Analysis of the Surface Drift Currents in the Bothnian Sea

摘要

Surface drift in the eastern part of the Bothnian Sea was studied by using satellite-tracked buoys. Two drifter experiments were performed during the Gulf of Bothnia Year 1991 in June and November. Both experiments were carried out near the Finnish coast and the buoy clusters drifted northwards almost parallel to the coastline. The dominant wind was from southeast to southwest. The mean drift speed in June and November was 10 cm/s and 14 cm/s and the mean geostrophic wind velocities were 6 m/s and 11 m/s, respectively. In the June experiment dispersion characteristics were more obvious than in the November experiment. In June an inertial-type motion was found at the period of about 13.9 h. Geostrophic surface wind fields were calculated from two different pressure analyses, one given by the HIRLAM and another by the ECMWF. A numerical interpolation method was used to define the geostrophic wind for the buoy site-Linear relationships between the wind and the buoy centroid drift speed were studied. Wind was the most remarkable factor generating the surface drift, but coastal and topographical effects were of importance as well. The latter effects were seen in the turning angle between the direction of the geostrophic wind and the drift. The drift speed was of 0.8-1.2 % of the geostrophic wind speed and was directed on the average some 30° left of the wind direction.