Quantifying the isotopic ‘continental effect’

Abstract

Since the establishment of the IAEA-WMO precipitation-monitoring network in 1961, it has been observed that isotope ratios in precipitation (δH2 and δO18) generally decrease from coastal to inland locations, an observation described as the ‘continental effect.’ While discussed frequently in the literature, there have been few attempts to quantify the variables controlling this effect despite the fact that isotopic gradients over continents can vary by orders of magnitude. In a number of studies, traditional Rayleigh fractionation has proven inadequate in describing the global variability of isotopic gradients due to its simplified treatment of moisture transport and its lack of moisture recycling processes. In this study, we use a one-dimensional idealized model of water vapor transport along a storm track to investigate the dominant variables controlling isotopic gradients in precipitation across terrestrial environments. We find that the sensitivity of these gradients to progressive rainout is controlled by a combination of the amount of evapotranspiration and the ratio of transport by advection to transport by eddy diffusion, with these variables becoming increasingly important with decreasing length scales of specific humidity. A comparison of modeled gradients with global precipitation isotope data indicates that these variables can account for the majority of variability in observed isotopic gradients between coastal and inland locations. Furthermore, the dependence of the ‘continental effect’ on moisture recycling allows for the quantification of evapotranspiration fluxes from measured isotopic gradients, with implications for both paleoclimate reconstructions and large-scale monitoring efforts in the context of global warming and a changing hydrologic cycle.