Merger of African easterly waves and formation of Cape Verde storms

There are two easterly wave tracks over West Africa, one to the south and the other to the north of the African Easterly Jet (AEJ). A northern wave sometimes merges with a southern wave near the coast of West Africa. The dynamic and thermodynamic evolution of the waves during the merger and the impacts of the merger on subsequent storm development are examined in this study. Three groups of storms are identified: non-merger developers, merger developers and merger non-developers. Relative to non-merger developers, merger developers have a weaker circulation near the surface prior to the merger, but the merger leads to a stronger and deeper wave pouch, which is more conducive to tropical cyclogenesis. The dryness of the northern disturbances does not seem to hinder the storm development. Composite analysis shows that merger developers are associated with a warmer and broader heat low over West Africa and a more extensive AEJ, which helps to maintain the northern waves over the East Atlantic. Merger developers and non-developers resemble each other prior to the merger, but dry air intrusion west of the wave trough in the middle and upper troposphere contributes to the non-development of some mergers. The dry air in non-developers is related to a strong and broad upper-tropospheric trough and the associated subsidence west of the low-level wave pouch. The trough advects dry air from the extratropical Atlantic in the middle and upper troposphere and also induces a strong westerly vertical shear. Composites of aerosol optical depth suggest that the Saharan Air Layer has a limited contribution to the dryness in the vicinity of the non-developers.