PNNL

Abstract

We conduct sensitivity experiments using a general circulation model that has an explicit water source tagging capability forced by prescribed composites of pre-industrial sea ice concentrations (SIC) and corresponding sea surface temperatures (SST) to understand the impact of sea ice anomalies on regional evaporation, moisture transport, and source–receptor relationships for precipitation over Antarctica. Surface sensible heat fluxes, evaporation, and column-integrated water vapor are larger over Southern Ocean areas with lower SIC, but changes in Antarctic precipitation and its source attribution with SICs reflect a strong spatial variability. Among the tagged source regions, the Southern Ocean (south of 50°S) contributes the most (40%) to the Antarctic total precipitation, followed by more northerly ocean basins, most notably the South Pacific Ocean (27%), South Indian Ocean (16%) and South Atlantic Ocean (11%). Comparing two experiments prescribed with high and low pre-industrial SIC, respectively, the annual mean Antarctic precipitation is about 150 Gt year-1 more in the lower SIC case than in the higher SIC case. This difference is larger than the model-simulated interannual variability of Antarctic precipitation (99 Gt year-1). The contrast in contribution from the Southern Ocean, 102 Gt year-1, is even more significant, compared to the interannual variability of 35 Gt year-1 in Antarctic precipitation that originates from the Southern Ocean. The horizontal transport pathways from individual vapor source regions to Antarctica are largely determined by large-scale atmospheric circulation patterns. Vapor from lower latitude source regions takes elevated pathways to Antarctica. In contrast, vapor from the Southern Ocean moves southward within the lower troposphere to the Antarctic continent, so the contribution of nearby sources also depends on regional coastal topography. The impact of sea ice anomalies on regional Antarctic precipitation also depends on atmospheric circulation changes that result from the prescribed composite SIC/SST perturbations. In particular, regional wind anomalies along with surface evaporation changes determine regional shifts in the zonal and meridional moisture fluxes that can explain some of the resultant precipitation changes. This study highlights the importance of better understanding changes in water transport toward Antarctica under natural variability.