On the structure of the regional-scale circulation over Central Africa: Seasonal evolution, variability, and mechanisms

Atmospheric circulation over central Africa is dominated by the tropical easterly jet, the African easterly jet, and the low-level westerly jet. In the lower troposphere, a zonal overturning cell occurs over central Africa, but the mechanisms driving its formation, seasonal evolution, and variability are still unclear. Here, using reanalyses (ERA-Interim, NCEP-2, and JRA-55) and the ECHAM5.3 atmospheric model forced by observed sea surface temperature, we highlight the existence, in the lower troposphere, of a separated single, closed, counterclockwise, and shallow zonal overturning cell, namely, the Congo basin cell. This Congo basin cell persists year round, with maximum intensity and width in August/September and minimum intensity and width in May. This shallow cell extracts heat from the warm central Africa landmass through latent and internal energies and transports it to the cold eastern equatorial Atlantic Ocean, reminiscent of the mixed Carnot-steam cycle. Indeed, the monsoon-like circulation triggered by the zonal surface pressure gradient between the warm central Africa landmass and surrounding cold oceans produces mass convergence at the Congo Air Boundary, providing necessary upward motion to air parcels to destabilize the atmosphere over central Africa. As result, convective updrafts depend on underlying moist static energy and the induced lowlevel westerly jet, controlled by the near-surface land-ocean thermal contrast through the zonal surface pressure gradient between the warm central African landmass and cold eastern equatorial Atlantic Ocean, rather than the midlevel easterly jet. This midlevel easterly jet is formed by the mechanical work that balances the convection associated with the saturation and rainfall. Furthermore, the efficiency of the Congo basin cell determines seasonality over central Africa.