Southern Hemisphere sea ice response to different flow regimes over the Antarctic Peninsula

Abstract Low-level winds over Antarctica are overwhelmingly controlled by the local orography. They, in turn, exert a large control over sea ice formation and transport. In this work, we investigate the relationship between Antarctic sea ice and the parameterized orographic gravity wave drag in the UK CMIP6 (Coupled Model Intercomparison Project Phase 6) climate model, HadGEM3-GC3.1-LL. We present results from sensitivity tests in which the partition between the "flow blocking'' and "gravity wave'' components of the total parameterized drag was altered to simulate different flow regimes. We show that sea ice strongly responds to changes in orographic drag. In the baseline historical simulations (1980-2015) a strong sea ice decline in the Weddell Sea is caused by the onset of periodic open water polynyas. The onset is delayed by 10-20 years in our flow blocking regime simulations, whilst in the flow over regime simulation, sea ice begins declining about 10 years earlier. The path through which orographic drag influences sea ice is via modifications of the flow regime over the Antarctic Peninsula, and thus the surface wind stress over the Weddell Sea, which in turn alters the occurrence of open ocean deep convection.