There is a well-known mode of rainfall variability associating opposite hydrological conditions over the Sahel region and the Gulf of Guinea, forming a dipole pattern. Previous meteorological observations show that the dipole pattern varies at interannual timescales. Using an EC-Earth climate model simulation for last millennium (850–1850 CE), we investigate the rainfall variability in West Africa over longer timescales. The 1000-year-long simulation data show that this rainfall dipole presents at decadal to multidecadal and centennial variability and long-term trend. Using the singular value decomposition (SVD) analysis, we identified that the rainfall dipole present in the first SVD mode with 60% explained variance and associated with the variabilities in tropical Atlantic sea surface temperature (SST). The second SVD mode shows a monopole rainfall variability pattern centred over the Sahel, associated with the extra-tropical Atlantic SST variability. We conclude that the rainfall dipole-like pattern is a natural variability mode originated from the local ocean–atmosphere-land coupling in the tropical Atlantic basin. The warm SST anomalies in the equatorial Atlantic Ocean favour an anomalous low pressure at the tropics. This low pressure weakens the meridional pressure gradient between the Saharan Heat Low and the tropical Atlantic. It leads to anomalous northeasterly, reduces the southwesterly moisture flux into the Sahel and confines the Gulf of Guinea's moisture convergence. The influence from extra-tropical climate variability, such as Atlantic multidecadal oscillation, tends to modify the rainfall dipole pattern to a monopole pattern from the Gulf of Guinea to Sahara through influencing the Sahara heat low. External forcing—such as orbital forcing, solar radiation, volcanic and land-use—can amplify/dampen the dipole mode through thermal forcing and atmosphere dynamical feedback.

在萨赫勒地区和几内亚湾，有一种众所周知的降雨变化模式，将相反的水文条件联系起来，形成偶极子模式。先前的气象观测表明，偶极子模式在年际时标上变化。使用上一个千年（公元850–1850年）的EC-地球气候模型模拟，我们研究了较长时间范围内西非的降雨变化。长达1000年的模拟数据表明，该降雨偶极子呈现十年到几十年和百年变化和长期趋势。使用奇异值分解（SVD）分析，我们确定在第一个SVD模式下存在的降雨偶极子具有60％的解释方差，并且与热带大西洋海表温度（SST）的方差相关。第二种SVD模式显示了以萨赫勒地区为中心的单极降雨变异性模式，与热带热带大西洋SST变异性相关。我们得出的结论是，降雨偶极状模式是一种自然变异模式，起源于热带大西洋盆地的局部海洋-大气-土地耦合。赤道大西洋的暖SST异常有利于热带低压异常。这种低压减弱了撒哈拉低气压和热带大西洋之间的子午压力梯度。它导致东北异常，减少了西南向萨赫勒地区的水分通量，并限制了几内亚湾的水分收敛。来自温带气候变化的影响，例如大西洋的多年代际振荡，倾向于通过影响撒哈拉低热，将几内亚湾到撒哈拉沙漠的降雨偶极子模式改变为单极子模式。外部强迫（例如轨道强迫，太阳辐射，火山和土地利用）可以通过热强迫和大气动力反馈来放大/减弱偶极子模式。