Influence of sea-ice anomalies on Antarctic precipitation using source attribution in the Community Earth System Model

Abstract

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 (SICs) and corresponding sea surface temperatures (SSTs) to understand the impact of sea-ice anomalies on regional evaporation, moisture transport and source–receptor relationships for Antarctic precipitation in the absence of anthropogenic forcing. Surface sensible heat fluxes, evaporation and column-integrated water vapor are larger over Southern Ocean (SO) areas with lower SICs. Changes in Antarctic precipitation and its source attribution with SICs have 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%), southern Indian Ocean (16 %) and South Atlantic Ocean (11 %). Comparing two experiments prescribed with high and low pre-industrial SICs, respectively, the annual mean Antarctic precipitation is about 150 Gt yr−1 (or 6 %) more in the lower SIC case than in the higher SIC case. This difference is larger than the model-simulated interannual variability in Antarctic precipitation (99 Gt yr−1). The contrast in contribution from the Southern Ocean, 102 Gt yr−1, is even more significant compared to the interannual variability of 35 Gt yr−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 along moist isentropes that are largely shaped by local ambient conditions and coastal topography. This study also highlights the importance of atmospheric dynamics in affecting the thermodynamic impact of sea-ice anomalies associated with natural variability on Antarctic precipitation. Our analyses of the seasonal contrast in changes of basin-scale evaporation, moisture flux and precipitation suggest that the impact of SIC anomalies on regional Antarctic precipitation depends on dynamic changes that arise from SIC–SST perturbations along with internal variability. The latter appears to have a more significant effect on the moisture transport in austral winter than in summer.

Future sea level contribution from Antarctica inferred from CMIP5 model forcing and its dependence on precipitation ansatz

“…All of this may indicate that the integrated precipitationdriven accumulation resulting from the RACMO precipitation reference field might be too large. However, the surface mass balance of RACMO agrees well with observational estimates (Wang et al, 2016), while the uncertainty of the surface mass balance (sea level equivalent of ∼ 0.25 mm yr −1 (Van Wessem et al, 2014)) is almost the same size as Antarctica's observation-based sea level contribution (∼ 0.2 mm yr −1 between 1992 and 2011; Shepherd et al, 2012;Wang et al, 2016). Additionally, recent satellite-based estimates clearly indicate that the Antarctic Ice Sheet has lost mass (sea level equivalent of 0.4 mm yr −1 ) in the period from 2011 to 2017 (Sasgen et al, 2019).…”

Future surface mass balance and surface melt in the Amundsen sector of the West Antarctic Ice Sheet

“…While Clausius-Clapeyron refers to the maximum saturation water vapour pressure, we suggest that decreasing coastal sea-ice cover makes surface air masses closer to their saturation level, as previously suggested by Gallée (1996) and Kittel et al (2018). This mechanism is also consistent with the modelling results of Wang et al (2020) who find that precipitation over the coastal Amundsen region mostly comes from evaporation occurring all the way from the Tropical Pacific to the Amundsen Sea. Another possible contributor to increased snowfall is the changing low-tropospheric circulation which shows a cyclonic anomaly in MAM favouring humidity transport towards the ice sheet (Fig.…”

Future ice-sheet surface mass balance and melting in the Amundsen region, West Antarctica

“…While Clausius-Clapeyron refers to the maximum saturation water vapour pressure, we suggest that decreasing coastal sea-ice cover makes surface air masses closer to their saturation level, as previously suggested by Gallée (1996) and Kittel et al (2018). This mechanism is also consistent with the modelling results of Wang et al (2020) who find that precipitation over the coastal Amundsen region mostly comes from evaporation occurring all the way from the Tropical Pacific to the Amundsen Sea. Another possible contributor to increased snowfall is the changing low-troposphere circulation, which shows a cyclonic anomaly in MAM, favouring humidity transport towards the ice sheet ( Fig.…”