Abstract This study analysed NCEP/NCAR reanalysis datasets of wind speed components at multiple pressure levels from 1956 to 2015 with a view to examining seasonal variations in wind shears in relation to convective precipitation over West Africa. It also estimated the West African Monsoon (WAM) wind shear index and related it to the WAM dynamics during different seasons (December to February: DJF; March to May: MAM, June to August: JJA; September to November: SON). The computed wind shear indices were compared with those derived from higher-resolution datasets of ERA5. Results revealed very similar spatial and seasonal distributions of wind patterns and significant positive correlations (0.75 ≤ r ≤ 0.99; p ≤ 0.05) between NCEP and ERA5 over West African onshore region. We obtained both positive and negative trends in wind shears (−0.001 to −0.05 units per year) and precipitation (−1.5 to −3.5 mm per year) over the entire study period and region. However, the trends were found to be statistically insignificant at 95% confidence level. The WAM wind shear index (WAMI) effectively captured the northward migration of the monsoon system over West Africa with peak values in JJA season. We established significant and strong associations between wind shear indices and precipitation climatology of the Climate Research Unit. The deep layer shear (0–6 km above ground level) and WAMI gave significant and positive correlation (0.75 ≤ r ≤ 0.95) with convective precipitation while fluctuation in precipitation was explained by 50–75% variations in these indices over most parts of the monsoon region. We concluded that convective precipitation was significantly influenced by seasonal variations in wind shears over West Africa.

中文翻译：

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西非风速、风切变和降水的季节性

摘要 本研究分析了 1956 年至 2015 年多个压力水平下风速分量的 NCEP/NCAR 再分析数据集，以检查与西非对流降水相关的风切变的季节性变化。它还估算了西非季风 (WAM) 风切变指数，并将其与不同季节（12 月至 2 月：DJF；3 月至 5 月：MAM，6 月至 8 月：JJA；9 月至 11 月：SON）的 WAM 动态相关联。计算出的风切变指数与来自 ERA5 的更高分辨率数据集的指数进行了比较。结果显示，在西非陆上地区，NCEP 和 ERA5 之间的风模式的空间和季节分布非常相似，并且显着正相关（0.75 ≤ r ≤ 0.99；p ≤ 0.05）。我们获得了风切变的正趋势和负趋势（-0.001 到 -0。05 个单位/年）和整个研究期间和区域的降水量（每年 -1.5 至 -3.5 毫米）。然而，在 95% 的置信水平下，发现这些趋势在统计上不显着。WAM 风切变指数 (WAMI) 有效地捕捉了西非季风系统在 JJA 季节的峰值向北迁移。我们在气候研究组的风切变指数和降水气候学之间建立了显着而强烈的关联。深层剪切（地平面以上 0-6 公里）和 WAMI 与对流降水呈显着正相关（0.75 ≤ r ≤ 0.95），而降水波动可以用这些指数在大部分地区的 50-75% 的变化来解释。季风区。