An analysis of multi-year ice distributions occurring in the Saglek Bank region of the Labrador Sea during 1979

Air photographs taken during six overflights of the Labrador marginal ice zone over Saglek Bank, during the 1979 ice season, have been visually examined to isolate the occurrence of multi-year floes. Along the flight lines, with extended seaward from the coast, the floes were significantly grouped into zones of high concentration with areas of few or zero occurrences lying between. Correlation between floe frequency and sum of diameters indicated that greater amounts of multi-year ice did exist in these groups and that a higher frequency was not due to an occurrence of smaller pieces. -- For flights of April 25, May 01, and May 12 most floes were observed in bands which crossed the flight line from top to bottom and which were deflected progressively shoreward through time. By May 26 individual bands had disappeared and the floes were found to occur in two main groups, while the center of the distribution had attained its farthest measured shoreward advance. On May 31 and June 07 the distribution was mainly scattered, however a return to the banded effect was observed near the ice edge. -- The banding phenomenon observed in the distribution of multi-year floes is thought to be a product of differential shear across a horizontal velocity gradient, occurring in a current regime dominated by laminar flow. Under these conditions, individual constituent floes of a hypothetically uniform floe field, which are located at opposite ends of an axis orthogonal to the direction of current flow, are moving in a common direction at unequal velocities. This creates differential rates of shear across the initially uniform field and causes it to elongate into a band. During this process, the originally orthogonal axis is progressively re-oriented until it is parallel to the direction of laminar flow. The subsequent attenuation of this banding phenomenon is probably due to a temporary transition from the laminar to a turbulent mode of flow within which random cross-flow components disperse the bands. -- The landward deflection of the centers of the multi-year floe distribution was probably due to a meandering of the thermal front which separates the cold water of the Labrador Current core from the warmer water offshore. The most severe disruption of banding, on May 26, was probably due to the passage of a cyclonic eddy along the frontal zone. A re-emergence of bands which occurred near the seaward ends of the May 31 and June 07 flight lines is considered normal because current velocities are greatest near the seaward edge of the bank, and the effects of turbulent events would be expected to disappear most quickly in this region. -- Seasonal trends in floe size occurrence suggest that larger floes deteriorate into smaller pieces by fracturing, which increases as the floes become weaker due to higher temperatures. Beyond a lower size limit fracture ceases and melt processes are dominant