A one-dimension model of hydrogen and oxygen isotopic ratios in the global hydrologic cycle

A one-dimensional model of meridional water vapor transport is used to evaluate the factors that control the spatial and temporal variations of oxygen (δ18O) and hydrogen (δ18O) isotopic ratios in global precipitation. The model extends Rayleigh descriptions of isotopes in precipitation by including (1) effects of recharge to air masses by evaporation and (2) horizontal transport by both eddy-fluxes and advection. Globally, spatial variations in precipitation δ 18O and δD depend on the ratio of evaporation to the product of horizontal moisture flux and horizontal temperature gradient. At low-latitudes, where this ratio is large, precipitation δ18O and δD are closely tied to the isotopic ratios of oceanic evaporation. At high-latitudes, the ratio is small and δ18O and δD are controlled by the ratio of advective transport to eddy-transport. Transport by eddy-fluxes induces less fractionation than transport by advection, resulting in a smaller spatial gradient of isotopic ratios with temperature.; The model predicted temporal relationships between δ18O (or δD) of Antarctic precipitation and temperature do not necessarily coincide with the modern spatial relationship and depend strongly on the proximity of the precipitation site to the ocean evaporation source. Sensitivity of δ 18O to temporal changes in local surface temperature is low at coastal sites and increases with distance inland. These results suggest a possible explanation of the apparent discrepancy between borehole temperature inversion estimates of glacial temperatures, and temperatures inferred from the modern spatial δ18O-surface temperature relationship.; A theoretical treatment of evaporation is incorporated in the model. Improvements to models of isotopic ratios in the hydrologic cycle depend heavily on understanding this process because the isotopic composition of evaporation determines δ18O and δD in low latitude precipitation and sets the isotopic ratio of water vapor from which high latitude precipitation is distilled. Results from the model indicate the relationship between δ 18O and δD (deuterium excess) in Antarctic precipitation is not directly related to the conditions at the main source region, but is heavily influenced by the temperature and relative humidity conditions of the atmosphere over the Southern Ocean.