African easterly waves: Evolution and relationship to Atlantic tropical cyclones

Every year, a majority of all Atlantic hurricanes originate from African easterly waves. Yet, less than 20% of all African waves originate tropical cyclones (including hurricanes). It is of interest to investigate if African waves involved in cyclogenesis possess special characteristics, or if the environment in which they are embedded is the sole factor that determines if cyclogenesis will occur. This study consists of assessment of the special characteristics of African easterly waves (AEW) associated with the formation of Atlantic tropical cyclones using wave composites of daily June--September (JJAS) NCEP/NCAR reanalysis data from 1972 to 2001. The average characteristics of the waves as they travel over western Africa and the Atlantic were determined from thirty-year JJAS composites, providing an opportunity to revisit their life cycle using a long-term homogeneous dataset. They were then used to aid in analyzing the remaining results AEW associated with cyclogenesis provide a favorable local environment with no vertical tilt, an upper level anticyclonic region, deep rising motion, a smaller and weaker cold core and widespread moisture. Waves that are not associated with the formation tropical cyclones show much stronger barotropic and baroclinic EKE production and their general characteristics are more similar to the average AEW characteristics but with a larger and stronger cold core. Thus, the characteristics of the individual waves will make them a favorable or unfavorable environment for cyclogenesis, independently of the mean environment.