A threshold in ice volume to reinforce the East Asian summer monsoon during a cool interglacial

Yin, Qiuzhen [UCL] Berger, André [UCL] Crucifix, Michel [UCL]

The loess in northern China, a sedimentary core in the eastern Tibetan Plateau and the palaeosols in southern China, all record an extremely strong East Asian Summer Monsoon (EASM) during the interglacial Marine Isotope Stage (MIS) 13 (about 500 000 years ago), actually the most intense one over the whole Quaternary. This was surprising because on the one hand, deep-sea and Antarctica ice cores show that this interglacial is significantly more glaciated (and cooler) than the most recent interglacials, and on the other hand, ice sheets are generally assumed to weaken the summer monsoon through their subsequent cooling of the continents (at least at the Last Glacial Maximum). To understand this seeming paradox of a strong EASM occurring during a cool interglacial, we have investigated the climate system response to prescribed GHG concentration, astronomical and ice sheet forcings. Primary climate modeling experiments show that this very strong MIS 13 EASM results not only from the astronomical forcing but also from the ice sheets themselves. As it was expected, Northern Hemisphere summer occurring at perihelion like at 506 ka BP leads indeed to an EASM stronger than during the Pre-Industrial time, but at the same time, the ice sheets reinforce also the EASM. Analyses convinced us that this reinforcement is through the propagation of a perturbation wave which is induced by the Eurasian ice sheet and is probably phase-locked by the Tibetan Plateau. This unexpected role played by the Eurasian ice sheet invites us to revisit our current understanding of monsoon dynamics and in particular its interaction with the ice sheets. A series of sensitivity experiments have been done related to the size, location, topography and albedo of the Eurasian and North American ice sheets. Among all the parameters, the total ice volume seems to be significant for the existence of a possible threshold in the response of the EASM.