North Atlantic Hurricanes Contributed by African Easterly Waves North and South of the African Easterly Jet

Abstract

A majority of tropical cyclones in the North Atlantic develop from African easterly waves (AEWs), which originate along both the southern and northern flanks of the midtropospheric African easterly jet (AEWS and AEWn, respectively). The purpose of this note is to identify the contribution of AEWSs and AEWns to North Atlantic tropical cyclones that develop from AEWs. Applying a manual backtracking approach to identify the genesis locations of AEWS, it was found that the population ratio of tropical cyclones formed from AEWSs to those formed from AEWns is 1:1.2. Because the population ratio of AEWSs to AEWns is 1:2.5, the conversion rate of the former AEWS to tropical cyclones is twice as effective as the latter waves. In addition, it was found that AEWns travel farther and take longer to transform into tropical cyclones than AEWSs, which is likely because the AEWns are drier and shallower than AEWSs. An analysis of various terms in the moisture and vorticity budgets reveals that the monsoon trough over West Africa provides moisture and enhances low-level vorticity for both AEWns and AEWSs as they move off the West African coast. The monsoon trough appears to be of particular importance in supplying AEWns with enough moisture so that they have similar properties to AEWSs after they have traveled a considerable westward distance across the tropical Atlantic.