The deep sea sediments off Oman contain records of the history of the Southwest Monsoon, as seen from lithology, paleontology, isotopes, geochemistry and magnetostratigraphy. Glacial-interglacial fluctuations in upwelling related to the Southwest Monsoon can be traced back 250 ka. In contrast, from 450 to 250 ka, carbon isotope differences indicating upwelling strength did not show such distinct cyclicity. Upwelling was strong during interglacial stages and had less fluctuation, and was weak during glacial stages with large fluctuation. This upwelling and climate can be traced back to the late Miocene. The strongest upwelling is estimated to have occurred in the Pliocene-Pleistocene time, based on calcareous nannofossil assemblages, isotopic indicators, and organic carbon content. During glacial stages several factors suggest that the Arabian Peninsula was humid, including a very high calculated rate of sedimentation based on oxygen isotope stratigraphy, several to ten times that of interglacial stages, and an increase in the absolute flux of fluvial sediments and variability of lithofacies. The Arabian Peninsula was separated by high mountains along its southwest margin from east Africa, which was arid during glacial stages.