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

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 (AGCM; ECHAM4.6) is utilized to examine the impact of two aspects of mid-Holocene ENSO characteristics on South American precipitation and precipitation δ 18 O: (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 precipitation change comparable to the effect of lower SHSI; however, the dipole pattern is not mirrored in precipitation δ 18 O. The experiments suggest that mid-Holocene western Amazon δ 18 O is influenced by reduced SHSI via decreasing precipitation seasonality which drives positive δ 18 O anomalies, while a La Niña-like mean state change suppress winter precipitation, leading to negative δ 18 O anomalies. These opposing effects further highlight the additional role of global SST feedbacks in shaping western Amazon δ 18 O. In contrast, in the northeastern Amazon, both SHSI and a more La Niña-like ENSO mean state directly influence precipitation δ 18 O, resulting in strong negative δ 18 O anomalies. Overall, these findings emphasize the combined effects of ENSO variability, tropical Pacific SSTs, and orbital forcing on South American precipitation and δ 18 O during the mid-Holocene.