Infrared satellite images indicate that a polyna, surrounded by a zone of loose pack ice, persisted in Terra Nova Bay, Antarctica through the winter of 1979; the feature may recur each year. The affected region (polynaya and loose pack) occupied 25000 km2 of the western Ross Sea. Throughout the winter the area of open water fluctuated quasi-periodically with a period of 15-20 days. Average polyna area was 1000 km2; maximum area was approximately 5000 km2. Fluctuations were associated with the magnitude of the zonal geostrophic wind, with a closing polynya being related to strong, persistent easterlies. An open, or opening, polynya was linked with persistent westerly or weakk easterly winds and the probable descent of adiabatically warming, drift-bearing air from the plateau. This air enters Terra Nova Bay through the Reeves Glacier valley, probably as katabatic surface winds. Polynya formation was probably due to the action of these winds on the coastal pack ice, while areal fluctuations reflected the interplay between the katabatic winds and synnoptic scale motions advecting ice toward and away from the bay.