Oceanic and Atmospheric Controls on Decadal Ice Shelf Basal Melt Variability around the Amundsen Sea

Abstract

Warm ocean waters are driving the rapid ice loss around the Amundsen Sea, Antarctica. Decadal variability in melt and on-shelf heat transport is thought to be controlled by zonal shelf break winds and Pacific sea surface temperatures, leading to the hypothesis that a strong El Ni ̃no in the 1940s initiated the present-day glacier retreat. This report investigates the controls on decadal melt variability by analysing simulations from a regional ocean model for the Amundsen Sea. This model was forced by an ensemble of 20 global climate model simulations for the years 1920 until 2014. Ice shelf basal mass loss has a strong connection with heat advection in seabed troughs in the simula- tions. Averaged over the ensemble, zonal shelf break winds and the El Ni ̃no Southern Oscillation (ENSO) are weakly correlated with basal mass loss. Individual members show a large spread, likely related to regional factors such as the Amundsen Sea Low and local factors such as sea ice and shelf water properties. Variability in baroclinic flow and salinity at the western shelf break suggest that melt could be sensitive to freshwater fluxes from ice shelves and sea ice. The occurrence of a large melt event in the 1940s depends on these factors. Finally, simulations with an advanced grounding line have minimal influence on decadal variability. Since ocean boundary conditions are fixed, the internal variability in the model is thus atmospherically forced. The results underscore the importance of using a model ensemble and long time scales when modelling this system. Furthermore, the limited influence of large scale atmospheric forcing on melt makes accurate future projections more difficult. Further research is necessary to disentangle internal feedbacks and forcings.