Milankovitch and albedo forcing of the tropical monsoons: a comparison of geological evidence and numerical simulations for 9000 yBP

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Abstract
Lake-level and palaeoecological evidence from Africa, Arabia and southern Asia for 9000 yBP suggests an intensification and increased poleward penetration of the northern monsoons. The vegetation belts shifted north by 4–6° latitude on the south side of the Sahara. In contrast, the monsoon over southern Africa was weaker than today. Calculations based on the new palaeogeographical map of Mali by Petit-Maireet al.(1988) indicated that the areaaveraged surface albedo decreased by 0·10–0·14 in the zone 16–24°N and that total annual precipitation increased by 150–320 mm north of the inland delta of the Niger (20–24° 15′N). Experiments with atmospheric general-circulation models suggest that this asymmetrical pattern of anomalies in the strength of the tropical monsoons can be explained in broad terms by the different orbital configuration of the Earth at 9000 yBP. Here, we describe a hitherto unpublished sensitivity experiment with the low-resolution (5° × 7·5°) version of the U.K. Meteorological Office 11-layer model, in which the albedo over Africa and Arabia between 15 and 30°N was reduced by between 0·04 and 0·06 to simulate the increase in vegetation cover at 9000 yBP. The results indicate that the surface-albedo change provides a significant positive feedback enhancing the direct climatic effects of Milankovitch forcing in the tropics.