The Tropical Easterly Jet over West Africa in Models and Observations and the Links to Sahel Rainfall

This talk compares the characteristics of the Tropical Easterly Jet(TEJ) and upper-level winds in six reanalysis products, compares them with soundings over West Africa, and examines the relationship between Sahel rainfall and the TEJ. The reanalysis products utilized are NCEP 1, ERA 40, ERA Interim, MERRA 2, CFSR, and 20th Century Reanalysis. For observational winds, pilot balloon and radiosonde data are evaluated at seven locations in West Africa. The jet characteristics assessed by MERRA2, NCEP 1, and the ERA products are similar, but CFSR and 20th Century Reanalysis are outliers in every analysis. They tend to overestimate jet wind speeds, being as much as 25% to 40% higher than for the other reanalyses. NCEP 1, the best performing product, is utilized for establishing a long-term climatology of the Tropical Easterly Jet and for comparisons with Sahel rainfall. Over the period 1948 to 2014, the correlation between rainfall and TEJ magnitude is .72. The results clearly demonstrate a strong relationship between the strength of the TEJ and Sahel rainfall and it appears to be a causal one: the strong jet leading to high rainfall. The factors that appear to control the strength of the jet include the sea-surface temperature (SST) contrast between the central equatorial Pacific and central equatorial Indian Ocean (correlation of -.73), SST contrast between the central equatorial and the southern subtropical Indian Ocean (correlation of +.56), the latitude of the shift between upper-tropospheric easterlies and westerlies in the Southern Hemisphere (correlation of -.84 at 150 hPa), the intensity of the Southern Hemisphere westerlies (correlation of +.56 at 200 hPa). This suggests considerable control on the TEJ by extra-tropical circulation in the Southern Hemisphere.