The relative impacts of tropical Pacific teleconnections and local insolation on mid-Holocene precipitation over tropical South America

Abstract. The study of El Niño–Southern Oscillation (ENSO)’s role in modifying past climates can provide valuable insights into the sensitivity of the present-day hydroclimate to changing ENSO characteristics. In this study, a water isotope-enabled atmospheric general circulation model (ECHAM4.6) is utilized to examine the impact of two aspects of mid-Holocene ENSO characteristics on South American precipitation and precipitation δ¹⁸O: 1) Reduced amplitude of ENSO variability (relative to present-day) and 2) a mid-Holocene La Niña-like mean state in tropical Pacific sea-surface temperatures (SST), but with modern-day ENSO variability. Additionally, we conducted a lower Southern Hemisphere summertime insolation (SHSI) experiment to investigate the role of orbital forcing. Our results show that decreased ENSO variability results in a minor change in the climatological precipitation. In contrast, a La Niña-like mean state results in an east–west dipole pattern of rainfall change comparable to the effect of lower SHSI; however, the dipole pattern is not mirrored in precipitation δ¹⁸O. The experiments suggest that mid-Holocene western Amazon δ¹⁸O is influenced by reduced SHSI by decreasing precipitation seasonality and drives positive δ¹⁸O anomalies, while ENSO mean state changes suppress winter precipitation, leading to negative δ¹⁸O anomalies. These opposing effects highlight the additional role of global SST feedbacks in shaping western Amazon δ¹⁸O. In contrast, in the northeastern Amazon, both SHSI and ENSO mean state changes directly influence precipitation δ¹⁸O, resulting in strong negative δ¹⁸O anomalies. Overall, these findings emphasize the combined effects of ENSO variability, tropical Pacific SSTs, and orbital forcing on South American precipitation and δ¹⁸O during the mid-Holocene.