The role of 25-90 day variability of tropical convection in the West African summer monsoon onset in 2006

This work presents an analysis of the main 25-90-day mode of variability of convection in the West African summer monsoon. Over Africa an extended pattern of enhanced convective develops from the east, covering most of West and Central Africa at its peak and finally maintaining itself over the western part of Africa. In parallel a Madden-Julian Oscillation (MJO) signal develops over the Indian ocean leading to a 40-day transition between an active and a break period in the Indian monsoon. Composed of a meridional OLR dipole structure, this signal is associated with a northern (southern) low-levels cyclonic (anticyclonic) circulation which propagates northward. A cyclonic Rossby wave grows from the Indian enhanced convective area and propagates westward over northern Africa, consistent with the enhanced convective activity of West and Central Africa. Once the break sequence occurs in the Indian sector, reverse anticyclonic circulation develops and contributes to progressively weaken westward the active convection sequence over Africa. A set of simulations with a global dry spectral primitive equation model of the Indian diabatic heating field impact on the atmospheric circulation confirms the scenario proposed from observational data sets. The initiation of such a 25-90-day signal occurrence and its impact on the African monsoon onset in 2006 is presented.