The end of the Middle Pleistocene Transition (MPT, ~ 800-670 thousand years before present, ka) was characterised by the emergence of large glacial icesheets associated with anomalously warm North Atlantic sea surface temperatures enhancing moisture production. Still, the direction and intensity of moisture transport across Eurasia towards potential ice-sheets is poorly constrained. To reconstruct late MPT moisture production and dispersal, 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 South-West Iberia, with records of East Asia summer monsoon (EASM) strength and West Pacific surface temperatures, and model simulations. Here we show that south-western European winter precipitation and EASM strength reached high levels during the Marine Isotope Stage 18 glacial. This anomalous situation was caused by nearly-continuous moisture supply from both oceans and its transport to higher latitudes through the westerlies, likely fuelling the accelerated expansion of northern hemisphere ice-sheets during the late MPT. Therefore, the nearly continuous moisture production in the North Atlantic and West North Pacific subtropical sectors and its progressive northward transport by the westerlies during the end of the MPT may have substantially contributed to the strongest glaciation of the last millions of years in Eurasia and North America during MIS 16, leading to the strong dominant 100,000-year ice age cycles.

 Previous paleoceanographic studies have shown that prior to MIS 16 the North Atlantic deep-water formation, releasing heat and humidity to the atmosphere and controlling the global climate, was located further south and west (Boreal heat pump) compared to the period after MIS 16 (Nordic heat pump)33. This position allowed the arrival of a substantial amount of moisture to southern Europe during the interval MIS 19-17 but does not explain the wetter conditions during the MIS 18 glacial compared to the MIS 19 interglacial

Pleistocene sea-surface temperature evolution: Early cooling, delayed glacial intensification, and implications for the mid-Pleistocene climate transition.