Professor John C. H. Chiang

Department of Geography University of California, Berkeley

Cave speleothem d18O records over East Asia reveal apparently large and rapid paleoclimate changes over the last several hundred thousand years. However, there is debate as to what the isotopic variation actually means for regional climate change. We examine the dominant spatiotemporal mode of interannual variation in precipitation-weighted d18O of precipitation (d18Op) over East Asia, using a isotope-enabled model simulation whose circulation is constrained by atmospheric reanalyses, to gain insight into isotope processes and links to the large-scale circulation. The mode has a coherent in-phase variation in d18Op across central and southeastern China co-located with the locations of major East Asian speleothem records. Primary contributions to the isotopic changes occurs between June and October, where the d18Op transitions from the isotopically heavy winter regime to the light summer regime; for enriched years, this transition is less pronounced, leading to a reduction in summer seasonality. Changes to d18Op are also shown to be strongly tied to meridional displacement of the upper-level westerlies straddling the Tibetan Plateau. For enriched years, the northward migration of the westerlies in the summer is less pronounced, consistent with the interpretation of reduced summer seasonality. A recently proposed hypothesis (‘Jet Transition Hypothesis’) argues that East Asian climate changes arise from a modulation in its seasonality, mediated by the northward jet migration across the Plateau; our result here thus specifically ties this hypothesis to d18Op over East Asia.

Our analysis suggests the following climate interpretation for East Asian speleothem d18O. For enriched years, moisture flux into East Asia is isotopically heavier, there is less moisture flux, and the moisture does not penetrate as far polewards. All these responses are consistent with a reduced summer seasonality. They also align with previously proposed hypotheses for speleothem d18O: reduced monsoon intensity (as measured by the winter-to-summer rainfall ratio), reduced monsoonal low-level flow, and reduced depletion from convection upstream. We argue that the ultimate cause of the reduced seasonality is the delayed northward jet migration across the Tibetan Plateau.