A Unified Interpretation of Variability in Precipitation Isotope Ratios

Abstract

Several mechanisms have been proposed to explain why the isotope ratios of precipitation vary in space and time and why they correlate with other climate variables like temperature and precipitation. Here, we argue that this behavior is best understood through the lens of radiative transfer, which treats the depletion of atmospheric vapor transport by precipitation as analogous to the attenuation of light by absorption or scattering. Building on earlier work by Siler et al., we introduce a simple model that uses the equations of radiative transfer to approximate the two-dimensional pattern of the oxygen isotope composition of precipitation (δp) from monthly mean hydrologic variables. The model accurately simulates the spatial and seasonal variability in δp within a state-of-the-art climate model and permits a simple decomposition of δp variability into contributions from gradients in evaporation and the length scale of vapor transport. Outside the tropics, δp is mostly controlled by gradients in evaporation, whose dependence on temperature explains the positive correlation between δp and temperature (i.e., the temperature effect). At low latitudes, δp is mostly controlled by gradients in the transport length scale, whose inverse relationship with precipitation explains the negative correlation between δp and precipitation (i.e., the amount effect). This suggests that the temperature and amount effects are both mostly explained by the variability in upstream rainout, but they reflect distinct mechanisms governing rainout at different latitudes.