Simulating the evolution of the Asian and African monsoons during the past30 Myr using an atmospheric general circulation model

At geologic timescales, many proxy data suggest a contrasting evolution of Asian and African monsoons since the Oligocene. The Asian summer monsoon in creases drastically around 8 Ma, whereas the African summer monsoon gradual ly weakens during the Miocene, Using an atmospheric general circulation mod el, we simulate most of the spatial evolutions of both monsoons only accoun ting for the changes of paleogeography, including continental drift, orogen y, and sea level change. The paleogeographic changes modify drastically the climate over central and southern Asia between the Oligocene and the prese nt. The retreat of an epicontinental sea warms central Eurasia in summer. T he heating of this area and the uplifts of the Tibetan plateau and of the H imalayas deepen the Asian low pressure cell and displace it northwest. This then shifts precipitation from Indochina toward the southern flank of the Himalayas. This is in good agreement with proxy data. Therefore our modelin g studies support a shift and a strengthening of the Asian monsoon during t he late Tertiary rather than a real "onset". We suggest that the increase i n seasonal precipitation and the strengthening of the number of days with h eavy rainfall over the Himalayas from 30 Ma to the present may be of critic al importance to explain the long- term evolution of physical erosion of th is area. We also investigate the respective impact of the Paratethys shrink age and of the Tibetan plateau uplift through sensitivity experiments and p rove that the Paratethys retreat plays an important role in monsoon evoluti on. The northward drift of the African continent confines summer monsoon pr ecipitation to a thin belt which favors the stretching of the subtropical d esert, in good agreement with data. We finally show that during the Oligoce ne, the African and Asian monsoon systems are clearly separated by the Teth ys seaway. The closure of this seaway and the evolution of the Asian monsoo n induce a connection between both monsoon systems in the low and middle tr oposphere.