The West African Monsoon Onset: A Concise Comparison of Definitions
The West African Monsoon Onset: A Concise Comparison of Definitions
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DOI：
10.1175/jcli-d-15-0265.1
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发表时间：
2015-11-01
影响因子：
4.9
通讯作者：
Parker, Douglas J.
中科院分区：
文献类型：
--
作者：
Fitzpatrick, Rory G. J.;Bain, Caroline L.;Parker, Douglas J.
西非季风(WAM)的爆发标志着当地和区域利益攸关方的关键时刻。虽然季风风的季节性进展以及与之相关的降水从几内亚海岸向苏丹/萨赫勒的迁移是显而易见的，但对于WAM开始意味着什么，存在着截然不同的人为定义。广义地说，发病可以在区域、当地或在指定的中间尺度上进行分析。在出版物中至少有18种不同的WAM发病定义，在比较从不同定义观察到的发病或比较不同数据集和分辨率上的发病实现方面做的工作很少。在这里，使用不同度量的不同分辨率的多个数据集计算了九个定义。研究发现，在多个数据集和不同定义中，平均区域发病日期是一致的。区域起伏期的年际变异性很低，这表明对起始点的区域季节性预报没有提供比气候学更多的好处。相比之下，局部起始点表现出很高的空间、年际和定义间的变异性。此外，研究发现，在整个西非，当地发病日期和区域发病日期之间几乎没有相关性，这意味着区域发病衡量标准与当地规模的经验之间存在不协调。这项研究的结果表明，对季节性季风爆发预报的评估远非一目了然。给出一个季节预报模式，只需选择季风爆发定义和用于比较的观测数据集，就有可能同时对季风爆发做出良好和糟糕的预测。
The onset of the West African monsoon (WAM) marks a vital time for local and regional stakeholders. While the seasonal progression of monsoon winds and the related migration of precipitation from the Guinea Coast toward the Sudan/Sahel is apparent, there exist contrasting man-made definitions of what the WAM onset means. Broadly speaking, onset can be analyzed regionally, locally, or over a designated intermediate scale. There are at least 18 distinct definitions of the WAM onset in publication, with little work done on comparing observed onset from different definitions or comparing onset realizations across different datasets and resolutions. Here, nine definitions have been calculated using multiple datasets of different metrics at different resolutions. It is found that mean regional onset dates are consistent across multiple datasets and different definitions. There is low interannual variability in regional onset, suggesting that regional seasonal forecasting of the onset provides few benefits over climatology. In contrast, local onsets show high spatial, interannual, and interdefinition variability. Furthermore, it is found that there is little correlation between local onset dates and regional onset dates across West Africa, implying a disharmony between regional measures of onset and the experience on a local scale. The results of this study show that evaluation of seasonal monsoon onset forecasts is far from straightforward. Given a seasonal forecasting model, it is possible to simultaneously have a good and a bad prediction of monsoon onset simply through selection of the onset definition and observational dataset used for comparison.