Forecast of Iceberg Ensemble Drift

ABSTRACT The objectives of the study are to gain a better understanding of the characteristics of iceberg motion and the factors controlling iceberg drift, and to develop an iceberg ensemble drift forecast system to be operated by the Canadian Atmospheric Environment Service. An extensive review of field and theoretical studies on iceberg behaviour, and the factors controlling iceberg motion has been carried out. Long term and short term behaviour of icebergs are critically examined. A quantitative assessment of the effects of the factors controlling iceberg motion is presented. The study indicated that wind and currents are the primary driving forces. Carioles Force and ocean surface slope also have significant effects. As for waves, only the higher waves have a significant effect. Iceberg drift is also affected by iceberg size characteristics. Based on the findings of the study a comprehensive computerized forecast system to predict the drift of iceberg ensembles off Canada's east coast has been designed. The expected accuracy of the forecast system is discussed and recommendations are made for future improvements to the system. 1 INTRODUCTION Offshore oil and gas development on Canada's East Coast iceberg-frequented waters has identified the need for research and development work on icebergs. Human error aside, icebergs probably pose the greatest threat to Canada's offshore oil exploration and production operations and related activities. Icebergs weighing up to 30 million tons, or more, present a threat to offshore structures, buried pipelines and bottom installations. The severity of the iceberg threat varies widely from year to year. For example 1,587 bergs crossed latitude 48°N in 1972 (the record year) while in 1966 no bergs were sighted south of this latitude. To protect drilling vessels, icebergs are deflected by towing while the untowable ones are avoided by moving the vessel. Both strategies require a warning system to provide information on approaching icebergs with sufficient lead time to take defensive action. Such warning can be provided by a reliable system to forecast iceberg numbers and locations in the vicinity of the shipping routes and drill ships. The International Ice Patrol forecasts the locations of icebergs (on an individual basis) in the area of the Grand Banks each spring. This paper describes a system currently being developed by the Atmospheric Environment Service (AES) of Environment Canada for forecasting the day-to-day drift of iceberg ensembles off Canada?s East Coast, and from which site specific forecasts of iceberg drift can be derived. 2 CHARACTERISTICS OF ICEBERG BEHAVIOUR In order to design a reliable forecast system for iceberg drift, we have to understand the short-term and long-term behaviour of icebergs and to correlate that behavior to environmental conditions and iceberg characteristics. Field studies of iceberg drift indicate that iceberg drift trajectories exhibit some peculiar features. Ball et al [1] presented typical examples of such iceberg trajectories. Icebergs were observed to move in loops and spirals. They also move erratically and reverse the direction of their motions rapidly.