General information concerning how the flow of the water within the Baltic Sea basin were of great importance to the Vikings. Knowing where rivers flowed into an inlet could give the Vikings an edge on their enemy when an attack was necessary. Escape through these outlets were enhanced by river flow into open bays. The Vikings also sailed with the currents when possible as this quickened their travel time, as the airlines use the upper level air currents in the sky (jet stream) when they fly as often as possible.
The Baltic Sea and the Danish Straits have numerous gulfs, bays, islands and fjords (inlets). The Baltic Sea is an intra-continental sea connected to the North Sea by the narrow Danish Straits. The influx (flow) of fresh water into the Baltic Sea exceeds the evaporation and results in a permanent salinity stratification (layer). The fresh-water supply to the Baltic Sea (from precipitation, ice, and river runoff) generates a brackish (mixture of fresh and seawater) out-flow of surface water. Denser, saline water enters the Baltic Sea along the bottom of the Skagerrak and over the sills of the Kattegat under the proper environmental conditions. The Baltic Sea extends to depths of 50m. at its deepest point with little variation in bottom topography.
The average flow of the water (current) is very weak. Water circulation is generally anti-clockwise and is driven (made to flow) by the difference in the amount of seawater content verses fresh water content and the “coriolis” effect. The average flow of the water is slightly more persistent along the east coast of the Baltic Sea than along the western portion due to the greater inflow of fresh water and coriolis effect. Low salinity surface water is concentrated along the coast of Sweden.
When environmental conditions are in place, a pulse of high salinity North Sea water moves into the Baltic Sea renewing Baltic bottom water. The outflow of cold, low salinity surface water is blocked by strong easterly winds. When the wind ceases or reverses direction, a pulse of cold, low salinity Baltic water flows out of the Kattegat and Skagerrak and into the North Sea.
The North Atlantic Drift, a continuation of the Gulf Stream, and the relatively warm waters of the Baltic Sea and the Gulf of Finland are the major factors tending to moderate the climate (moderate – have an effect on the incoming weather conditions) of the region, but does not directly flow into the Baltic Sea but as described above.
The principal changes in the water level in the Baltic Sea and Gulfs of Finland and Bothnia, are caused by meteorological conditions. The entire water mass of the Baltic Sea can be set in oscillation (motion) by variations in winds or atmospheric pressure distribution. The natural period of oscillation of the Baltic Sea and Gulf of Finland is about 27 hours. Water levels have been recorded to rise as much as 10 feet above normal due to pressure and wind variation in the Baltic Sea.