Reinforced precipitation in Eurasia as an important feedback mechanism contributing to the Middle Pleistocene Transition ice-sheet expansion

The late Middle Pleistocene Transition (MPT, ~ 800-670 thousand years before present, ka) was characterised by the emergence of large glacial ice-sheets associated with anomalously warm  mid North Atlantic sea surface temperatures (SST) enhancing moisture production. Still, the moisture transport across Eurasia towards high northern latitudes is poorly constrained despite its potential role as feedback mechanisms feeding the ice caps. To reconstruct late MPT moisture production and spreading, we combine records of upper ocean temperature and pollen-based Mediterranean forest cover, a tracer of westerlies and precipitation, from a subtropical drill-core collected off SW Iberia Margin, with records of East Asia summer monsoon (EASM) strength and West Pacific surface temperatures, and compare them with the iLOVECLIM model simulations. We observe that the strongest Mediterranean forest development occurred during Marine Isotope Stage (MIS) 17, centered at 700 ka, reflecting a high amount of regional winter precipitation. In contrast, MIS 19 (~785 ka), under the influence of both similar ice volume and higher atmospheric CO₂ concentration, is marked by limited forest expansion indicating lower winter precipitation in SW Europe compared to MIS 17. More interestingly, the MIS 18 glacial was more forested, reflecting stronger winter rainfall, compared to the preceding MIS 19, despite that the latter interglacial was characterised by higher insolation, sea level, atmospheric CO₂ concentrations and similar warm SST. The long-term increasing trend in winter precipitation in SW Europe parallels the trend of the EASM strength that reached high levels during MIS 18. The model results show high amount of winter rainfall in SW Europe and enhanced EASM (based on the modelled East Asian δ¹⁸O_calcite and summer precipitation) for the three MISs. Similar SW European tree fraction percentages are also modelled during MIS 18 and MIS 19, as inferred from the pollen data. In contrast to the proxy data, the simulated tree fraction is the weakest during MIS 17. The simulated winter rainfall is the highest during MIS 17, but the simulated EASM is the lowest during MIS 18. This mismatch between model and proxy reconstructions could be explained by the difficulty in quantitatively estimating the forest cover from pollen data and/or the result of a feedback process that is not well reproduced in the model such as the poor prediction of the intensity and position of the oceanic moisture source despite a robust SST simulation. Here the data show that SW European winter precipitation and EASM strength reached high levels during the MIS 18 glacial. We explained that this anomalous situation was caused by nearly-continuous moisture supply from both Pacific and Atlantic oceans and its transport to higher latitudes through the westerlies, likely fueling the accelerated expansion of northern hemisphere ice-sheets during the late MPT.