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.

Earth System Models’ complex land components simulate a patchwork of increases and decreases in surface water availability when driven by projected future climate changes. Yet, commonly-used simple theories for surface water availability, such as the Aridity Index (P/E0) and Palmer Drought Severity Index (PDSI), obtain severe, globally dominant drying when driven by those same climate changes, leading to disagreement among published studies. In this work, we use a common modeling framework to show that ESM simulated runoff-ratio and soil-moisture responses become much more consistent with the P/E0 and PDSI responses when several previously known factors that the latter do not account for are cut out of the simulations. This reconciles the disagreement and makes the full ESM responses more understandable. For ESM runoff ratio, the most important factor causing the more positive global response compared to P/E0 is the concentration of precipitation in time with greenhouse warming. For ESM soil moisture, the most important factor causing the more positive global response compared to PDSI is the effect of increasing carbon dioxide on plant physiology, which also drives most of the spatial variation in the runoff ratio enhancement. The effect of increasing vapor-pressure deficit on plant physiology is a key secondary factor for both. Future work will assess the utility of both the ESMs and the simple indices for understanding observed, historical trends.

The regional to global responses of the Standardized Precipitation-Evapotranspiration Index, Palmer Drought Severity Index, and Aridity Index to future global warming tend to be much more pervasively and strongly negative than the responses of comprehensive land model runoff and bulk soil-moisture outputs to the same warming. We term these systematic differences “index-impact gaps.” Some studies have assumed that these gaps arise because land-surface models include water-saving CO2-plant effects that the dryness indices do not, but recently published work makes clear that the gaps largely persist even in model simulations in which these effects are switched off. Thus, the main reason(s) for the index-impact gaps are still unclear, making it difficult to trust either the common dryness indices or the comprehensive land-surface models under climate change. In this study, we are investigating several postulated causes of these index-impact gaps using sensitivity experiments with the state-of-the-art Community Land Model version 5.0. In addition to CO2-plant effects, we are testing the roles of stomatal closure driven by high vapor-pressure deficits, short-term runoff enhancement due to sharper concentration of rain in time with warming, and annual-scale runoff enhancement due to changes in the seasonality of precipitation and/or infiltration with warming. If CLM5.0’s runoff and bulk soil-moisture responses start to agree with the dryness-index responses much more after eliminating these pathways, it will imply that the dryness indices are in fact a useful theoretical baseline for understanding the comprehensive model responses. However, if the index-impact gaps still remain wide, it will imply either that dryness-index responses are fundamentally different from runoff and soil-moisture responses to climate change, or else that CLM5.0’s evapotranspiration is not sensitive enough to rising temperatures. Further experiments will be required in that case.