Fingerprinting the Pacific Walker Circulation Using Precipitation δ18O

The Pacific Walker Circulation (PWC) influences climate far beyond the Pacific region, such that future changes in Earth's climate will be impacted by both natural and anthropogenically-driven changes in the PWC. But observational records of the PWC are short, and single-site proxy records for changes in the strength of the PWC during the last millennium offer contrasting interpretations. This has led to a critical gap in our understanding of the PWC's long-term natural variability and how it will respond to anthropogenic forcing.

Water isotopes should be ideal tracers of the PWC because they encapsulate information about circulation-dependent processes including moisture source, transport, and delivery. Given this, the new PAGES Iso2k database of hundreds of published palaeo-isotopic proxy records (Konecky et al., 2020) provides unprecedented potential to infer past changes in the PWC over the past millennium. However, to realise this potential we must first understand the spatio-temporal isotopic fingerprint of the PWC and its variability.

We collated publicly available measurements of modern precipitation δ ¹⁸ O (δ ¹⁸ O _P ), and investigated relationships between PWC strength and δ¹⁸O_P , precipitation amount, sea level pressure, and winds. We tested and employed multiple data processing and statistical techniques to maximise information from short, temporally and spatially discontinuous δ¹⁸O_P measurements. We find that the PWC is the leading mode of variability in global δ¹⁸O_P observations, with both strong and weak PWC anomalies associated with distinct spatial patterns in δ¹⁸O_P . Regional isotopic patterns reflect changes in atmospheric circulation rather than local temperature or precipitation amount, suggesting a direct role for the PWC in driving the isotopic anomalies. Using EOF-based techniques, we identify approximately five regions with a robust isotopic signal of the PWC and then consider probable mechanisms for these relationships. The identified regions are found to be ideal targets for palaeo-PWC reconstructions. We thus use water isotope proxy records to derive preliminary reconstructions of the PWC over the past millennium.