SENSITIVITY OF SIMULATED ASIAN AND AFRICAN SUMMER MONSOONS TO ORBITALLY INDUCED VARIATIONS IN INSOLATION 126, 115 AND 6KBP

We have conducted four numerical experiments with an atmospheric gener al circulation model (AGCM) to investigate the sensitivity of Asian an d African monsoons to small changes (-5 to +12%), with respect to pres ent-day, in incoming solar radiation at the top of the atmosphere. We show that, during the mid-Holocene (6 kBP where kBP means thousands of years before present-day) and the last interglacial (126 kBP), the No rthern Hemisphere seasonal contrast was increased, with warmer summers and colder winters. At the time of glacial inception (115 kBP) howeve r, summers were cooler and winters milder. As a consequence, Asia and tropical North Africa experienced stronger (weaker) summer monsoons 6 and 126 kBP (115 kBP), in agreement with previous numerical studies. T his present study shows that summer warming/cooling of Eurasia and Nor th Africa induced a shift of the main low-level convergence cell along a northwest/southeast transect. When land was warmer (during the summ er months 6 and 126 kBP), the monsoon winds converged further inland b ringing more moisture into northern India, western China and the south ern Sahara. The southern tips of India, Indochina and southeastern Chi na, as well as equatorial North Africa became drier. When land was coo ler (during the summer 115 kBP), the main convergence zone was located over the west Pacific and the wet (dry) areas were those that were dr y (wet) 6 and 126 kBP. The location and intensity of the simulated pre cipitation maxima were therefore very sensitive to changes in insolati on. However the total amount of monsoon rain in Asia as well as in Afr ica remained remarkably stable through the time periods studied. These simulated migrations of convective activities were accompanied by cha nges in the nature of precipitation events: increased monsoon rains in these experiments were always associated with more high precipitation events (>5 mm day(-1)), and fewer light showers (less than or equal t o 1 mm day(-1)). Rainy days with rates between 1 and 5 mm day(-1) were almost unchanged.