Abstract
Abstract. In this study, we use a sophisticated high-resolution atmosphere-ocean coupled climate model, EC-Earth, to investigate the effect of Mid-Holocene orbital forcing on summer monsoons on both hemispheres. During the Mid-Holocene (6 ka), there was more summer insolation on the Northern Hemisphere than today, which intensified the meridional temperature and pressure gradients. Over North Africa, monsoonal precipitation is intensified through increased landward monsoon winds and moisture advection as well as decreased moisture convergence over the oceans and more convergence over land compared to the pre-industrial simulation. Precipitation also extends further north as the ITCZ shifts northward in response to the stronger poleward gradient of insolation. This increase and poleward extent is stronger than in most previous ocean-atmosphere GCM simulations. In north-westernmost Africa, precipitation extends up to 35° N. Over tropical Africa, internal feedbacks completely overcome the direct warming effect of increased insolation. We also find a weakened African Easterly Jet. Over Asia, monsoonal precipitation during the Mid-Holocene is increased as well, but the response is different than over North-Africa. There is more convection over land at the expense of convection over the ocean, but precipitation does not extend further northward, monsoon winds over the ocean are weaker and the surrounding ocean does not provide more moisture. On the Southern Hemisphere, summer insolation and the poleward insolation gradient were weaker during the Mid-Holocene, resulting in a reduced South American monsoon through decreased monsoon winds and less convection, as well as an equatorward shift in the ITCZ. This study corroborates the findings of paleodata research as well as previous model studies, while giving a more detailed account of Mid-Holocene monsoons.
Highlights
Monsoon systems play a key role in the climate of the tropics and subtropics
There is ample evidence from paleodata that the enhanced seasonal cycle of insolation on the Northern Hemisphere coincided with intensified monsoonal summer precipitation, while the reduced seasonal cycle on the Southern Hemisphere corresponded to weaker monsoons
For North Africa and Asia we focus on the months JulyAugust-September (JAS), when insolation changes over the Northern Hemisphere (NH) tropics are largest
Summary
Generated by the asymmetric heating of continents and oceans, they are characterised by seasonal reversals in atmospheric circulation and associated precipitation (Hastenrath, 1991; Webster, 1987a; Webster et al, 1998). Even larger changes occur on time scales of (tens of) thousands of years, when variations in the Earth’s orbit induce fluctuations in the distribution of incoming solar radiation on Earth (Berger, 1978). On these orbital time scales, monsoons are primarily controlled by precession, which modifies the seasonality of insolation and, thereby, changes the asymmetric heating of continents and oceans (Ruddiman, 2007). Pollen- and plant macrofossil-based reconstructions of the 6 ka vegetation distribution in NorthAfrica show that steppe extended up to 23◦ N in the Sahara, compared to roughly 16◦ N today, and that lake levels were higher than they are today (e.g. Street-Perrott and Perrot, 1993; Jolly et al, 1998; Kohfeld and Harrison, 2000), Published by Copernicus Publications on behalf of the European Geosciences Union
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