ENSO and Indo-Pacific Water Isotopes: Observations, Modeling, and Implications for Proxy Reconstructions

Abstract:

Water isotope ratios (δ¹⁸O, δD) in precipitation, surface waters, and seawater comprise a majority of paleo-hydroclimate proxies in the tropical Indo-Pacific. The δD and δ¹⁸O of precipitation and water vapor are powerful tracers of atmospheric circulation processes, and can help to illuminate fundamental interactions between ENSO and the atmospheric water cycle. Throughout the tropics, past changes in δD and δ¹⁸O are often attributed to ENSO-driven changes in rainfall amount (via the tropical "amount effect") and related circulation processes. However, recent studies have revealed that ENSO’s impact on rainfall δD in many Indo-Pacific locations deviates substantially from the classic amount effect, with other fractionation and circulation processes becoming more important (e.g. Conroy et al., 2013). In order to accurately reconstruct past changes in Indo-Pacific hydroclimate using water isotopes, an improved understanding is required of the dynamical and microphysical processes controlling δ¹⁸O_precip and δD_precip, and their relationships with ENSO and other climate phenomena.

In this study, we examine the controls on δD_precip and δD_vapor in the Indo-Pacific and their relationships to ENSO in ground observations, data from the Troposheric Emission Spectrometer (TES) on NASA’s Aura satellite, and a new simulation with the isotope-equipped Community Atmosphere Model v. 5 (iCAM5). In both the model and in observations, we find that the dominant processes driving monthly to interannual δD_vapor anomalies differ substantially across the Indo-Pacific. Prominent differences are evident between the Indo-Pacific Warm Pool and the Central/Eastern Pacific (CEP), particularly in the relative importance of precipitation efficiency, detrainment, and rain evaporation on water isotope ratios. The dominant influence of the classic amount effect is spatially variable, with other processes playing important roles especially in the CEP. ENSO’s influence on δD_precip and δD_vapor reflects the different controls on fractionation in each region. These findings may challenge the interpretation of some proxy records simply in terms of precipitation amount, while also opening up the possibility for novel reconstructions of other key aspects of atmospheric circulation using water isotopes.