The Tropical Easterly Jet over Africa, its representation in six reanalysis products, and its association with Sahel rainfall
The Tropical Easterly Jet over Africa, its representation in six reanalysis products, and its association with Sahel rainfall
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DOI：
10.1002/joc.6623
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发表时间：
2020-06
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通讯作者：
S. Nicholson;D. Klotter
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作者：
S. Nicholson;D. Klotter
本文比较了六个再分析产品中热带东风急流（TEJ）和高空风的特征，并将其与西非七个地点的探测进行比较，研究了萨赫勒降雨与 TEJ 之间的关系，并研究了影响 TEJ 的因素。 MERRA2、NCEP 1、JRA 55 和 ERA 5 评估的射流特性相似。 CFSR 和 20 世纪再分析几乎在所有分析中都是异常值，与其他再分析相比，它们高估了风速高达 25% 到 40%。 1948年至2014年期间，降雨量与TEJ震级之间的相关性为0.72。基于观测和建模研究的论据提供了证据，表明在年际尺度上，TEJ 的变化不是由降雨造成的，而是大尺度因素驱动了 TEJ。讨论了强急流导致高降雨量的因果关系的潜在机制。然而，需要进一步的建模工作来最终确定 TEJ/萨赫勒降雨联系是否是共同强迫因素的结果。控制急流强度的因素包括赤道中太平洋和中赤道印度洋之间的海表温度（SST）对比（相关性为-.64）、中赤道和南副热带印度洋之间的海温对比（相关性为-.39）、南半球对流层上层东风带和西风带之间的移动纬度（相关性为-.84） 150 hPa），以及南半球西风强度（200 hPa 相关性为+.52）。这表明南半球温带环流对 TEJ 有相当大的控制。
This paper compares the characteristics of the Tropical Easterly Jet (TEJ) and upper‐level winds in six reanalysis products, compares them with soundings at seven West African locations, examines the relationship between Sahel rainfall and the TEJ, and examines factors influencing the TEJ. The jet characteristics assessed by MERRA2, NCEP 1, JRA 55, and ERA 5 are similar. CFSR and 20th Century Reanalysis are outliers in nearly every analysis, overestimating wind speeds by as much as 25 to 40% compared to other reanalyses. Over the period 1948 to 2014, the correlation between rainfall and TEJ magnitude is .72. Arguments based on observations and modelling studies provide evidence that on interannual scales changes in the TEJ are not forced by rainfall, that large‐scale factors drive the TEJ. Potential mechanisms are discussed for a causal relationship such that a strong jet leads to high rainfall. However, further modelling efforts are needed to conclusively determine whether the TEJ/Sahel rainfall link is a result of common forcing factors. The factors that appear to control jet strength include sea‐surface temperature (SST) contrast between the central equatorial Pacific and central equatorial Indian Ocean (correlation of −.64), SST contrast between the central equatorial and the southern subtropical Indian Ocean (correlation of −.39), the latitude of the shift between upper‐tropospheric easterlies and westerlies in the Southern Hemisphere (correlation of −.84 at 150 hPa), and the intensity of the Southern Hemisphere westerlies (correlation of +.52 at 200 hPa). This suggests considerable control on the TEJ by extra‐tropical circulation in the Southern Hemisphere.