What caused the lag between oxygen-18 and deuterium excess in atmospheric vapor and precipitation during the earlier summer season in southwest China?

Abstract

A combined interpretation of oxygen-18 (δ18O) and deuterium excess (d-excess) in various archives in the Indian monsoon-dominated region may add value to the isotopic climate significance. Yet, coupling water δ18O and d-excess together is rarely discussed. Here, we present measurements for both δ18O and δ2H in water vapor and precipitation samples collected in the Nujiang River valley, an important moisture corridor for the Indian monsoon in southwestern China, aiming for an improved understanding of the hydrological cycle on seasonal scales. The observation found a significant drop occurred in the earlier summer season for δ18O and d-excess in vapor and precipitation. However, an approximately one-month lag between vapor δ18O and vapor d-excess is observed in seasonal variations, revealing different drivers for the seasonal patterns of vapor d-excess and δ18O. Spatial correlation analysis found that d-excess is mainly modulated by the relative humidity over the core moisture source over the Bay of Bengal (BOB), while the sharp decrease of vapor δ18O corresponds to the onset of the Indian summer monsoon associated with large-scale convective activities, which significantly depleted the vapor heavy isotopes. The finding of the phase difference in the seasonal isotope signals and the underlying mechanism will benefit the study in particularly using both δ18O and d-excess for an improved understanding of the hydrological processes in the Indian monsoon-dominated region, and will provide new insight into studies of paleoclimate rebuilding by using isotope-based proxy as well.