Imprint of the Pacific Walker Circulation in Global Precipitation d¹⁸O

Characterizing variability in the global water cycle is fundamental to predicting impacts of future climate change; understanding the role of the Pacific Walker circulation (PWC) in the regional expression of global water cycle changes is critical to understanding this variability. Water isotopes are ideal tracers of the role of the PWC in global water cycling because they retain information about circulation-dependent processes including moisture source, transport, and delivery. We collated publicly available measurements of precipitation δ¹⁸O (δ¹⁸O_P) and used novel data processing techniques to synthesize long (34 yr), globally distributed composite records from temporally discontinuous δ¹⁸O_P measurements. We investigated relationships between global-scale δ¹⁸O_P variability and PWC strength, as well as other possible drivers of global δ¹⁸O_P variability—including El Niño–Southern Oscillation (ENSO) and global mean temperature—and used isotope-enabled climate model simulations to assess potential biases arising from uneven geographical distribution of the observations or our data processing methodology. Covariability underlying the δ¹⁸O_P composites is more strongly correlated with the PWC (r 5 0.74) than any other index of climate variability tested. We propose that the PWC imprint in global δ¹⁸O_P arises from multiple complementary processes, including PWC-related changes in moisture source and transport length, and a PWC- or ENSO-driven ‘‘amount effect’’ in tropical regions. The clear PWC imprint in global δ¹⁸O_P implies a strong PWC influence on the regional expression of global water cycle variability on interannual to decadal time scales, and hence that uncertainty in the future state of the PWC translates to uncertainties in future changes in the global water cycle. SIGNIFICANCE STATEMENT: Anthropogenically driven climate change has repercussions beyond global warming. One of the most impactful changes is to the global water cycle, but future changes to regional precipitation patterns on land are not well constrained. The Pacific Walker circulation (PWC) affects weather and climate far beyond the Pacific Ocean, and thus likely plays a role in global-scale precipitation patterns. But traditional approaches for assessing the PWC’s role in the global water cycle do not capture the full range of circulation-dependent processes including variability in moisture source, transport, and delivery. Here, we used water isotopes as a novel water cycle tracer, and found a strong PWC imprint in global water isotope patterns.