Observational studies show that there is a positive correlation between summer rainfall over East Africa and India, and a negative correlation between West African (e.g., the Guinea Coast) and East African rainfall during the summer. The purpose of this study is to understand the underlying mechanisms that connect rainfall in these three regions.

One potential mechanism is heating over the East African Highlands. To explore this idea, a mesoscale climate model (MCM) developed from the PSU/NCAR MM5 is modified to alter the low-level heating over East Africa by removing the topography of the African continent. As a result, summer rainfall rates decrease over East Africa since the rainfall here is intensified by orographic uplift. Consistent with the observations, the decrease in East African precipitation is associated with a decrease in precipitation over India, and an increase in precipitation along the Guinean Coast.

One consequence of removing the elevated heat source over the East African Highlands is cooler lower-tropospheric temperatures. This decreases the East Africa?Indian Ocean thermal gradient. Lower-tropospheric cross equatorial flow (i.e., the Somali Jet) weakens and this leads to a decrease in moisture convergence and rainfall over India. Another effect is a reduction in the cyclonic flow over East Africa. This is associated with a reduction in the zonal component of the West African monsoonal flow and increased precipitation along the Coast.

The results of these simulations are verified using additional model simulations in which the heating field is perturbed without removing the topography (e.g., by modifying the surface boundary conditions).