The Baltic Sea is the largest brackish water basin in the world. Its water is a mixture of salty water from the ocean and fresh water supplied by numerous rivers. In the Southern Baltic Sea salinity is as high as 20 parts per thousand, but it is as low as six ppt in the Northern Baltic Sea. The water is almost fresh in river estuaries, for example near St. Petersburg.
Shape, Area and Volume : The current surface area of the Baltic Sea is approx. 422 000 km2, its volume is 21 000 km3. The extent of its catchment area exceeds 1 700 000 km2. Therefore, the catchment area is approx. five times as large as the surface area itself. The average depth of the Baltic Sea is just 55 m, compared to other landlocked seas such as the Mediterranean whose average depth is 1000 m. Ocean average depths reach several kilometres. The greatest depth of the Baltic Sea is only 450 m.
It rains a lot at these northern latitudes, and the hundreds of rivers running through the Baltic Sea catchment area bring approx. 450 km3 of fresh water annually. In addition, 100 km3 of precipitation in the form of water or snow, falls on the surface of the sea itself annually. The same amount, about 100 km3 evaporates into the atmosphere. The annual fresh water surplus is therefore approx. 2% of the total volume of the Baltic Sea. Since the volume of the Baltic Sea remains stable in the long term, the surplus runs to the North Sea via the Danish Straits. The Baltic Sea would gradually become a fresh water sea, unless an occasional influx of salty water would occur in the opposite direction.
In addition to the main basin of the Baltic Sea proper, Finland is surrounded by broad but shallow gulfs.
Gulf of Bothnia : A threshold formed by the continuation of the Salpausselkä end moraines protects the Gulf of Bothnia. The water which enters it is mainly slightly salty surface water from the Baltic Sea proper. The salinity of Bothnia Sea water is lower than that of the Baltic Sea proper, and it is also less stratified. Oxygen can enter the bottom through almost regular spring and autumn circulations, and there is no oxygen deficiency. The oxygen content of Bothnia Sea near bottom water is going down, however. Since the Quark is so shallow, it further prevents salty water from entering Bothnia Bay, so that the oxygen situation in the Bothnia Bay remains good.
Gulf of Finland : There is no threshold between the Baltic Sea proper and the Gulf of Finland, so that the characteristics of the Baltic Sea proper are reflected in the conditions of the Gulf of Finland. The oxygen situation has long been weak in the near bottom water of the Gulf of Finland. Nevertheless, oxygen deficiency has been a rarely observed phenomenon. The effects of the great influxes of salty water from the North Sea is readily observed as far as the central areas of the Gulf of Finland, when the old water from the central basin, which has been displaced by the influx, is pushed eastward in the near bottom water layer. This near bottom water has a low oxygen content, which increases the dissolution of phosphates from the sediment into the water mass itself.
The phosphate content of Gulf of Finland surface water has greatly increased from last year. An obvious reason for this is the upwelling of near bottom water. The deep, near bottom water of the Gulf of Finland is strongly tied to the stratification of the water mass. The salinity of the near bottom water can change rapidly, and consequently its oxygen content also changes quickly. When salinity increases, the oxygen exchange between the surface and deep water layers is obstructed, and the oxygen content decreases and in some areas it runs out altogether. As the oxygen content decreases, phosphates bound in sediment start to dissolve back into the water. If this kind of a stagnation phase lasts a long time, considerable amounts of phosphates will accumulate in deep near bottom water.
The strong stratification of the Gulf of Finland can be disrupted especially when storms mix the water, and also when strong northern winds drive surface water from the Finnish southern and south-eastern coasts out to the open sea. This water is replaced by cold Gulf of Finland near bottom water, which has become rich in phosphates during the long stagnation phase. Since the spring algal bloom uses nutrients, mainly nitrates and phosphates in a particular ratio, it is possible to calculate how much phosphate remains in the surface water waiting for the late summer algal blooms which rise to the surface. These only require phosphate dissolved in water, since they absorb the nitrogen they need directly from the atmosphere.
Water level variation in the Baltic : Most important factors influencing water level in the Baltic are atmospheric pressure, wind, currents through the Danish straits and, during winter, the extent of ice cover and its effects. Tidal forces cause only few centimetres of waterlevel variation in Finnish coastal areas. Wind piles up water to certain areas of the Baltic, especially inner bays, and the highest amplitudes of water level can be found in these areas. Wind can also have local effects.
High atmospheric pressures pushes water surfaces down. A density gradient of one millibar equals to one centimeter water, normal changes in atmospheric pressures can thus shift water levels tens of centimeters.
Water flow in and out of the danish straits changes the total volume of Baltic water and thus to the water level all around the Baltic. Currents are caused by waterlevel differences between the Atlantic and the Baltic and strong winds in the area.
Ice cover : The severity of ice winter can be measured by the maximum extent of ice cover at a given year. There is a clear relationship between ice cover and waterlevel; more extensive ice cover means lower water levels. Ice cover influences also to the amplitude of water level changes; more ice means less variation.
There is an annual cycle in the Baltic water levels caused by a parallel cycle in atmospheric pressures and thus winds. The mean water level is highest during December and lowest during April-May. Chenges in water levels are strongest during November-January and weakest during May-July. Single years can vary a lot and during some years the kind of cycle described is not present.
Temperature : The annual temperature variation in surface waters of the Baltic is great. During the winter the temperature in the layer above the permanent halocline is only a few degrees above freezing. In general ice forms in marine waters when temperatures are below zero on the celsius grade, exact freezing temperature depending on the salinity of the water; more saline water freezes at lower temperatures. Because of this Deep sea water freezes at ca.-0.20o C in the Bothnian Bay but at -0.45o C in the more salt Baltic Proper.
During the spring the sun starts to warm the sea surface and first the warming water turns denser and sinks deeper. This is because of the density maximum of freshwater is at 4oC and for the brackish Baltic seawater at 2.3-3.5oC. When the watermasses above the halocline are all within the temperatures of highest density this vertical mixing caused by the temperature gradient stops. From this point on warming decreases the density and the thus heat is transported to the deeper layers by the forces of wind and waves. This is the way the summer thermocline forms dividing the upper waterlayer in two. Temperatures may drop 10oC within a few meters in thermocline depths, shallower in the spring but at around 15-20m during the end of August.
During summer the water below the thermocline usually remains as cold as during the melting period in early spring or 2-4oC. Below the halocline salinity is the dominant factor in determining density, temperature is fluctuating less and stays at ca.4-6oC.
As the atmospheric temperatures above the surface start cooling during the fall the sea starts to transfer heat energy to the colder air. Water cools as a result and sinks until it meets water having temperatures within the density maximum. Thermocline and thus the density differences in the upper layer disappear and wave and wind action mixes finally the whole layer above the halocline. As Baltic seawater reaches its freezing point -1...-0.1oC it turns into ice.
The varying Baltic currents influence horisontal and vertical distributions of seawater temperatures and salinity. Average water transportation is counterclockwise, this can be observed in the slightly lower surface salinities along the Finnish Bothnian Sea coast compared to the Swedish coast at similar latitudes.
During summer the phenomenon called upwelling can induce a drastic drop in coastal surface temperatures. Upwelling is caused by wind pushing water off the coast. Cooler and saltier deepwater flows up to replace it.
The Baltic Sea ice winter :
Ice winter means the time when the ice is present in the Baltic Sea. The season normally takes place from October-November to May-June. The annual maximum ice extent occurs between January and March, normally in late February – early March. The ice covers on average about 200,000 km², which is almost a half of the total area of the Baltic Sea. During extremely mild seasons the maximum extent is well below 100,000 km². The minimum extent was reached in 1989 with only 52,000 km².
The extent of ice varies greatly from year to year.
Freezing : In the beginning of the season prediction of the severity of the ice winter is almost impossible, and quite accurate forecast cannot be given before late January. During mild winters, the Bothnian Sea does not freeze at all, and the Gulf of Finland only partly.
Ice formation begins in the northernmost parts of the Bothnian Bay and in the most easternmost parts of the Gulf of Finland in October – November. Next to freeze are the Quark, the Bothnian Bay totally and the coastal areas of the Bothnian Sea. During average winters also the Botnian Sea, the Archipelago Sea and the Gulf of Finland are totally frozen, and the northern part of the Baltic Sea Proper partly. In severe winters ice occurs also in the Danish Straits and the Baltic Sea Proper. The sea area northeast of Bornholm is the last to cover with ice.
The ice break-up starts in April.
In spring, within up-growing sun radiation, the ice starts to melt from the south to the north. Normally in beginning of April the northern Baltic Sea Proper is open. By the early May the ice exists only in the Bothnian Bay, where it melts completely by early June. On average the duration of ice winter in the northern Baltic Sea Proper is about 20 days. In the northern Bothnian Bay it can be more than six months.
The ice in the Baltic Sea exists as fast ice and drift ice. Fast ice is situated in coastal and archipelago areas, where the depth is less than 15 metres. It develops during early ice season, and remains stationary to the melting period.
Drift ice and pack ice : The drift ice has a dynamic nature being forced by winds and currents. Drift ice can be level, rafted or ridged, and its concentration could be 1-100%. In media, drift ice is occasionally called pack ice. Pack ice is drift ice with concentration more than 80%. The term has no dynamic meaning.
The consequences of ice drift : Drift ice movements are large: in stormy conditions thin drift ice field can move 20-30 km in a single day. The motion results in uneven and broken ice field with distinct floes up to several kilometres in diameter, leads, and cracks, slush and brash ice barriers, rafted ice and ridged ice.
The ridges and brash ice barriers are the most significant obstructions to navigation in the Baltic Sea. Powerful, ice-strengthened vessels can break through ice up to almost one metre thick, but they are not capable of navigate through ridges without icebreaker assistance. Ice dynamics affects navigation considerably – high pressure in the ice fields can be dangerous to the vessels, and it may at least cause time delays from hours up to days.
Severity of the Baltic sea ice season : The Finnish Ice Service of the FIMR classifies the severity of the Baltic Sea ice seasons into five classes: extremely mild, mild, average, severe, and extremely severe. Classification is done according to maximum extent of ice cover in 1720-1996.
The maximum ice extent has been calculated for the day when the annual maximum has been reached. In calculations ice concentration, thickness or ice deformation degree have not been taken in account. The classification is based on the area of the total ice extent.
Mild ice season could not be easy in perspective of ice navigation, and on the other hand, severe seasons might not be difficult. Average seasons, like 2003, are the hardest for ice navigation. The warm and windy periods between cold periods cause ice drift followed by ice pressure and ridging in the ice fields. Cold and calm periods increase amount of ice, which will with next windy period drift against the close drift ice edges, and brash ice barriers are formed, difficult to force.