时间。先前的研究指出，在全球变暖的模型预测中，海洋上的振幅增强和陆地上的相位延迟，这以一阶物理原理为基础。然而，尚不清楚这些变化是否已在观察中出现。在这里，我们使用网格降水数据集来报告 4.1 ± 1.1 和 4.2 ± 0.9 天的季节性延迟（P < 0.05) 在 1979-2019 年期间分别在北部热带土地和萨赫勒地区。大部分延迟是由外部强迫驱动的，主要是温室气体 (GHG) 和人为气溶胶 (AER)。近几十年来，温室气体的增加和 AER 的降低通过产生更潮湿的大气延迟了降雨，从而增加了其对季节性太阳强迫的反应滞后。随着 GHG 增加和 AER 减少，预计这些季节性延误在未来将进一步扩大。