Understanding Paleoclimatic Inference of Stable Water Isotopes using iTRACE Simulation

Stable water isotopes (δ¹⁸O) in precipitation are one of the most abundant paleoclimate proxies and have been used to infer temperature changes at high latitude and hydrological changes in the tropics. In spite of much progress, however, fundamental questions on the paleoclimate interpretation of stable water isotopes still remain open. Combing water isotope observations and an isotope-enabled TRAnsient ClimatE simulation of the last 21,000 years (iTREACE-21), I will discuss some recent progresses towards the understanding of paleoclimatic inferences of  δ¹⁸O.

I will first discuss the δ¹⁸O for the pan-Asian monsoon region. We show that the widespread δ¹⁸O variability that is coherent over the Asian monsoon continental region is accompanied by a coherent hydroclimate footprint, with spatially opposite signs in rainfall. This footprint is generated as a dynamically coherent response of the Asian monsoon system to meltwater forcing and insolation forcing, reinforced by atmospheric teleconnections. As such, a widespread δ¹⁸O depletion in the Asian monsoon region is accompanied by a northward migration of the westerly jet and enhanced southwesterly monsoon wind, as well as increased rainfall from South Asia to northern China, but decreased rainfall in southern China. 

I will then discuss the temperature effect of polar ice core δ¹⁸O, quantitatively, in a new framework called the Unified Slope Equations (USE) that illustrates the general relationship between spatial and temporal δ¹⁸O-temperature slopes. The application of USE to the Antarctica in model simulations and observations shows that the comparable Antarctica-mean spatial slope with deglacial temporal slope in δ¹⁸O-surface temperature is caused accidentally by the compensation responses between the δ¹⁸O-inversion layer temperature relation and the inversion layer temperature itself.  This finding further leads us to propose a paleothermometer that is more accurate and robust than the spatial slope as the present day seasonal slope of -inversion layer temperature, suggesting the possibility of reconstructing past polar temperature changes using present observations.

I will finally discuss the climate interpretation of tropical alpine ice core δ¹⁸O by combining proxy records with climate models, modern satellite measurements and radiative-convective equilibrium theory. I show that the tropical ice core δ¹⁸O is an indicator of the temperature of the middle and upper troposphere, with a glacial cooling of ~7oC . Furthermore, it severs as a Goldilocks indicator of global mean surface temperature change, providing the first estimate of glacial stage cooling that is independent of marine proxies as ~6oC .