The potential causes of megadroughts over North China (NC) in the preindustrial period were investigated using eight climate models evolved using CMIP5/PMIP3 and multiple proxy data. The full forcing experiments driven by both external forcing and internal variability show that megadroughts in NC were accompanied by extensive decadal drying over the Northern Hemisphere (NH), particularly in most of North America and Europe. The spatial pattern of hydroclimate matched well with the proxy-based illustration. However, such hydroclimate conditions over NH were difficult to reproduce in control experiments driven solely by internal variability. This finding suggests that external forcing may be a crucial factor leading to megadroughts in NC and simultaneous decadal drying over the NH. Moreover, external forcing may lead to climate cooling. Climate cooling weakened the pressure gradient from the West Pacific to East Asia and hence reduced the water vapor to land; meanwhile, local vertical downward movement occurred in the cooling climate background, which was unfavorable to air convection over NC. Both reduced water vapor and weakened air convection led to less precipitation. Meanwhile, there was a negative NAO, which intensified the eastward wave train at mid-latitudes. Consequently, the North American trough and European trough deepened, the northerlies in the post trough pattern intensified, and the water vapor from the high-latitude continent or polar areas was low, which was unfavorable to precipitation. This study implies that the megadrought in NC and simultaneous hydroclimatic spatial pattern over the NH may be underestimated in the CMIP5 climate prediction, which excludes external forcing.

使用 CMIP5/PMIP3 和多个代理数据演变的八个气候模型研究了工业化前时期华北 (NC) 特大干旱的潜在原因。由外部强迫和内部变化驱动的完全强迫实验表明，北半球的特大干旱伴随着北半球 (NH) 的大范围十年干旱，尤其是在北美和欧洲的大部分地区。水文气候的空间格局与基于代理的插图很好地匹配。然而，NH 上的这种水文气候条件很难在仅由内部变异驱动的对照实验中重现。这一发现表明，外强迫可能是导致北卡罗来纳州发生特大干旱和北卡罗来纳州同时发生十年干旱的关键因素。此外，外力可能导致气候变冷。气候变冷削弱了从西太平洋到东亚的压力梯度，从而减少了陆地的水汽；同时，在降温的气候背景下发生了局部垂直向下运动，不利于北卡罗来纳州的空气对流。减少的水蒸气和减弱的空气对流导致降水减少。同时，NAO为负值，加剧了中纬度东向波列。因此，北美海槽和欧洲海槽加深，后海槽格局中的北风加强，高纬度大陆或极地地区的水汽偏低，不利于降水。这项研究意味着 CMIP5 气候预测中可能低估了北卡罗来纳州的特大干旱和北半球同时出现的水文气候空间格局，