Speleothem oxygen isotope records (δ18O) of tropical South American rainfall in the late Quaternary show a zonal “South American Precipitation Dipole” (SAPD). The dipole is characterized by opposing east-west precipitation anomalies compared to the present—wetter in the east and drier in the west at the mid-Holocene (∼7 ka), and drier in the east and wetter in the west at the Last Glacial Maximum (LGM; ∼21 ka). However, the SAPD remains enigmatic because it is expressed differently in western versus eastern δ18O records and isotope-enabled climate model simulations usually misrepresent the magnitude and/or spatial pattern of δ18O change. Here, we address the SAPD enigma in two parts. First, we re-interpret the δ18O data to account for upwind rainout effects that are known to be pervasive in tropical South America, but are not always considered in Quaternary paleoclimate studies. Our revised interpretation reconciles the δ18O data with cave infiltration and other proxy records, and indicates that the centroid of tropical South American rainfall has migrated zonally over time. Second, using an energy balance model of tropical atmospheric circulation, we hypothesize that zonal migration of the precipitation centroid can be explained by regional energy budget shifts, such as changing Saharan albedo associated with the African Humid Period, that have not been modeled in previous SAPD studies. This hypothesis of a migrating precipitation centroid presents a new framework for interpreting δ18O records from tropical South America and may help explain the zonal rainfall anomalies that predate the late Quaternary.

In the tropics, oxygen isotope signals of past climate change range from less than 1‰ to upwards of 7‰ or more. Regardless of the amplitude, these signals are often interpreted to reflect changes in local rainout. However, stable isotopes in precipitation can carry information about rainout across thousands of kilometers, making it hard to parse the local and non-local effects. Here, we present a framework that links the amplitude of tropical isotope signals to spatial patterns of climate change that cause them. Using three models of varying complexity, we show that the largest signals require coherent hydrologic change across ~1,000 to 8,000 kilometers. This pattern can be explained by the balance of vapor being rained out versus replenished as it moves over space. Within ~1,000 kilometers, upwind changes in rainout are too localized for a large isotope shift to emerge. Beyond ~8,000 kilometers, the rainout signal is overwhelmed by more locally-sourced vapor. We find that rainout in this ~1,000-8,000 km upwind window causes the largest isotope shifts in tropical paleoclimate, even when the isotope composition is strongly correlated with local precipitation amount. Our results indicate that large amplitude isotope signals are reliable tracers of large scale hydrologic change, and their link to local precipitation amount is tenuous.