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Florida State University (2005)

African Easterly Waves and their relationship to rainfall on a daily timescale

Baum, Jeffrey DuWayne

Titre : African Easterly Waves and their relationship to rainfall on a daily timescale.

Auteur : Baum, Jeffrey DuWayne

Université de soutenance : Florida State University

Grade : Master of Science 2005

Résumé partiel
African Easterly Waves (AEWs) are mesoscale synoptic features that form in West Africa and propagate westward. The relationship of AEWs to rainfall in the region has been examined in previous studies but no scientific consensus has been reached. This relationship, however, has been found to be complex and to involve numerous feedback mechanisms such as the African Easterly Jet (AEJ) fluctuations. Other mechanisms like latent heat release and the temperature gradient at the surface between the dry Sahara Desert and the moist maritime region along the Gulf of Guinea also play a role in the relationship between rainfall and AEWs. This study will investigate the relationship of AEWs to rainfall through objective analysis of the waves and an EOF analysis of daily rainfall over a prescribed region. Two classification schemes along with statistical techniques were used to investigate the spatiotemporal variability of the relationship between African Easterly Waves (AEWs) and the rainfall over West Africa. Two varying methods were used because of the highly subjective manner of tracking AEWs. This study was limited to four wet years (1958 ñ1961) and four dry years (1982-1985) during the summer months of June, July, August and September. The first technique was a new three-step method designed by Dr. A. Fink that makes use of 700mb and 850mb streamline plots and band-pass filtered meridional wind data. The second method uses the vertical structure of the band-pass filtered meridional wind to identify wave passage. The methods were compared and contrasted with one another, and both were shown to be adequate for tracking AEWs. However, for examining the relationship of rainfall with AEWs the second method appropriately named the Vertical Profile Method (VPM) was found to be preferable. Using the VPM the difference between wave characteristics and rainfall variability were examined on a daily timescale. This involved creating daily rainfall datasets for four different quadrants over the Sahel and relating any changes in rainfall activity to the AEWs as they both propagated across the quadrants. The differences in AEW and rainfall characteristics were examined by making comparisons within two distinct timeframe perspectives. The first perspective examined differences between wet and dry years and the second perspective examined differences between the 1st two months (June- July) and the last two months (August-September). For each category the strength of the waves, number of waves, location of the north and south regime, rainfall distribution and rainfall amounts were compared.

Mots-clés : Daily Rainfall ; African Easterly Waves ; EOF

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Page publiée le 29 avril 2011, mise à jour le 24 décembre 2018