An Analysis of West African Dynamics Using a Linearized GCM*

Abstract

This study utilizes a linear, primitive equation spherical model to study the development and propagation of easterly wave disturbances over West Africa. Perturbations are started from an initial disturbance consisting of a barotropic vortex and the governing equations are integrated forward in time. The perturbations are introduced into basic states corresponding to the observed dynamical and thermodynamical characteristics of two wet years in the Sahel and two dry years.The model simulations show consistent contrasts in wave activity between the wet and dry years. The waves are markedly stronger in the wet years and show a barotropic structure throughout the troposphere. The waves tend to extend throughout the troposphere to the level of the tropical easterly jet (TEJ) in the wet years, but not in the dry years. The upper-tropospheric shear, which is stronger in wet years, appears to be a key factor in wave development. This shear is dependent on the intensity of the TEJ, suggesting that the TEJ is an important factor in interannual variability in the Sahel. When the overall shear is weak, vertical development is suppressed.Another contrast is that in the dry years the growth rates show a single maximum around 3000-4000 km, but in the wet years there is a second, around 6000-7000 km. This suggests that both synoptic-scale and planetary-scale waves are active in the rainy season of some wet years. Imposing considerations of potential vorticity, the generation of planetary-scale waves implies a strong link between the surface and the TEJ in wet years. Such a link is absent in the dry years. This is likely a major factor in the interannual variability of rainfall in the Sahel.

The West African Sahel: A Review of Recent Studies on the Rainfall Regime and Its Interannual Variability

“…This permits the development of waves on the TEJ. Such waves have been observed over West Africa [135] and their characteristics match those predicted by a numerical simulation of African wave activity [136]. Such waves may play a role in the development of extreme precipitation events in the western Sahel.…”

“…The predicted waves are of planetary scale with a period of 5 to 6 days and they develop to the northwest of the mid-level AEW. Observations showed that such a wave developed in July of 1950, a very wet year (Nicholson et al [136], with characteristics similar to the predicted TEJ waves. Potential vorticity theory suggests that the waves on the TEJ develop via interactions between the surface and the TEJ.…”

“…The waves also appear to modulate annual precipitation amounts [136,144]. Taleb and Druyan [199], in contrast, find no correlation between the number of waves and interannual variability.…”

“…Nevertheless, their structure and development can be largely understood in terms of dry dynamics [68,119]. Nicholson et al [136] for example used a dry model to simulate wave development and produced realistic waves for dry years and wet years that matched observed characteristics in those years. Once a wave precursor is triggered, the state of the AEJ still plays a critical role in determining whether or not a wave develops [193].…”

A physical basis for the interannual variability of rainfall in the Sahel

“…This location most commonly produces the configuration of rainfall anomalies represented by a dipole in which the Sahel is anomalously wet and the Guinea Coast is anomalously dry. As noted in Section 2.2, strong, deep and extensive equatorial westerlies and a strong vertical shear zone over the Sahel characterize the Sahel mode Nicholson et al, 2007). The shear indicates that baroclinic instability is probably important in the Sahel in the wet years.…”