Séminaire
In monsoonal regimes, most of the annual precipitation occurs in few months, when the prevailing lower-level winds reverse their direction. The West African monsoon (WAM) is a classic monsoon generated by the sea-land thermal contrast. The onset of WAM occurs in early June when the ITCZ lands at the coast of Guinea, to move further inland towards the end of June. The South Asian monsoon (SAM) is a mixed marine-land monsoon. It originates over the Tropical Indian Ocean at the beginning of the boreal summer, to move northwards in few days, making its landfall on the Indian subcontinent. The East African Monsoon (EAM) is an ancillary monsoon of the SAM. It acts as a pivot about which the easterly winds over the Tropical Indian Ocean veer westerly, to be redirected backwards the Indian subcontinent. Monsoons are characterized by a large upper anticyclone and a lower smaller cyclone, dynamically connected by a strong mid-tropospheric updraft, balanced by a vast weak subsidence. These flows are analyzed in terms of Kelvin, mixed and planetary modes. In summer, the Mediterranean, North Africa and the Tropical Atlantic are under the combined influence of the SAM and the African monsoonal systems. The SAM overpowers the African monsoonal system by driving a strong Walker-like cell with rising air over South
Asia and subsiding air over the African deserts and the eastern Mediterranean. The African monsoonal system stretches its influence from the Tropical Atlantic to the Tropical Indian Ocean. It results that
easterly winds, driven by this system over the Tropical Indian Ocean, fuel moist air into the SAM by turning westerly when they hit East Africa.
marco.gaetani@latmos.ipsl.fr