Physical Environment in the Gulf of Bothnia

The Gulf of Bothnia contains about 29% of the whole water volume of the Baltic Sea. The water exchange between the Baltic proper and the Gulf is good, which results in a water renewal time of about 4 to 5 yrs. The dynamics and thermodynamics of the Gulf of Bothnia are guided by hydrological, meteorological and, partly, topographical factors, such as freshwater supply, wind over sea, sun radiation, sea ice and sill depth between the sub-basins. The water mass is vertically divided into two layers with a halocline at 50 to 60 m depth. The deep water, below 60 m, mainly originates from the upper layer of the Baltic proper and has a salinity of 6-7 psu. In the surface layer, a thermocline develops each summer. This upper well-mixed layer is, on average, 15 m deep. The north to south water transport mainly occurs in this surface layer along the coasts, with a tendency to cyclonic circulation. It is, however, strongly wind-dependent and thus intermittent in time. On a long time scale, pulse-like wind events produce on average an eastbound current drift in the open sea. The river runoff causes a slow southbound current drift of diluted freshwater along the western region of the Bothnian Sea. Distribution and deposition of dissolved and particulate matter occurs with several types of transport mechanisms. Our measurements during 1991 show that in the surface water, substances can be distributed right across the Bothnian Sea in about one to two weeks, but perhaps more normally in about one month. The geographical extension of the accumulation bottoms is most often limited to areas deeper than 40 m. Transport of particulate matter can however also occur below this critical depth especially during periods of strong winds, forcing wind waves of sufficient wave length and amplitude to erode bottom particulate matter.