Pacific Interannual and Multidecadal Variability Recorded in δ18O of South American Summer Monsoon Precipitation

The South American summer monsoon (SASM) generates important hydroclimatic impacts in (sub‐)tropical South America and isotopic tracers recorded in paleoclimatic archives allow for assessing its long‐term response to Pacific variability prior to modern observations. Stable oxygen isotopes in precipitation integrate hydroclimatic changes during the SASM mature phase from December to February (DJF) in response to the Interdecadal Pacific Oscillation (IPO) and El Niño—Southern Oscillation (ENSO), respectively. Here, results from the isotope‐enabled Community Atmosphere Model v.5 are compared with highly resolved and precisely dated isotopic records from speleothems, tree rings, lake and ice cores during the industrial era (1880–2000 CE) and validated against observations from the International Atomic Energy Agency (IAEA) network. Pacific sea surface temperatures (SSTs) are coupled to the isotopic composition of SASM precipitation through perturbations in the Walker circulation associated with low‐ (IPO) and high‐frequency (ENSO) variability, impacting convective activity over tropical South America and the tropical Atlantic. Changes in convection over this monsoon entrance region ultimately control the downstream oxygen isotopic composition of precipitation recorded in paleoclimate archives. Overall, model results, paleoclimate records and IAEA data agree on the isotopic response to Pacific SST forcing. These results highlight the potential for long isotopic paleoclimate records to reconstruct Pacific climate variability on both high‐ and low‐frequency timescales. Furthermore, the isolation of the IPO signal in a diverse set of isotopic archives invites the reinterpretation of other paleoclimate proxies for identifying this historically overlooked forcing. Plain Language Summary The summertime rainfall associated with the South American monsoon is important for the hydropower and agricultural sectors in South America as well as to the traditions of many Amazonian indigenous cultures. The amount and spatial extent of the monsoon rainfall patterns are influenced by a variety of factors, including changes in Pacific Ocean sea surface temperatures. While evaluating this link has been mostly limited to meteorological observations, the analysis of heavy and light oxygen atoms, called isotopes, in natural archives such as cave deposits, lake sediments, glacier ice, and tree rings, can help extend this relationship further back in time. In combination with climate mo