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

Abstract

African Easterly Waves (AEWs) are synoptic-scale weather systems influencing Northern Africa, with distinct northern and southern tracks affected by the African Easterly Jet (AEJ), as noted in previous research. These waves are most active in the lower troposphere around 700-850 hPa. This study explores the spatial distribution of AEWs and their precipitation using the Tropical Easterly Waves Climatology and the Integrated Multi-satellite Retrievals for Global Precipitation Measurement (IMERG). First, a spatial filter is used to limit 2001-2021 tropical easterly waves to only waves over the northern Africa region. The resulting AEWs are then split into categories based on the level of maximum vorticity (850 hPa, 700 hPa or vertically collocated) and their location with respect to 15°N (northern and southern). The spatial distribution of these waves shows that the 700 hPa AEWs initiate farther east at about 51ºE and extend farther to the north to about 45ºN. The 850 hPa AEWs have a similar spatial distribution but are fewer in number. Analysis of latitudes and longitudes reveals that most AEWs at 700 hPa occur between latitudes 8ºN to 12ºN and longitudes 12ºW to 12ºE, while AEWs at 850 hPa are more concentrated between latitudes 10ºN to 20ºN and longitudes 10ºW to 8ºE. Next, precipitation composites are generated for the six wave types, and there is a clear distinction in precipitation patterns between the northern and southern track AEWs, with southern track AEWs producing precipitation along an elongated north-south axis and northern track waves producing very little precipitation near the wave center. The study also applies shape metrics to assess the AEW precipitation structure at thresholds of 2 mm hr⁻¹ and 5 mm hr⁻¹. The Mann-Whitney U-test indicates significant differences between 700 hPa, 850 hPa, and vertically collocated waves in terms of area, closure, and solidity, which are metrics quantifying the size and shape of the AEW precipitation. Similar to the composite analysis, the shape metric analysis indicates that vertically collocated waves produced higher precipitation rates over larger areas and more solid precipitation regions, suggesting that vertical alignment of the vorticity maxima leads to enhanced AEW-associated precipitation.