Seasonal and spatial variation of 17Oexcess and dexcess in Antarctic precipitation: insights from an intermediate complexity isotope model

Abstract:

The sensitivity of water isotope ratios in precipitation to climate variations in the Southern Hemisphere is investigated with an intermediate complexity isotope model (ICM) that includes updated equilibrium and kinetic fractionation factors for temperatures below zero. NCEP2 reanalysis data, with permutations to the surface temperature and humidity fields, are used as model boundary conditions. Decreases in ocean surface relative humidity result in increased ¹⁷O_excess and d_excess, with a uniform response over the ocean and Antarctic continent. The response of ¹⁷O_excess to a global temperature change is insignificant over the ocean, but there is a strong ¹⁷O_excess response over the ice sheet, particularly in East Antarctica. The simulated seasonal cycle in ¹⁷O_excess for East Antarctica is positively correlated with δ¹⁸O and of large magnitude (~50 per meg), in agreement with observations at Vostok. The seasonal cycle in ¹⁷O_excess for West Antarctica is predicted to be smaller in magnitude (~12 per meg) and of opposite sign. The ¹⁷O_excess seasonal cycle over the ocean is only ~3 per meg for the West Antarctic sector and ~8 per meg for the East Antarctic sector, neither of which are large enough to explain the full seasonal changes over the ice sheet produced by the ICM or observed in ice cores. The sensitivity of ¹⁷O_excess to local site temperature, and both the modern spatial distribution and seasonal variability of ¹⁷O_excess and d_excess over the ice sheet, reflect the proportion of equilibrium and kinetic fractionation during snow formation. To simulate the present-day ¹⁷O_excess observations requires that the sensitivity of supersaturation to temperature to be relatively high. Evaporative recharge is also found to be an important process that reduces the variability of ¹⁷O_excess and d_excess over the ocean. Evidence is provided that the simulation of kinetic fractionation during snow formation may require the inclusion of a term that accounts for turbulence.