A New Look At Sahel Rainfall Variability Over Two Centuries

该项目是利用提供该地区长期覆盖的数据集重新审查萨赫勒降雨变异性的动态，其中一个数据集可以追溯到两个多世纪以前。重新审查的动机是，目前的认识强烈依赖于20世纪后半叶的观测，当时该区域经历了长时间的潮湿(1950-1967年)和干旱(1968-1997年)时期，年复一年。与这种气候学相比，21世纪萨赫勒降水的行为在某些方面是非典型的，因此，较长期记录提供的背景可能被证明是有价值的。最近几年与上个世纪后半叶的一个重要区别是，在最近的记录中，后期降雨(8月和9月)似乎每年都不如6-7月的降雨稳定，因此雨季这两个部分的重要性可能会随着时间的推移而转移。在最近的记录中受到质疑的另一个假设是，萨赫勒降雨量在该区域相当大的纵向范围内具有空间上的连贯性。特别是，萨赫勒西部地区从20世纪末干旱中恢复的势头更为强劲。研究的重点是降水与大气环流特征之间的关系，包括赤道附近的低空非洲西风急流(AWJ)和高层对流层热带东风急流(TEJ)。季风雨带位于两股急流的轴线之间，因此对它们的波动很敏感。夏季风和夏季风的变化导致萨赫勒降雨量的两种不同模式：季风雨带的南北移动，这主要是由夏季风的经向位移驱动的；降雨强度的总体变化，由夏季风的速度决定。鉴于萨赫勒地区容易受到长期干旱和随之而来的粮食短缺和相关社会破坏的影响，这项工作具有更广泛的社会影响。PI作为美国机构、非洲政府和气象服务机构的顾问有着长期的记录，这里所做的工作可能对开发经验性的干旱预测技术有价值。此外，这项工作将支持和培训两名研究生，从而为这一研究领域的未来劳动力做出贡献。

This project is a re-examination of the dynamics of Sahel rainfall variability using datasets which provide long-term coverage of the region, in one case going back more than two centuries. The re-examination is motivated by the strong dependence of current understanding on observations over the latter half of the 20th century, when the region experienced prolonged wet (1950-1967) and dry (1968-1997) periods with strong year-to-year persistence. The behavior of Sahel rainfall in the 21st century is in some ways atypical compared to this climatology, thus the context provided by a longer-term record may prove valuable. One significance difference between the recent years and the second half of the last century is that in the recent record the late rains (in August and September) appear to be less stable from year to year than the June-July rains, thus the importance of the two parts of the rainy season may be shifting over time. A further assumption which is challenged in the recent record is that Sahel rainfall variability is spatially coherent over the considerable longitudinal extent of the region. In particular, the recovery from the late 20th century drought was stronger in the western portion of the Sahel. The research focuses on the relationship between rainfal and atmospheric circulation features, including a low-level African Westerly Jet (AWJ) near the equator and the upper-tropospheric Tropical Easterly Jet (TEJ). The monsoon rain belt lies between the axes of the two jets and is thus sensitive to their fluctuations. Variations of the TEJ and AWJ lead to two distinct modes of Sahel rainfall variability: a north-south shift in the monsoon rain belt which is primarily driven by meridional displacements of the AWJ; and an overall change in the intensity of the rains which is governed by the speed of the TEJ.The work has societal broader impacts given the susceptibility of the Sahel to prolonged drought and consequent food shortages and related societal disruption. The PI has a long record of service as an advisor to US agencies and African governments and meteorological services, and the work performed here may be of value for developing empirical drought forecasting techniques. In addition, the work will support and train two graduate students, thereby contributing to the future workforce in this research area.