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 (AGCM; ECHAM4.6) is utilized to examine the impact of two aspects of mid-Holocene ENSO characteristics on South American precipitation and precipitation 18O: (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 18O. The experiments suggest that mid-Holocene western Amazon 18O is influenced by reduced SHSI via decreasing precipitation seasonality which drives positive 18O anomalies, while a La Niña-like mean state change suppress winter precipitation, leading to negative 18O anomalies. These opposing effects further highlight the additional role of global SST feedbacks in shaping western Amazon 18O. In contrast, in the northeastern Amazon, both SHSI and a more La Niña-like ENSO mean state directly influence precipitation 18O, resulting in strong negative 18O anomalies. Overall, these findings emphasize the combined effects of ENSO variability, tropical Pacific SSTs, and orbital forcing on South American precipitation and 18O during the mid-Holocene.