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Keywords:

  • West African monsoon;
  • moist convection;
  • diabatic heating;
  • reanalysis comparison

Abstract

This study examines regional differences in the structure of African easterly waves (AEWs) at four locations between East Africa and the East Atlantic in a 12 yr (1998–2009) climatology. We focus on the cold-core convectively active portions of the AEWs, which are typically found between 5 and 15°N. African easterly wave composites are produced from three reanalyses using the tracks of synoptic-scale middle tropospheric vorticity maxima. The vertical structure of temperature and vorticity in the composite AEWs are related to regional differences in the climatological and vortex-centred diabatic heating and diabatic potential vorticity (PV) generation profiles from the reanalyses. In addition, differences in AEW structure and activity between the three reanalyses are related to differences in rainfall and diabatic heating between the reanalyses. The climatological diabatic heating over East Africa, which is associated mostly with latent heating, peaks at 450 hPa with diabatic cooling below 750 hPa. In contrast, the convectively active West African coast and the East Atlantic have increased diabatic heating in the lower troposphere. Due to these regional differences in diabatic heating, diabatic PV generation peaks near 700 hPa over the continent and below 800 hPa over the East Atlantic. Moving westwards from East Africa to the East Atlantic, AEW composites have weaker lower tropospheric cold cores and significantly increased lower tropospheric vorticity within the convectively active troughs. The shape of the diabatic heating and PV generation profiles within the trough of the regional AEW composites are similar to the climatological profiles in each respective region. This suggests that regional differences in moist convection, particularly the land-ocean contrast, provide a strong constraint on the diabatic process within the AEW trough and therefore the vertical structure of the AEWs.