Under global warming, summer precipitation over East China was projected to increase by current state-of-the-art climate models, but a large inter-model spread exists. Here we try to reduce the projection uncertainty by imposing constraints. Our procedure consists of first decomposing the projected future precipitation into inter-model principal components. The two leading modes (region-wide uniform monopole and north-south dipole, accounting for 55% of variability), by emergent constraints, are then linked to the simulation of historical precipitation in the northwest Pacific and the tropical Pacific (constraining areas). This allows us to reduce the uncertainties by 23% and obtain a smaller increase of projected precipitation in East China, relative to previous multi-model ensemble projections. Quasi-uniform increases, although weak, are obtained for the first mode, while the second mode shows a contrast pattern with a decrease in the south and an increase in the north, which both contribute to the spatial structure of constrainted projection. It is also shown that the emergent relations of both modes are physically consistent, with an enhancement of future zonal land-sea thermal contrast and a La Niña-like pattern, respectively. The use of emergent constraints inspires more confidence in the future regional precipitation projection and helps policymakers and stakeholders adjust their management policies.

在全球变暖的情况下，根据目前最先进的气候模型预估华东地区夏季降水量将增加，但模式间存在较大的传播。在这里，我们尝试通过施加约束来减少投影的不确定性。我们的程序包括首先将预测的未来降水分解为模型间的主成分。然后，通过紧急约束，两种主要模式（区域范围的均匀单极子和南北偶极子，占变率的 55%）与西北太平洋和热带太平洋（限制区域）历史降水的模拟相关联。这使我们能够将不确定性降低 23%，并且相对于以前的多模式集合预估，华东地区的预估降水增加幅度较小。第一种模式虽然较弱，但获得了准均匀的增加，而第二种模式则表现出南方减少和北方增加的对比模式，这两者都有助于约束投影的空间结构。研究还表明，两种模态的涌现关系在物理上是一致的，分别增强了未来的纬向陆海热对比和类似拉尼娜的模式。紧急约束的使用激发了对未来区域降水预测的更多信心，并帮助政策制定者和利益相关者调整其管理政策。