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

This research 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 prescribed, observed sea surface temperatures (SSTs) and sea-ice extents, while in the perturbation simulations sea-ice is reduced. The simulations are restarts of an AMIP (Atmospheric Model Intercomparison Project) configured simulation, and two summers (1979/80 and 1984/85) have been selected for the study.

The results show that a reduced sea-ice extent causes an increase in surface 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°S and 50°S, which is associated with a strengthening and southward extension of the subtropical high pressure belt. The change in vertical velocities supports these results showing an intensification of the ascending limb of the Ferrell 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 middle/high latitudes (45°S-65°S), and a decrease in the westerly flow in the subtropics (30°S-40°S). The amplitude of wave number 1 decreases in the perturbations 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°S-70°S), and a decrease further north (40°S-60°S). The general response for the summer of 1985 is positioned slightly south of that in 1980, perhaps related to the less extensive sea-ice in 1985. The results show that the atmosphere is sensitive to relatively small ice changes.