Analysis of African Easterly Wave Structures and Their Role in Influencing Tropical Cyclogenesis

Abstract

Composite structures of African easterly waves (AEWs) that develop into named tropical cyclones in the Atlantic are compared and contrasted with nondeveloping AEWs using the 40-yr ECMWF Re-Analysis (ERA-40) data and satellite brightness temperature between 1979 and 2001. Developing AEWs are characterized by a more distinctive cold-core structure two days before reaching the West African coast. As they move westward, the convective activity increases further in the vicinity of the Guinea Highlands region. At the same time the AEW trough increases its vorticity at low levels consistent with a transformation toward a more warm-core structure before it reaches the ocean. As the AEW moves over the ocean convection is maintained in the trough, consistent with the observed tropical cyclogenesis. The nondeveloping AEW has a similar evolution before reaching the coast except that the amplitudes are weaker and there is less convective activity in the Guinea Highlands region. The nondeveloping AEW composite has a more prominent dry signal just ahead of the AEW trough at mid- to upper levels. It is argued that the weaker west coast development (i.e., reduced convective activity and reduced spinup at low levels) combined with the closer proximity of the trough to mid- to upper-level dry air aloft are consistent with the nondevelopment. The most intense nondeveloping AEWs were characterized by more intense convection and stronger mid- and low-level synoptic circulations at the West African coast than the developing AEWs. The analysis strongly suggests that the lack of development was due to the presence of dry mid- to upper-level air just ahead of the AEW trough that may have been enhanced because of equatorward advection of dry air by the AEW itself.

Reevaluating the Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution

“…A composite study of AEWs by Hopsch et al (2010) found results consistent with the results of the present study. They composited wave structures for developing and nondeveloping waves for the time of passage of AEWs at 158W (the west coast of Africa) as well as 2 days before and after.…”

Projected changes in African easterly wave intensity and track in response to greenhouse forcing

“…In addition, AEWs characterized by stronger low-level (850 mb) circulations and high relative humidity at the West African coast are more likely than weaker AEWs to undergo tropical cyclogenesis (7). However, strong winds associated with southern-and northern-track AEW vortices can advect dry Saharan air downstream of the AEW and impede development into a tropical cyclone entirely, or can delay tropical cyclogenesis until the wave reaches favorable environmental conditions farther west in the Atlantic basin (7,28). Further, although there is much evidence that northern-track waves can influence tropical cyclogenesis, in the present climate, many of the northern-track waves weaken as they reach the eastern Atlantic, or track north of the MDR (7,27).…”

“…However, compared with the southern vortices, the northern vortices are less efficient at undergoing tropical cyclogenesis. As the northern vortices propagate westward into the Atlantic, they often advect relatively dry Saharan air, known as the Saharan Air Layer, downstream of the wave trough (7). As a result of the dry air advection, northern-track vortices that do transition to tropical cyclones often do not undergo tropical cyclogenesis until they reach the Western Atlantic, where environmental conditions are more favorable for development (28).…”

Anatomy of an observed African easterly wave in July 2006

“…AEWs play a significant role in the modulation of rainfall (e.g. Fink and Reiner, 2003) over West Africa and also act as a precursor for tropical storms and hurricanes in the Atlantic (Landsea and Gray, 1992;Avila and Pasch, 1992;Thorncroft and Hodges, 2001;Zawislak and Zipser, 2010;Hopsch et al, 2010). Hence, the current lack of forecast ability has direct consequences on the forecasting of these events also.…”