Session: Warm Climates (Mid-Holocene, Last interglacial, Deep-time, Pliocene)
Author: Kenji Izumi / Kenji.Izumi@lsce.ipsl.fr / LSCE/IPSL
Co-author: Sandrine Bony, LMD/IPSL;
Masa Kageyama, LSCE/IPSL;
Pascale Braconnot, LSCE/IPSL;
Paleoenvironmental data, in particular vegetation and lake-status in Sahara shows that at mid-Holocene (6,000 years ago) African monsoon extended much further north than today. Much of this change results from the changes in insolation driven by precession of the Earth’s orbit, but in the state-of-the-art climate models, this factor alone is insufficient to explain the magnitude of the change. Previous studies showed that ocean and vegetation feedbacks affect the mid-Holocene monsoon and that the incorporation of these feedbacks in models improves the simulation of the hydrological cycle. However, it is not sufficient to reduce the discrepancies between simulated and reconstructed surface climates.
In this study, we investigate the impacts of atmospheric cloud radiative effects (ACRE) on tropical rain belts during the mid-Holocene. This is done by running a general circulation model with and without cloud-radiation interactions using the IPSL model. The ACRE impacts include (1) a small northward shift of the tropical rain belts, (2) a decrease in tropical precipitation, (3) a narrowing and a strengthening of the ascending motions of the tropical overturning circulation, and (4) an intensification of the African easterly wave activity, but a contraction of tropical rain belts and decrease in precipitation over West Africa. Although the last impact in the mid-Holocene simulation is much larger than one in the control simulation, it is not enough to represent observed hydrological cycle over West Africa.