The Tropical Easterly Jet Over Africa, Its Representation In Six Reanalysis Products, And Its Association With Sahel Rainfall

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.