Antarctic sea ice sensitivity to the orographic gravity wave drag in a fully coupled climate model 

Low-level winds over Antarctica are overwhelmingly controlled by the local orography. They, in turn, exert a large control on sea ice formation and transport. In Global Circulation Models, the influence of orography on the climate system is modelled via orographic gravity wave drag (OGWD) parameterizations. Models usually partition the drag exerted on the atmosphere by the sub-grid scale orography into two components due to flow blocking and gravity waves. In this work, we investigate the relationship between Antarctic sea ice and the parameterized OGWD in the UK Earth System Model (UKESM). We present results from sensitivity tests performed using the UKESM-CMIP6 historical runs.In these simulations, the partition between the “flow-blocking” component and the “gravity wave” component of the OGWD parameterization was altered to simulate “flow-over” and “flow-blocking” regimes. These experiments show that sea ice strongly responds to changes in the orographic gravity wave drag. The strong sea ice decline simulated by the control run from 1980 to 2015, not matched by the observational record, is halted and is delayed by 15-20 years (across the ensemble members) in our flow-blocking regime simulation. Conversely, in the flow-over regime simulation, sea ice begins declining about 10 years earlier than in the control run. The systematic response of the coupled system suggests the existence of a dynamical relationship between sea ice and OGWD. The pan-Antarctic signal for sea ice decline derives from the Weddell Sea sector. The pathway through which OGWD influences sea ice is via modifications of the flow regime across the Antarctic Peninsula, and thus the surface wind stress across the Weddell Sea sector, which in turn alters the occurrence of oceanic deep convection. This happens because the flow regime across the Antarctic Peninsula is critical in determining the strength and pattern of the surface winds on both the windward side (Bellingshausen and Amundsen Seas sector) and the lee side (Weddell Sea sector) of the mountain ridge.