A New Look At Sahel Rainfall Variability Over Two Centuries

该项目利用提供该地区长期覆盖范围的数据集（其中一个数据集可追溯到两个多世纪以前），重新审视了萨赫勒地区降雨变化的动态。重新考察的动机是目前的认识对20世纪下半叶观测的强烈依赖，当时该地区经历了长时间的湿润期（1950-1967年）和干旱期（1968-1997年），具有很强的年持续性。与这种气候学相比，21世纪萨赫勒地区的降雨行为在某些方面是非典型的，因此，长期记录提供的背景可能是有价值的。近年来与上个世纪下半叶的一个重要区别是，在最近的记录中，晚雨（8月和9月）似乎比6月至7月的降雨更不稳定，因此雨季的这两个部分的重要性可能会随着时间的推移而变化。在最近的记录中受到挑战的另一个假设是，萨赫勒地区的降雨变异性在该地区相当大的纵向范围内在空间上是一致的。特别是，萨赫勒西部地区从20世纪末的干旱中恢复的势头更为强劲。重点研究了赤道附近低空非洲西风急流（AWJ）和对流层上层热带东风急流（TEJ）的降水与大气环流特征的关系。季风雨带位于两股喷流的轴线之间，因此对它们的波动很敏感。TEJ和AWJ的变化导致萨赫勒地区降雨变化的两种不同模式：季风雨带的南北移动主要是由AWJ的经向位移驱动的；以及降雨强度的整体变化，这是由TEJ的速度决定的。鉴于萨赫勒地区容易遭受长期干旱和随之而来的粮食短缺以及相关的社会混乱，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服务记录，在这里进行的工作可能对开发经验干旱预测技术有价值。此外，这项工作将支持和培养两名研究生，从而为该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.