Atlantic ITCZ: A Wall or a Leaky Barrier for African Aerosol?

In order to evaluate the role of the ITCZ in aerosol distributions over Africa and the tropical Atlantic Ocean, we derived three-dimensional seasonal occurrence probabilities of aerosol from the Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) Vertical Feature Mask (VFM) product. Our frequency of occurrence structures, when combined with rainfall and wind fields, illustrate aerosol transport pathways, boundaries, and external mixing states. There are clear separation boundaries of pure dust and smoke in the ITCZ vicinity during boreal summer (JJA); dust is confined north of the ITCZ while smoke is bounded to its south. Polluted dust, an external mixture of dust and smoke, is prevalent in this area. The structure of the atmospheric circulation is the reason for the apparent separation of dust and smoke. Three distinct meridional regimes are present: over Africa, Africa in the north and the Atlantic in the south, and over the Atlantic. In the presence of deep convection, precipitation washes out the majority of transported dust and smoke no matter the regime. Over Africa in the absence of deep convection, a shallow meridional circulation associated with the Indian Monsoon transports smoke northward, while outflow of the Saharan heat low transports dust southward. To the west, with Africa in the north and the Atlantic to the south, the Saharan low outflow is still present, but the shallow meridional circulation is associated with the West African Monsoon instead of the Indian Monsoon. Over ocean, there are two shallow meridional circulations: one north of the ITCZ transporting dust southward and one south of the ITCZ transporting smoke northward. In all regimes, the area of the dust/smoke intersection, and thus their lowest frequency of occurrence, is the most probable location of polluted dust created from the external mixing of the two. Interestingly, no smoke is observed north of the ITCZ, but dust is observed to its south. The altitude of the cross-ITCZ transport is responsible. North of the ITCZ, dust is lofted and transported southward above shallow convection. To its south, smoke is transported northward at the base of the circulation where wet deposition, cloud processes, and external mixing occur. To show that dust is able to penetrate and cross the ITCZ, we used NOAA's HYSPLIT trajectory model to create seasonal trajectory frequency climatologies originating from the Sahara Desert. The trajectory climatologies agree with the seasonal occurrence probabilities in both the southward extent and transport altitudes of possible dust transport. The transport across the ITCZ is evident. We find that almost 15% of the trajectories initiated over the Sahara Desert cross the ITCZ at 8°N, the approximate location of maximum ITCZ rainfall. We present schematics for the three regimes to illustrate the transport and mixing mechanisms. As dust and smoke are advected by the dominant zonal wind, they continue to mix during the westward transport by the shallow meridional circulations in the absence of deep convection. The alternation between deep and shallow convection makes the ITCZ a leaky barrier for African aerosol.