Iceberg severity off the east coast of North America in relation to upstream sea ice variability: An update

Abstract

Earlier examination of strong correlations between mid-winter spatial extents in Davis Strait and large annual variations in the estimated numbers of icebergs passing south of 48°N motivated detailed studies of the origins of variability in iceberg numbers was conducted over twenty years ago [1]. This work established the critical role played by the processes that control the cyclonic movement of the icebergs from their primary West Greenland calving ground to the northern perimeter of Baffin Bay and, subsequently past the coastal shelves of North America. It was demonstrated that the effectiveness of these processes and year to year variations in their timing tended to overwhelm interannual variations in Greenland iceberg calving rates. A major connection between such fluxes and upstream ice extent was established through the effectiveness of extensive pack ice in lowering deterioration and melt rates of freely drifting icebergs during the last, late winter through early summer, segments of their drift trajectories. This role is exercised through the effectiveness of sea ice in lowering the local water temperature and reducing sea state parameters which essentially determine iceberg lifetime in these segments. Comparisons between historical International Ice Patrol (IIP) records of annual south of 48N iceberg numbers and a shorter record (post-1960's) of sea ice extents indicated strong correlations between the former and the January spatial extent of ice in Baffin Bay and Davis Strait north of 67N. Measures of this extent, designated as the Davis Strait ice index (DSII) were subsequently used on their own or with modifications as a basis for early season assessments and apportionments of resources by the IIP and others with interests in East Coast navigation. The iceberg severity off Newfoundland is revisited in the light of several major efforts to document and reorganize East Coast iceberg data-taking and analyses as well as the availability of, roughly, 20 more years of higher quality sea ice and iceberg data. The updated and extended analysis includes assessing impacts of suspected over-counting in early iceberg surveys and access to improved Canadian Ice Service (CIS) digital ice chart data, available from 1971 to the present, which allows better resolution of the sea ice data in marine areas upstream of 48N both in terms of areas of interest and in time. Examination of the 1971-2014 sea ice concentrations in Davis Strait for mid-January reveal that the post 2000 ice extents are consistent with the iceberg numbers being lower than those observed and inferred from the 1980-1999 sea ice concentrations. This extended analysis also discriminates between sea ice types, in particular first year ice vs. the thinner new to young ice categories for regions with three-tenths or more in ice concentration in the DSSII area of interest.