在西非季风多雨年期间撒哈拉浅经向环流的减弱和移动
Weakening and Shifting of the Saharan Shallow Meridional Circulation During Wet Years of the West African Monsoon
摘要：
在观测和全球气候模型中，萨赫勒地区降雨量增加与撒哈拉地区地面压力减少之间的相关性已得到充分证实，并被用来暗示萨赫勒地区降雨量增加是由撒哈拉地区上空较强的浅层经向环流造成的。这项研究使用两个大气再分析，以检查萨赫勒降雨和撒哈拉SMC，其中包括向北近地面流穿越萨赫勒进入撒哈拉和向南流近700百帕的撒哈拉的年际变化。在萨赫勒湿润的年份，撒哈拉SMC向极地移动，导致撒哈拉上空的低层位势和表面气压下降。从统计学上消除低层位势向极移动的影响，消除了这种位势和萨赫勒降水之间的显着相关性。随着撒哈拉SMC向极地移动，其对流层中层辐散流出减少，表明其翻转质量通量减弱。向极移动和减弱的撒哈拉SMC在湿萨赫勒年复制在一个理想化的模式，西非;广泛的强加的海表温度和陆地表面的扰动在这个模型中产生一个更大的范围SMC的变化，但有类似的定量协会与萨赫勒降雨的再分析。这些结果反驳了萨赫勒降雨量增加是由于撒哈拉SMC加强的想法。相反，这些结果是一致的假设，一个较强的SMC抑制萨赫勒地区的降雨，也许是通过平流对流层中部的温暖和干燥的空气进入降水最大。
英文摘要：
The correlation between increased Sahel rainfall and reduced Saharan surface pressure is well established in observations and global climate models, and has been used to imply that increased Sahel rainfall is caused by a stronger shallow meridional circulation (SMC) over the Sahara. This study uses two atmospheric reanalyses to examine interannual variability of Sahel rainfall and the Saharan SMC, which consists of northward near-surface flow across the Sahel into the Sahara and southward flow near 700 hPa out of the Sahara. During wet Sahel years, the Saharan SMC shifts poleward, producing a drop in low-level geopotential and surface pressure over the Sahara. Statistically removing the effect of the poleward shift from the low-level geopotential eliminates significant correlations between this geopotential and Sahel precipitation. As the Saharan SMC shifts poleward, its mid-tropospheric divergent outflow decreases, indicating a weakening of its overturning mass flux. The poleward shift and weakening of the Saharan SMC during wet Sahel years is reproduced in an idealized model of West Africa; a wide range of imposed sea surface temperature and land surface albedo perturbations in this model produce a much larger range of SMC variations that nevertheless have similar quantitative associations with Sahel rainfall as in the reanalyses. These results disprove the idea that enhanced Sahel rainfall is caused by strengthening of the Saharan SMC. Instead, these results are consistent with the hypothesis that the a stronger SMC inhibits Sahel rainfall, perhaps by advecting mid-tropospheric warm and dry air into the precipitation maximum.
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