The Role of Remote Forcing for the TEJ Variability over West Africa on Interannual to Multidecadal Time Scales

The Tropical Easterly Jet (TEJ) is an integral part of the summer-time West African Monsoon (WAM) circulation. The TEJ is an upper-tropospheric (100-200 hPa) easterly current which originates in the South Asian Monsoon system, extends westwards to Africa and decays over the tropical Atlantic. The intensity of the TEJ over West Africa was shown to be strongly correlated with summer Sahel rainfall on decadal and interannual time scales. Here we investigate whether the variability of the regional TEJ over West Africa on such time scales is mainly forced directly by WAM rainfall changes or whether the TEJ is substantially influenced by remote atmospheric forcing. We conduct a variety of numerical simulations with an AGCM with dry dynamics driven by atmospheric diabatic heating patterns derived from reanalyses/observations or output from dedicated model experiments. To disentangle the effects of remote versus regional forcing, we prescribe varying remote diabatic forcing patterns while keeping the regional diabatic forcing unchanged over Africa. It is found that a major part of TEJ variability over West Africa can be explained by remote planetary-scale diabatic heating anomalies which are strongly associated with well-known modes of SST variability, in particular ENSO. In most cases, SST anomaly pattern which were shown to enhance Sahel rainfall, also lead to a more intense TEJ over West Africa. WAM rainfall anomalies alone affect the TEJ as well, but the changes are comparably small. In synthesis, the performed idealized model experiments strongly suggest that the high correlation between the TEJ and Sahel rainfall on interannual and decadal time scales is less explained by the influence of the regional rainfall variability on the regional TEJ, but more by the simultaneous effect of the remote forcing on the regional rainfall and the TEJ.