The influence of midlatitude and tropical overturning circulation on the isotopic composition of atmospheric water vapor and Antarctic precipitation

摘要

While isotope records from ice cores are known to reflect temperature, this must be associated with zonally symmetric circulation. A new conceptual depiction of the isotopic cycling is established by considering the overturning circulation in isentropic coordinates. In this depiction, poleward transport of air and water vapor is non-diffusive, in a way that is similar to that depicted by Rayleigh models. However, the equatorward return flow is also important since it is this which is supplied with water by surface evaporation. The isotopic state emerges as a balance between evaporative supply and poleward advection, and removes the need to assume some initial source condition for an open distillation. Model experiments that simulate a wide range of circulation strengths show the isotopic composition of Antarctic snow is strongly linked to the strength of midlatitude (eddy driven) circulation, which in turn is driven by meridional temperature differences. Antarctic isotopes are largely independent of the tropical (Hadley) circulation because the rate of advective transport from the tropics to the polar region exceeds the rate at which surface sources replenish the poleward moving air stream. Across all simulations and between seasons, the relationship between δ 18O in Antarctica above 1500 m and local surface air temperature is found to be remarkably robust at around 0.69‰/K in winter, 0.85‰/K in summer and with a seasonal slope of 0.60‰/K. Because these slopes result from changes in circulation, isotope records from the continent interior can be considered indicative of the history of overturning circulation.