A decrease in the ratio of 18O to 16O (delta18O) of sedimentary carbonate from the Bolivian Altiplano has been interpreted to indicate rapid surface uplift of the late Miocene Andean plateau (AP). Here we report on paleoclimate simulations of Andean surface uplift with an atmospheric general circulation model (GCM) that tracks oxygen isotopes in vapor. The GCM predicts changes in atmospheric circulation and rainfall that influence AP isotopic source and amount effects. On eastern AP slopes, summer convective precipitation increases by up to 6 millimeters per day (>500%) for plateau elevations that are greater than about 2000 meters. High precipitation rates enhance the isotope amount effect, leading to a decrease in precipitation delta18O at high elevations and an increase in delta18O lapse rate. Our results indicate that late Miocene delta18O depletion reflects initiation and intensification of convective rainfall.