Lagrangian modelling of precipitation and speleothem proxy oxygen isotope systematics in the East Asian Summer Monsoon region

Abstract

The Asian Summer Monsoon generates intense seasonal precipitation across India, China and Indochina, comprising Earth’s largest monsoonal climate regime, and this vital component of the global energy and water cycles directly impacts the world’s most populous regions. Accurate palaeomonsoon reconstructions are required to investigate natural climate variability beyond the coverage of instrumental records and inform predictions of future monsoon trends. Stable oxygen isotope ratios (δ18O) are an important proxy for hydroclimate variability and stalagmite δ18O is widely used to investigate East Asian palaeoclimate, typically interpreted as a semi-quantitative measure of precipitation amount. However, recent studies suggest δ18O instead reflects multiple hydroclimatic processes, warranting a detailed understanding of precipitation and proxy δ18O systematics. This thesis (i) presents a quantitative Lagrangian study of atmospheric moisture transport and precipitation across central and eastern China, a continental region affected by the East Asian Summer Monsoon (EASM); (ii) investigates the hydroclimatic drivers of precipitation δ18O variability at Wanxiang Cave, an important site for palaeomonsoon reconstruction located near to the northerly EASM limit; (iii) evaluates the extent to which seasonal- to decadal-scale proxy δ18O variability reflects terrestrial moisture fluxes by constructing a pseudoproxy record for Wanxiang.

Present-day precipitation across monsoonal China is primarily derived from the northern Indian Ocean and recycled intensely over the East Asian continent; Pacific Ocean moisture export peaks during winter. A geographically variable isotopic source effect is apparent in present-day mean monthly precipitation δ18O data. Wanxiang Cave precipitation δ18O variability during AD 2001-2002 is successfully reproduced by a recently-developed Lagrangian model which simulates air masses’ isotopic evolution along three-dimensional atmospheric trajectories, though winter values are ~2 ‰ over-depleted. Contributions of Tibetan Plateau- and free troposphere-derived moisture are identified as synoptic-scale isotopic depletion processes. Overall, land-derived moisture causes isotopic enrichment during summer, thus moderating depletion due to strengthened EASM circulation or increased precipitation amount. As such, periods of elevated summer surface temperatures may attenuate the seasonal EASM signal in precipitation and proxy δ18O. An idealised Rayleigh-type isotope model is unable to capture these effects accurately, emphasising the importance of regional moisture fluxes. A pseudoproxy record, driven by terrestrial moisture fluxes, replicates seasonal- to decadal-scale stalagmite δ18O variability at Wanxiang. The work presented in this thesis offers new insights into the influence of atmospheric moisture transport dynamics on precipitation and proxy δ18O variability across central and eastern China. This constitutes an important advancement in our ability to use δ18O to reconstruct past climate variability quantitatively in the EASM region.