The Seasonality of Deuterium Excess in Non‐Polar Precipitation
Zhengyu Xia, J. M. Welker, Matthew Winnick
Abstract
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.