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Zenodo
2021
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| Online Access: | https://doi.org/10.5281/zenodo.16746227 |
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Table of Contents:
- <p>This thesis focuses on two aspects. The first part focuses on examining the African easterly waves (AEW) activity’s modulation by convectively coupled Kelvin waves (CCKWs) over eastern Atlantic and West Africa. The second part focuses on examining the regional differences in the structure of CCKWs and AEWs and the diurnal cycle’s influence on AEWs and CCKWs.</p> <p>CCKWs modify the background environment in which AEWs get either amplified or suppressed. The CCKW convective phase enhances the magnitudes of the meridional<br>gradient of mean zonal wind to the south of the African easterly jet (AEJ). Thus enhanced barotropic energy conversions and more AEW southern storm track growth in the CCKW convective phase than in the CCKW suppressed phase. Besides, AEWs are more convectively active in the CCKW convective phase than in the suppressed phase. The AEW northern track enhanced more in the CCKW convective phase than in the suppressed phase primarily through enhanced baroclinic energy conversions. The CCKW convective phase enhances the vertical gradient’s magnitudes of the mean zonal wind and associated lower-troposphere meridional gradients of temperature below and the poleward side of the AEJ.</p> <p>The AEW southern storm track diabatic heating profile over central Africa exhibits a top-heavy structure. In contrast, near coastal West Africa and the eastern Atlantic, the<br>diabatic profile displays a bottom-heavy structure. The AEW diabatic heating vertical structure matches with the climatological diabatic heating structure. The bottom-heavy mean diabatic heating structure over eastern Atlantic and coastal West Africa is associated with mean horizontal convergence below 800 hPa and divergence above. The top-heavy mean diabatic heating structure over central Africa is associated with elevated mean horizontal convergence above 850 hPa and divergence above 350 hPa and below 850 hPa. AEWs become less cold-core as they move westward towards coastal west Africa while these cold anomalies shift to higher altitudes over the eastern Atlantic. This study argues the importance of large-scale circulations in constraining the AEWs diabatic heating vertical mode.</p> <p>CCKWs typical structure over the eastern Atlantic matches with that observed over the Pacific ocean. CCKWs, as they move towards coastal west Africa, their structure is distorted from other influences such as topography and land-ocean contrast. They intensify and regain their typical structure as they move towards central Africa. The lower-troposphere cold anomalies intensify as they move from the eastern Atlantic towards central Africa. To the east of Greenwich meridian, the upper-troposphere positive temperature perturbations and positive diabatic heating perturbations become in-phase, suggesting more wave growth.</p> <p>The CCKWs and AEWs, when in phase with the diurnally enhanced convection, intensifies and produce more rainfall than when they are in phase with the diurnally suppressed convective phase.</p>