New gridded currents for the International Ice Patrol operations area

For over twenty years researchers have used satellite tracked buoys to study oceanic currents. This ever growing data base offers an exceptional opportunity to improve the mean current data bases used for a wide range of purposes, from oil spill contingency planning to predicting the movement of search objects. The origin of currents now used is a patchwork of ship-drift and hydrographic data of widely varying reliability and spatial resolution. Of course the drifter data set comes with its own list of problems, the most serious of which are: the large variation in drifter and drogue design, differing drogue depths and, indeed, whether or not there is a drogue attached. Although there is some variability in the quality of the buoy position data, most researches use ARGOS tracking services, so the accuracy of the positions is reasonably well known, a distinct advantage over the bulk of the ship drift data. This paper describes the first steps of International Ice Patrol's (IIP) efforts to use drifter data to develop a new gridded data base for its iceberg drift model. In 1995 IIP set out to obtain all available drifting buoy data in the western North Atlantic, its area of operations, and blend these data into a single representation of the near surface currents in the region. Ice Patrol acquired data from the two major archival centers, the National Oceanographic Data Center (NODC) and Canada's Marine Environmental Data Service (MEDS). In addition, IIP obtained data from several principal investigators whose buoy tracks were not yet in the archives. The resulting drifter data set contains 243 trajectories for a total of 30,308 buoy-days, over half of which were from IIP's own drifter program. Daily averaged current vectors were calculated from the interpolated drifter tracks. These vectors were then resolved into their u and v components, which were placed in 1 degree of latitude by 1 degree of longitude bins over the area 37-61 N and 35-60 W. For each bin the mean and variance were calculated. Although some areas, notably the offshore branch of the Labrador Current, are well resolved there are vast areas where there are relatively few observations.