Water Isotope Tracers of Indo-Pacific Atmospheric Circulation: A Modern Take on Past Dynamics

Abstract:

Stable oxygen and hydrogen isotope ratios (δ¹⁸O, δD) in precipitation, terrestrial water bodies, groundwater, and surface seawater are powerful integrators of the atmospheric water cycle. As such, proxy archives of δ¹⁸O and δD form the basis for much of our understanding of past changes in Indo-Pacific climate. Water isotope studies of the past millennium suggest that both internal variability and forced changes in global temperature drove decadal to centennial changes in monsoons, the Intertropical Convergence Zone, ENSO, and other modes of variability. However, recent observations as well as proxy data have shown that δ¹⁸O and δD signatures are far more complex than previously believed. Testing hypotheses about the drivers of past Indo-Pacific hydroclimate therefore requires an improved understanding of modern-day isotope ratios.

In this study, we present new analyses of Indo-Pacific climate/isotope relationships from satellite and in situ observations, as well as new simulations with water isotope-enabled components of the Community Earth System Model. We evaluate the mechanisms that reinforce or weaken the tropical amount effect, which is often invoked in interpreting paleo-isotope data as hydroclimate proxies. We find that the amount effect is highly variable through space and time. Generally, it is strongest at sites with large-amplitude variations in the seasonal cycle. Circulation and moisture convergence play key roles in determining the strength of the amount effect, although cloud processes such as Rayleigh distillation and rain evaporation are still important, especially in determining initial isotope ratios of transported moisture. The relative influence of these mechanisms on vapor δ¹⁸O and δD varies in different parts of the tropics, affecting how regional archives record ENSO and other circulation patterns. We discuss these differences, and their implications for reconstructing Indo-Pacific atmospheric variability on decadal and longer timescales.