Roy Indrani, Troccoli Alberto
University College London (UCL), UK.
World Energy and Meteorology Council (WEMC), UK.
Sci Total Environ. 2024 Mar 1;914:169615. doi: 10.1016/j.scitotenv.2023.169615. Epub 2023 Dec 29.
Monsoon rainfall plays a crucial part in Africa's socio-economic structure and its year-to-year variability has profound implications for agricultural, energy, and other societal sectors. The current study focuses on two of the major climate drivers of the east African rainy season during October-November-December (OND), which is when the season starts for a large portion of east Africa (e.g. Tanzania and Malawi). Such drivers could be different in early austral summer from the rest of the year, due to the relative positioning of the Intertropical convergence zone, which passes through this region - hence regions of east Africa and OND season are the focus here. The two drivers of Monsoon viz. El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are considered: both independently indicate strong connections with monsoon OND rain. Not only is there a strong significant positive correlation in the OND season as a simultaneous relation, but the signal is also there even with the lag of a few months. This has been tested using various data sources, detrending data beforehand, analysing either recent time periods or earlier time periods - covering two decades each, and using regression analyses. To further strengthen the results, a compositing approach is applied that can additionally identify strong rainfall signals to that from years when opposite combinations of ENSO and IOD phases act as confounding factors. Results of precipitation anomaly for OND for compositing, when IOD and ENSO are both negative (positive) in July-August-September(JAS) indicate a deficit (excess) in rainfall in that region. The Walker circulation seems to play a major part via altering ascending to descending branch in two situations, when both drivers are in same phase. In the last near thirty years period, a total of 9 years matched the criteria when both drivers were negative and suggested a deficit in OND rainfall; more recently, that criteria occurred also in 2022 (JAS) and was again associated with a rainfall deficit in OND 2022. Based on this analysis, it is possible to deliver an estimation of cumulative rain in terms of median value, range and distribution, one season in advance, at a point location or average over a region. Some results of compositing are confirmed for longer record (1940-2021) too and further classifications based on threshold of drivers are tested. Rainfall (OND) variability at intra-decadal, decadal and multi-decadal scales are studied by applying the method of centered moving averages of 5-year, 11-year and 21-year respectively. Our results have implications for future planning in optimizing agricultural and energy outputs, mitigating severe consequences and losses associated with droughts and excess rain and will impact the livelihoods of millions of east Africans.
季风降雨在非洲的社会经济结构中起着至关重要的作用，其年际变化对农业、能源和其他社会部门有着深远影响。当前的研究聚焦于10月至11月至12月（OND）期间东非雨季的两个主要气候驱动因素，这是东非大部分地区（如坦桑尼亚和马拉维）雨季开始的时间。由于穿过该地区的热带辐合带的相对位置，这种驱动因素在南半球夏季早期可能与一年中的其他时间不同，因此东非地区和OND季节是这里的重点。考虑了季风的两个驱动因素，即厄尔尼诺南方涛动（ENSO）和印度洋偶极子（IOD）：两者都独立显示出与OND季风降雨有很强的联系。不仅在OND季节作为同步关系存在很强的显著正相关，而且即使有几个月的滞后，信号也存在。这已通过使用各种数据源进行了测试，事先对数据进行去趋势处理，分析最近时间段或更早时间段（每个时间段涵盖二十年），并使用回归分析。为了进一步加强结果，应用了一种合成方法，该方法可以额外识别出与ENSO和IOD相位相反组合作为混杂因素的年份的强降雨信号。当IOD和ENSO在7月至8月至9月（JAS）均为负（正）时，OND合成降水异常结果表明该地区降雨不足（过剩）。沃克环流似乎在两种情况下通过改变上升支和下降支起主要作用，即当两个驱动因素处于同一相位时。在过去近三十年期间，共有9年符合两个驱动因素均为负的标准，表明OND降雨不足；最近，该标准在2022年（JAS）也出现了，并且再次与2022年OND降雨不足相关。基于此分析，可以提前一个季节在一个点位置或一个区域的平均值上，以中位数、范围和分布的形式给出累积降雨量的估计。合成的一些结果在更长的记录（1940 - 2021年）中也得到了证实，并对基于驱动因素阈值的进一步分类进行了测试。分别应用5年、11年和21年的中心移动平均方法研究了年代内、年代际和多年代际尺度上的降雨（OND）变化。我们的结果对优化农业和能源产出、减轻与干旱和降雨过多相关的严重后果和损失的未来规划具有重要意义，并将影响数百万东非人的生计。
