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An Intense Summer Monsoon in a Cool World, Climate and East Asian Monsoon during Interglacials with a special emphasis on the Interglacials 500,000 years ago and before

Final Report Summary - EMIS (An Intense Summer Monsoon in a Cool World, Climate and East Asian Monsoon during Interglacials with a special emphasis on the Interglacials 500,000 years ago and before)

Asian monsoon is a spectacular occurrence in the climate system. What make it so powerful are the combination of thermal contrast between the World’s largest landmass (Eurasian continent) and ocean basin (the Indo-Pacific Ocean) and the presence of the World’s largest ridge, the Tibetan Plateau. The economy, culture and rhythms of life of a large part of humanity are critically influenced by its evolution and variability. Moreover, as we are in an interglacial which is projected to be exceptionally long and intense, the need to better understand the climate and monsoon leads inevitably to the close inspection of their behaviour during the interglacials of the last million years to provide a long-term perspective from which any future change may be more effectively assessed.

Using climate models of different complexities, a number of sensitivity experiments to the astronomical forcing, the greenhouse gas concentrations (GHG), the Eurasian and North American ice sheets, the Tibetan Plateau and the Ocean helped to understand the seeming paradox of the exceptionally intense East Asian summer monsoon (EASM), actually the strongest over the last one million years, which occurred during the relatively cool interglacial MIS-13 (marine isotope stage 13), 500,000 years ago as revealed by the Chinese loess-soils sequences. At MIS-13, the strong insolation forcing imposed a strong land-ocean thermal contrast, resulting in a particularly strong EASM which could further be reinforced by a possible Eurasian ice sheet. The wave train induced by this ice sheet is phase-locked over East China by the Tibetan Plateau and the summer thermal Low over Asia. In addition, a warmer warm tropical Pacific pool was shown to enhance the summer precipitation in South China but to reduce it in North China. The temperature anomaly over the tropical Pacific also helped to reinforce EASM through atmospheric teleconnections. Finally, the long duration of MIS-13 seems to be critical for explaining its unusually strong EASM.

To infer its exceptional character, this monsoon of MIS-13 and its climate have been compared with the climate and monsoon which occurred during the other interglacials of the last million years. The relative importance of CO2 and of insolation differs from one interglacial to the other, in regions and in climatic variables. GHG plays a dominant role on the variations of the annual mean temperature of both the Globe and the southern high latitudes, whereas insolation plays a dominant role on the variations of tree fraction, of monsoon precipitation and of the northern high latitude temperature and sea ice. The finding that an insolation-induced Mid-Brunhes Event (MBE, around 430 ka BP) in the South Ocean ventilation and deep-sea temperature might be a first step towards understanding the magnitude change of the interglacial CO2 concentration around the MBE. It suggests that the MBE might have resulted from a series of individualistic interglacial responses to various combinations of insolation conditions rather than being necessarily a "real" event. In response to both insolation and GHG forcing, it was shown also that boreal winters and the Southern Hemisphere play important role in the warming of the post-MBE interglacials as compared to the pre-MBE ones, a result which should be assessed when investigating the underlying causes of the higher CO2 concentration during the post-MBE interglacials. All simulation results have been compared with the available proxy records, in particular-but not exclusively-those coming from the loess-soil sequences in China.

As far as analogues to the Holocene are concerned, the intercomparison between the nine interglacials shows that MIS-11 and MIS-19 are in very close agreement with MIS-1,but that the warmest interglacials MIS-5 and MIS-9 have to be considered with great care when compared to present-day climate and its future given their relatively larger astronomical and greenhouse forcing.