Improving the simulation of the West African Monsoon (WAM) system is paramount to increasing confidence in the projections of the region’s monsoon rainfall change. This work aims to thoroughly analyze the representation of the WAM system in two state-of-the-art, high-resolution (~ 25 km) regional climate models (RCMs) in order to highlight the causes of models’ biases through a process-oriented evaluation approach. Model results generally feature a north (Sahel) - south (Guinea Coast) dipole-like rainfall bias, although, sometimes, positive or negative rainfall biases are evident almost over the whole of West Africa. Our analysis shows that biases in the sea- and land-surface temperature on the one hand, and biases in the sea-level and land-surface pressure, on the other hand, lead to biases in the simulated temperature and pressure contrasts between the west African landmass and the eastern Atlantic ocean. As a consequence, biases appear in the modeled monsoon flow strength, which, in turn, lead to errors in the amount of advected moisture in the interior of the continent via southwesterlies and the West African westerly jet (WAWJ) on the one hand, and the extent of deepening of the monsoon flux inland on the other hand. In addition, the African easterly jet (AEJ) is underestimated, inducing an underestimation of the African easterly waves (AEWs) activity and a weakening of the cyclonic convective circulation resulting from the AEWs’ troughs, leading to a decrease in the southwesterly flow feeding mesoscale convective systems (MCSs) embedded within the AEJ. The modeled equatorward or northward shifting of the AEJ is likewise found to contribute to the models' wet or dry biases over the Sahel. Finally, there is no consistency between models and reanalyses on the one hand, and between RCM experiments on the other hand, in the way, the simulated atmospheric instability/stability modulates the convection, especially over the Sahel.

改进西非季风 (WAM) 系统的模拟对于提高对该地区季风降雨变化预测的信心至关重要。这项工作旨在彻底分析 WAM 系统在两个最先进的高分辨率（约 25 公里）区域气候模型（RCM）中的表示，以通过一个过程突出模型偏差的原因 -导向的评价方法。模型结果通常具有北（萨赫勒）-南（几内亚海岸）偶极子样降雨偏差，尽管有时在整个西非几乎可以看到正或负降雨偏差。我们的分析表明，一方面是海面和地表温度的偏差，另一方面是海平面和地表压力的偏差，导致西非大陆和东大西洋之间模拟温度和压力对比的偏差。因此，模拟的季风流动强度出现偏差，这反过来又导致通过西南风和西非西风急流（WAWJ）在大陆内部的平流水分量出现误差，并且另一方面，内陆季风通量加深的程度。此外，非洲东风急流（AEJ）被低估，导致对非洲东风波（AEWs）活动的低估，以及由AEWs槽引起的气旋对流环流减弱，导致西南流向中尺度流动减少嵌入 AEJ 中的对流系统 (MCS)。模拟的 AEJ 向赤道或向北移动同样被发现有助于模型在萨赫勒地区的干湿偏差。最后，一方面模型和再分析之间没有一致性，另一方面 RCM 实验之间也没有一致性，模拟的大气不稳定性/稳定性调节对流，特别是在萨赫勒地区。