Indo-Pacific Walker circulation drove Pleistocene African aridification

A record of flow-speed variations from an ocean core shows that a key component of Indian Ocean circulation rapidly accelerated 2.1 million years ago, coincident with intensification of Pacific Walker circulation.Today, the eastern African hydroclimate is tightly linked to fluctuations in the zonal atmospheric Walker circulation(1,2). A growing body of evidence indicates that this circulation shaped hydroclimatic conditions in the Indian Ocean region also on much longer, glacial-interglacial timescales(3-5), following the development of Pacific Walker circulation around 2.2-2.0 million years ago (Ma)(6,7). However, continuous long-term records to determine the timing and mechanisms of Pacific-influenced climate transitions in the Indian Ocean have been unavailable. Here we present a seven-million-year-long record of wind-driven circulation of the tropical Indian Ocean, as recorded in Mozambique Channel Throughflow (MCT) flow-speed variations. We show that the MCT flow speed was relatively weak and steady until 2.1 +/- 0.1 Ma, when it began to increase, coincident with the intensification of the Pacific Walker circulation(6,7). Strong increases during glacial periods, which reached maxima after the Mid-Pleistocene Transition (0.9-0.64 Ma; ref. (8)), were punctuated by weak flow speeds during interglacial periods. We provide a mechanism explaining that increasing MCT flow speeds reflect synchronous development of the Indo-Pacific Walker cells that promote aridification in Africa. Our results suggest that after about 2.1 Ma, the increasing aridification is punctuated by pronounced humid interglacial periods. This record will facilitate testing of hypotheses of climate-environmental drivers for hominin evolution and dispersal.