Sahel Rainfall–Tropical Easterly Jet Interaction on Intraseasonal Time Scales with a Focus on the Role of Jet-Induced Upper-Level Divergence for Mesoscale Convective Systems

The Tropical Easterly Jet (TEJ) is a characteristic upper-level feature of the West African Monsoon (WAM) circulation. The TEJ was shown to be strongly correlated with summer Sahel rainfall on decadal and interannual time scales. As of yet it is not clear whether or to which degree the regional TEJ over West Africa can affect Sahel rainfall by inducing upper-level divergence. This comprehensive statistical analysis, using a variety of temporally high resolved measurement and reanalysis data, shows that convection anomalies are in most cases leading changes in the TEJ by one to three days indicating a passive role for the TEJ. Because mesoscale convective systems (MCSs) provide the lion's share of Sahel rainfall, the effect of the TEJ on MCS development is studied for a sufficiently large number of cases to obtain statistically robust results. In most cases, no significant TEJ anomalies and/or jet-induced upper-level divergence are observed when MCSs are forming. Furthermore, no statistical evidence is found that an anomalous TEJ might affect intensity, size and lifetime of MCSs. In synthesis, this study robustly shows that the TEJ in most cases only reacts to convection variations and is typically not a cause of intraseasonal Sahel rainfall variability via its effect on upper-level divergence.