Space and time variation of δ¹⁸O andδD in precipitation: Can paleotemperature be estimated from ice cores?

A one‐dimensional model of meridional water vapor transport is used to evaluate the factors that control the spatial and temporal variations of oxygen (δ 18 O) and hydrogen (δD) 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 18 O 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 δ 18 O and δD are closely tied to the isotopic ratios of oceanic evaporation. At high latitudes the ratio is small, and δ 18 O 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 gradient of isotopic ratios with temperature. The model‐predicted temporal relationships between δ 18 O (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 18 O 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 δ 18 O—surface temperature relationship.