- African Easterly Waves and Their Relationship to Rainfall on a Daily Timescale.
Baum, Jeffrey D., Nicholson, Sharon E., Krishnamurti, T. N., Kim, Kwang-Yul, Department of Earth, Ocean and Atmospheric Sciences, Florida State University
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...
Show moreAfrican 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 â1961) 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 time-frame 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. During the wet years (1958 -1961) the number of waves that propagated across West Africa averaged 59 waves per year using the Vertical Profile Method (VPM) and 49 using the Andreas Fink Method (AFM). The number of AEWs tracked during the dry years (1982-1985) averaged about 63 with the VPM and 54 with the AFM. The difference in the average number of AEWs between the methods was probably a result of the limitations of each method. More waves in the dry years supports previous conclusions that stronger but less frequent AEWs occur in wet years and weaker but more frequent AEWs occur in the dry years. Tracking of the AEWs also allowed for the separation of the AEWs into two distinct wave regimes north and south of the African Easterly Jet (AEJ). Throughout all eight years the separation became apparent as the summer progressed with the separation reaching a maximum. This coincides with the fluctuations in AEJ intensity, which also has a maximum in the month of August. Rainfall within the tracked AEWs was found to be bimodal with the region of maximum northerly flow containing the most rainfall. This region is ahead of the trough and is typically associated with squall line convective activity. The maximum region was the same for both wet and dry years and also for the two month season comparison. An EOF analysis of daily rainfall was completed for 1958 (wet year) and 1983 (dry year) to determine if any relationship to AEWs could be identified in the rainfall time series. The resulting eigenvalues show the influence of topography along the Guinea coast with positive values: however, no discernable link to AEWs was present. The power spectrum of the first two principle components did identify a significant peak at the same period associated with AEWs. This power spectrum analysis demonstrated that rainfall is linked to AEWs, but a more detailed dynamical investigation is needed.
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