An extremely strong East Asian summer monsoon during MIS-13 in response to the astronomical and ice sheets forcings

Deep-sea and ice cores show significantly reduced amplitude before about 430 ka ago, with less warm (cooler) interglacials and generally less cold glacials, and with Marine Isotope Stage 13 (about 500 ka ago) being the most glaciated interglacial of the last one million years. But at the same time, different proxy records from China show that MIS-13 was quite warm and humid, with the strongest East Asian summer monsoon (EASM) over the whole Quaternary. In parallel, unusually high rainfall events have been found over other monsoon regions, like North Africa and India. This is very surprising because usually strong monsoon is expected to occur during warm climate with high greenhouse gases concentration. To understand the possible causes of an extremely strong EASM during MIS-13, in particular the impacts of the astronomical forcing and of the reconstructed ice sheets, different sensitivity experiments have been made using LOVECLIM, an Earth system model of intermediate complexity. First, the Greenland and Antarctica ice sheets are assumed to keep their present-day volume and the Eurasian and North American ice sheets are assumed to exist with different ice volumes ranging from the largest ones (amounting respectively 11.9 and 24.2 106 km3) up to 0. Equilibrium experiments show that the astronomical forcing is the most important (Yin et al., 2008, 2009), but also that the Eurasian ice sheet has a tendency to reinforce the EASM whatever their sizes when northern hemisphere (NH) summer occurs at perihelion. When NHS is at aphelion, there is a threshold in the ice volume beyond which the ice sheets start to reduce the precipitation over East China (Yin et al., 2009). This underlies the importance of insolation in shaping the ice sheet impact on the precipitation over the EASM region. This might be interesting for further studies of glacials. Second, some proxy records (Guo et al., 2009) show that there is a high probability that actually the ice volume signal of MIS-13 is mainly due to the southern hemisphere (SH) ice sheets. We therefore analyze the response of the climate system to boundary conditions where the Greenland ice sheet is smaller (even disappeared) and the Antarctica ice sheet is larger (up to its LGM size) than today. In this case, precipitation over East China increases all the year around but in summer remains not exceptional. The EASM during MIS-13 becomes as exceptional as seen in proxy records when either ice sheets excess in NH or in SH as soon as the length of MIS-13 is taken into account. It is remains however that a choice must be done between the two cases of ice sheets excess in NH and in SH