a1 Alfred-Wegener-Institut für Polar- und Meeresforschung, 2850 Bremerhaven, FRG
a2 Department of Atmospheric Sciences, AK-40, University of Washington, Seattle, Wash. 98195 USA
a3 Scott Polar Research Institute, University of Cambridge, Lensfield Road, Cambridge CB2 1ER UK
During a late winter and early spring oceanographic voyage south into the Weddell Sea the icebreaker RV Polarstern first encountered patches and bands of loose floes in 58°S; these increased over the next 150 km to form closed ice pack which extended 1000 km to the coast. Along the coast the ship encountered almost continuous shore leads and polynyas that formed repeatedly despite persistently low air and sea temperatures. These areas of open water, which are generally visible in USA NOAA and USSR METEOR satellite photographs, form under the action of strong offshore winds that carry the main body of pack ice west and southwest. Grease ice, pancake ice and nilas spreading over the open water are rafted and ridged by windgenerated stresses to double thickness or more; these kinds of ice were continually driven westward, accumulating in a distinctive zone along the eastern edge of the pack ice. Polynyas and leads narrow and disappear temporarily only when winds with northerly or westerly components bring the pack ice toward the land, and reform as soon as offshore winds predominate. Open water, often more than 15 km wide, was present close to the ship throught the spring voyage, facilitating oceanographic work as far south as 77°S. Polarstern's full icebreaking capacity was needed only occasionally when winds temporarily pressed the pack ice against the coast. The presence throughout early spring of both fast and pack ice, separated by a zone of thin ice or open water, is essential to large populations of Weddell seals, emperor penguins and whales in the area. The transect from the continent included ice pack that was undergoing early summer decay, characterized by differential expansion and melting which brought about a gradual decrease in concentration toward the ice edge.
(Received July 1987)