The atmospheric response to a reduction in summer Antarctic sea-ice extent

This paper examines the response of an atmospheric general circulation model (GCM) to a reduction in Antarctic sea-ice extent during summer. The control simulations are forced by pre- scribed, observed sea surface temperatures (SSTs) and sea-ice extents, while in the perturbation sim- ulations sea-ice is reduced. The simulations are restarts of an AMIP (Atmospheric Model Intercom- parison Project) configured simulation, and 2 summers (1979/80 and 1984/85) were selected for the study. The results show that a reduced sea-ice extent causes an increase in surface air temperatures in the regions where sea-ice was removed, and an associated decrease in pressure at high latitudes (around 60° S). The greatest increase in surface air temperatures are found north of the Weddell and Ross Seas. There is an increase in pressure between 30 and 50° S, which is associated with a strength- ening and southward extension of the subtropical high pressure belt. The change in vertical veloci- ties supports these results showing an intensification of the ascending limb of the Ferrel cell and a southward extension of the descending limb of the Hadley cell. In response to the perturbation there is an increase in wind speeds in mid/high latitudes (45 to 65° S), and a decrease in the westerly flow in the subtropics (30 to 40° S). The amplitude of circumpolar wave number 1 decreases in the pertur- bations compared to the controls in both years. This may be a result of reduced asymmetry of the SST distribution about the pole and the southward shift of the subtropical high-pressure belt. A cyclone analysis shows an increase in the number of midlatitude cyclones around Antarctica (60 to 70° S) and a decrease further north (40 to 60° S). The general pattern of changed circulation for the summer of 1984/85 is positioned slightly south of that in 1979/80, perhaps related to the less extensive sea-ice in 1984/85.