In the late spring or early summer, a discontinuity in the northward excursion of the precipitation maximum over West Africa is typically observed when, over the course of a few days, the latitude of maximum rainfall moves rapidly from the Guinean coast (at about 5 degrees N) into the Sahel (at about 12 degrees N). This event is known as the West African monsoon jump. After the jump, Sahelian precipitation rates generally exceed Guinean coast precipitation rates. Previous modeling results associate the occurrence of the jump with the development of inertial instability above the boundary layer over the Guinean coast region. When a region of the atmosphere is inertially unstable, meridional perturbations of the flow result in divergence.

Regional model simulations of the spring and summer of 7 years (2000 – 2006) are diagnosed to improve our understanding of the development of inertial instability over the Guinean coast region prior to the jump. The model simulations capture the monsoon jump, including differences in the timing of the jump from one year to the next, with reasonable accuracy. Analysis of the springtime evolution of absolute vorticity provides insight into the development of the inertial instability and its relationship to precipitation.