Spatial Patterns of African Easterly Waves and their Precipitation in Northern Africa

African easterly waves (AEWs) are synoptic-scale disturbances that exert a profound influence on rainfall patterns in West Africa. Previous studies have investigated AEW structure, propagation, and genesis, as well as their impacts in Africa. For example, Gu et al. (2004) assessed AEW meridional winds and precipitation and found that northern track waves (15-25°N) have a stronger meridional wind signature while southern track waves (5-15°N) have a stronger precipitation signature. White et al. (2021) investigated how the presence of orography (terrain features) impacts the motion of northern and southern track AEWs. However, no published work has used spatial methods to study how AEWs influence the spatial structure of precipitation in Tropical Africa. This study quantifies precipitation associated with AEWs in their northern and southern tracks along the African easterly Jet (AEJ). We use the following datasets: tropical easterly wave (TEW) tracks from Mahollis 2023 (https://doi.org/10.5281/zenodo.10076724), satellite derived rain rates from Integrated Multi‐satellitE Retrievals for the Global Precipitation Measurement (IMERG), and reanalysis data from ECMWF Atmospheric Reanalysis Fifth Generation (ERA-5). First, we sorted AEWs from the TEW track dataset using a north and south of the AEJ criteria and their vertical tracking level. Previous studies have diagnosed AEW activity north and south of the AEJ with their traditional vertical level of maximum vorticity at 850 hPa and 700 hPa, respectively. Specifically, we have six categories of AEWs based on (1) whether the wave is located north or south of 15°N and (2) whether the wave’s vorticity center is tracked at 700 hPa, 850 hPa or both levels (vertically collocated) (Table 1). For each category, we make composites of AEW precipitation structure. Next, we use shape metrics (e.g., area, elongation, dispersion; see Zick 2023) to identify the differences in AEW precipitation structure among the categories. The shape of a wave is hypothesized to be related to the strength and intensity of the precipitation corresponding to it. For our results, we expect the AEWs south of the AEJ to correspond with higher precipitation (intensity and frequency) and have a more compact, circular precipitation structure compared to AEWs north of the AEJ. Future work will determine the influence of environmental relative humidity and temperature on AEW precipitation.

Vertical level used for tracking the wave	North of AEJ	South of AEJ
700hPa	Untraditional Northern track AEW	Traditional Southern track AEW
850hPa	Traditional Northern track AEW	Untraditional Southern track AEW
700 and 850 hPa	Vertically collocated northern track AEW	Vertically collocated southern track AEW

References

Gu, G., Adler, R. F., Huffman, G. J., & Curtis, S. (2004). African easterly waves and their association with precipitation. Journal of Geophysical Research: Atmospheres, 109(D4). https://doi.org/10.1029/2003JD003967

Hollis, M. (2023).TEW Tracks (Version v1) [Computer software]. Zenodo. https://doi.org/10.5281/zenodo.10076724

White, J. D., Aiyyer, A., & Russell, J. O. H. (2021). The Impact of Orography on the African Easterly Wave Stormtrack. Journal of Geophysical Research: Atmospheres, 126(9), e2020JD033749. https://doi.org/10.1029/2020JD033749

Zick, S. E. (2023). Anatomy of a storm: A review of shape analysis research that fuses form and function in weather forecasting and analysis. Progress in Physical Geography: Earth and Environment, 47(1), 152–170. https://doi.org/10.1177/03091333221133098