The Seasonality of Deuterium Excess in Non-Polar Precipitation

Abstract

The deuterium excess (d-excess) of precipitation varies seasonally at sites across the globe, an observation that has often been linked to seasonal changes in oceanic evaporation conditions, continental moisture recycling, and subcloud raindrop re-evaporation. However, there have been very few studies to quantify and evaluate the relative importance of these processes. Here, we revisit the mechanisms of precipitation d-excess seasonality in low-latitudes and mid-latitudes through a new analysis of precipitation isotope databases along with climate reanalysis products and moisture tracking models. In low-latitudes, the raindrop re-evaporation effect, indicated by local relative humidity, exerts a strong and prevalent control on observed d-excess seasonality and overprints the effect of oceanic evaporation conditions. In mid-latitudes, the effect of oceanic evaporation conditions becomes stronger and seems dominant in the observed d-excess seasonality. However, the ultimate d-excess signals are produced after complex modulations by several reinforcing or competing processes, including prior distillations, moisture recycling, supersaturation in snow formation, and raindrop re-evaporation. Among these processes, substantial increases in the proportion of recycled moisture during the warm and dry season do not produce higher precipitation d-excess in mid-latitude continental interiors. We develop a simple seasonal water storage model to show that contributions of previously evaporated residual water storage and higher transpiration fractions may lead to relatively low d-excess in evapotranspiration fluxes during periods of enhanced continental moisture recycling. This study underscores the ubiquitous nonconservative behavior in d-excess throughout the water cycle, as opposed to using d-excess as a simple tracer for remote conditions at oceanic moisture sources.