Abstract
The role of the Tropical Easterly Jet (TEJ) in West African rainfall climatology has received little attention in the research community to date. Therefore, this dissertation will examine the instabilities and wave activity associated with the TEJ and their implications regarding interannual rainfall variability over western Africa. First, the instability of the TEJ is examined using potential vorticity (PV) concepts to contrast wet and dry years in West Africa. Analyses of the meridional PV gradient indicate an abrupt shift in both location and magnitude of the instability associated with the TEJ during the transition from wet to dry years in the Sahel. Additionally, the signs of the climatological anomalies of PV at the TEJ level strongly reflect the four primary modes (wet, dry, wet dipole, and dry dipole) of interannual rainfall variability in West Africa. Several examples of PV perturbation analyses at the TEJ level confirm that the upper-level development of African Easterly Waves (AEWs) differs considerably between the two periods. These results support recent observations and modeling studies that suggest that the interaction between the TEJ and the African Easterly Jet (AEJ) plays an important role in the development and structure of AEWs.
In addition to the observational study, a multi-layer primitive equation model is utilized to examine easterly wave activity and vertical motion patterns based on the juxtaposition of the three primary jets located over western Africa. Idealized simulations based on the basic states of the low-level westerly jet (LLWJ), African Easterly Jet (AEJ), and the Tropical Easterly Jet (TEJ) for several anomalously wet and dry years in the Sahel are studied. Results are compared to several linearized GCM simulations that are initialized with NCEP observational data. Results show that the location, intensity, and scale of wave perturbations are sensitive to the position and intensity of the jets. Vertical motion patterns also indicate that maximum upward motion is generally located between the cores of the AEJ and TEJ and was more intense in the wet years. These results generally agree with observational and other modeling studies, further emphasizing that jet position and intensity are important factors in determining interannual rainfall variability across western Africa.
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