The Gulf of Bothnia Year 1991 project was a multi-disciplinary programme aiming at a better understanding and knowledge of the ecological system of the northern part of the Baltic Sea, including also the physical processes affecting the system. This report describes the Finnish-Swedish-Canadian physical sub-programme for studies of sea currents and hydrography of the Bothnian Sea. General results of the field experiments are given as well as the marine meteorological, river runoff and ice conditions during the experiment.
Coastal-open sea exchange and deep-sea mixing processes were investigated using moored current meters and thermistor chains, satellite-tracked drifting buoys and satellite images. The measurements from the coastal transect moorings and some surface layer drifter experiments indicate a weak cyclonic circulation within a coastal boundary layer, the extent of which depends on the depth. The width of the boundary layer varies from a few kilometres at deep
coasts to a couple of tens of kilometers in shallow coasts. This circulation is superimposed on burst-like events of alongshore currents, strongly varying both in time and direction and highly wind-dependent. The open sea currents were dominated by inertia and wind-driven currents.
The drifter experiments and wind observations from the Swedish coast in the northem Bothnian Sea indicate an eastward drift. The typical drift velocity of the surface waters were found to be roughly 1.6 % of the wind speed, indicating that the surface waters can mix horizontally within a couple of months. Cluster drifter experiments performed in the surface layer indicate stronger horizontal dispersion characteristics during quite calm surnmer conditions than in the autumn during strong winds. The lateral dispersion coefficients varied from 105 cm2 s-1 to 104 cm2 s-1 respectively. In general more complicated spreading characteristics were found in the northwestern part of the Bothnian Sea than above the gently sloping south-eastern part of the basin.
The deep sea velocity measurements with current meters and with a drifter experiment showed also northward currents in the central Bothnian Sea and Bothnian Bay. lnterestingly enough in the very deep parts of the basin, 5 meters above the sea floor, high speed burst-like events were obseived with velocities up to 30 cm/s.
The hydrography of the Gulf of Bothnia was studied using a network of hydrographic stations, dense both in time and space. Some intensive hydrographic open sea stations and coastal sections were visited once a month for investigations of the time evolution of the hydrographic conditions. The temperatures of the Bothnian Sea were during the field year higher than the monthly mean values from the two preceding decades. The winter of 1990/91 was mild and the weather was warm in July, August and September. The salinities showed negative anomalies at all occasions.