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Secular spring rainfall variability at local scale over Ethiopia: trend and associated dynamics

机译：埃塞俄比亚地方尺度上的长期春季降雨变化：趋势和相关动力学

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摘要

Spring rainfall secular variability is studied using observations, reanalysis, and model simulations. The joint coherent spatio-temporal secular variability of gridded monthly gauge rainfall over Ethiopia, ERA-Interim atmospheric variables and sea surface temperature (SST) from Hadley Centre Sea Ice and SST (HadISST) data set is extracted using multi-taper method singular value decomposition (MTM-SVD). The contemporaneous associations are further examined using partial Granger causality to determine presence of causal linkage between any of the climate variables. This analysis reveals that only the northwestern Indian Ocean secular SST anomaly has direct causal links with spring rainfall over Ethiopia and mean sea level pressure (MSLP) over Africa inspite of the strong secular covariance of spring rainfall, SST in parts of subtropical Pacific, Atlantic, Indian Ocean and MSLP. High secular rainfall variance and statistically significant linear trend show consistently that there is a massive decline in spring rain over southern Ethiopia. This happened concurrently with significant buildup of MSLP over East Africa, northeastern Africa including parts of the Arabian Peninsula, some parts of central Africa and SST warming over all ocean basins with the exception of the ENSO regions. The east-west pressure gradient in response to the Indian Ocean warming led to secular southeasterly winds over the Arabian Sea, easterly over central Africa and equatorial Atlantic. These flows weakened climatological northeasterly flow over the Arabian Sea and southwesterly flow over equatorial Atlantic and Congo basins which supply moisture into the eastern Africa regions in spring. The secular divergent flow at low level is concurrent with upper level convergence due to the easterly secular anomalous flow. The mechanisms through which the northwestern Indian Ocean secular SST anomaly modulates rainfall are further explored in the context of East Africa using a simplified atmospheric general circulation model (AGCM) coupled to mixed-layer oceanic model. The rainfall anomaly (with respect to control simulation), forced by the northwestern Indian Ocean secular SST anomaly and averaged over the 30-year period, exhibits prevalence of dry conditions over East and equatorial Africa in agreement with observation. The atmospheric response to secular SST warming anomaly led to divergent flow at low levels and subsidence at the upper troposphere over regions north of 5(A degrees) S on the continent and vice versa over the Indian Ocean. This surface difluence over East Africa, in addition to its role in suppressing convective activity, deprives the region of moisture supply from the Indian Ocean as well as the Atlantic and Congo basins.
机译：使用观测，重新分析和模型模拟研究了春季降雨的长期变化。使用多锥度方法奇异值分解提取了埃塞俄比亚网格月尺度降水的联合相干时空长期变化，ERA-临时大气变量和哈德利中心海冰和海表温度（HadISSST）数据集（MTM-SVD）。同时使用部分格兰杰因果关系来检验同期关联，以确定任何气候变量之间是否存在因果联系。该分析表明，尽管亚热带太平洋，大西洋部分地区的春季降雨，SST具有强烈的长期协方差，但只有西北印度洋的长期SST异常与埃塞俄比亚的春季降雨和非洲的平均海平面压力（MSLP）有直接的因果关系。印度洋和MSLP。长期的高降雨方差和统计学上显着的线性趋势一致地表明，埃塞俄比亚南部的春季雨水大量减少。这与MSLP在东非，东北非洲（包括阿拉伯半岛的部分地区，中部非洲的某些地区）以及南海（除ENSO区域以外）所有海盆的变暖同时大量积聚同时发生。响应印度洋变暖的东西压梯度导致阿拉伯海，东非中部非洲和赤道大西洋上空出现东南风。这些流量减弱了阿拉伯海上的东北气候流，而赤道大西洋和刚果盆地则向西南流，春季向东部非洲地区供应水分。由于东风的长期异常流，低水平的长期发散流与高水平的收敛同时发生。在东非的背景下，使用简化的大气总环流模型（AGCM）和混合层海洋模型，进一步探讨了印度洋西北部SST异常调节降雨的机制。由印度洋西北部长期SST异常强迫并在30年内平均的降雨异常（相对于控制模拟而言）表明东部和赤道非洲的干旱状况普遍存在。大气对长期SST增温异常的响应导致了该大陆5°A以北地区低层的低散流和对流层上层的沉降，而印度洋则相反。东非上空的这种表面分流，除了抑制对流活动外，还剥夺了印度洋以及大西洋和刚果盆地的水供应。

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  • 来源
    《Theoretical and applied climatology》 |2017年第2期|91-106|共16页
  • 作者

    Tsidu Gizaw Mengistu;

  • 作者单位

    Botswana Int Univ Sci & Technol, Dept Earth & Environm Sci, Priv Bag 16, Palapye, Botswana|Addis Ababa Univ, Dept Phys, POB 1176, Addis Ababa, Ethiopia|Karlsruhe Inst Technol, Inst Meteorol & Climate Res, D-76344 Karlsruhe, Germany;

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