Genesis of Atlantic tropical storms from African Easterly Waves—a comparison of two contrasting years

Using automatic tracking techniques to follow coherent vorticity centers in ECMWF reanalyses, we have identified large interannual-to-decadal variability in the number that emanate from the West African coast. While the low-frequency variability is well correlated with Atlantic tropical cyclone activity and other large-scale variations (e.g. West African rainfall and SSTs), their variation on interannual timescales is not. In contrast, however, a significant positive correlation was found between the interannual variation of the 2-6 day filtered meridional wind (a synoptic-scale measure of African Easterly Wave activity), and Atlantic tropical cyclone activity.

Toward helping us interpret this result we consider two contrasting years, 1988 and 1989, which contained the least active and most active seasons in the storm track data over the Guinean Highlands area, one of the most important source regions of storm tracks. While relatively few vorticity centers were found over the land in 1988, more vorticity centers appeared to be generated further west over the tropical Atlantic, westward of a cold SST anomaly. In contrast, the ERA40 tracks for 1989 showed that many vorticity centers were generated over the Guinea Highlands and the adjacent tropical Atlantic. While seven named Atlantic tropical cyclones formed within the MDR in both years, the location of genesis differed markedly. Namely, the tropical cyclones were formed further west during 1988, whereas they formed directly offshore of West Africa in 1989.

A more detailed analysis for the two seasons, including description of environmental influences and the nature of the weather systems themselves will be provided.