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The transport pathways of carbon monoxide (CO) in the African UpperTroposphere (UT) during the West African Monsoon (WAM) is investigatedthrough the assimilation of CO observations by the Aura Microwave LimbSounder (MLS) in the MOCAGE Chemistry Transport Model (CTM). The assimilationsetup, based on a 3-D First Guess at Assimilation Time (3-D-FGAT) variationalmethod is described. Comparisons between the assimilated CO fields andin situ airborne observations from the MOZAIC program between Europeand both Southern Africa and Southeast Asia show an overall good agreementaround the lowermost pressure level sampled by MLS (~215 hPa). The 4-Dassimilated fields averaged over the month of July 2006 have been used todetermine the main dynamical processes responsible for the transport of CO inthe African UT. The studied period corresponds to the second AMMA (AfricanMonsoon Multidisciplinary Analyses) aircraft campaign. At 220 hPa, the COdistribution is characterized by a latitudinal maximum around 5° Nmostly driven by convective uplift of air masses impacted by biomass burningfrom Southern Africa, uplifted within the WAM region and vented predominantlysouthward by the upper branch of the winter hemisphere Hadley cell. Above150 hPa, the African CO distribution is characterized by a broad maximum overnorthern Africa. This maximum is mostly controlled by the large scale UTcirculation driven by the Asian Summer Monsoon (ASM) and characterized by theAsian Monsoon Anticyclone (AMA) centered at 30° N and the TropicalEasterly Jet (TEJ) on the southern flank of the anticyclone. Asian pollutionuplifted to the UT over large region of Southeast Asia is trapped within theAMA and transported by the anticyclonic circulation over Northeast Africa.South of the AMA, the TEJ is responsible for the tranport of CO-enriched airmasses from India and Southeast Asia over Africa. Using the high timeresolution provided by the 4-D assimilated fields, we give evidence that thevariability of the African CO distribution above 150 hPa and north of the WAMregion is mainly driven by the synoptic dynamical variability of both the AMAand the TEJ

Transport pathways of CO in the African upper troposphere during the monsoon season: a study based upon the assimilation of spaceborne observations