The water of the Baltic Sea is constantly moving

Seawater is always in motion in the Baltic Sea. Movement can be observed in many ways. Currents are also important for the nature of the sea, because they transport oxygen to deeper parts of the sea. Read more about sea water movements!

Water is set in motion due to winds, differences in air pressure, as well as density differences between different water masses. Together, all of these factors create a complex flow field, in which eddies or whirlpools occur. The size of such eddies varies from a few kilometres wide to tens of kilometres in diameter.

Currents in the Baltic Sea are primarily dependent on the weather conditions and weather changes. Therefore, currents are also highly variable. The permanent currents typically found in oceans do not occur in the Baltic Sea, where even tidal currents are negligible. When viewed from the perspective of their flow direction, the rotation of the globe turns currents on the open sea towards the right in the northern hemisphere. The coasts, islands and seabed direct these currents. Compared to coastal areas, the variation in current directions is much greater on the high seas.

Surface currents may even reach one metre per second

Winds and freshwater flows into the Baltic Sea affect the sea surface, where they can produce brief currents, typically at speeds of five to ten centimetres per second.

In heavy storms, surface currents speed may reach 50 cm per second. Even greater speeds of over one metre per second, have been measured in narrow straits between basins, e.g. in the narrows between the Åland- and Bothnian Seas.

As a general rule of thumb, surface water flow rates measure approximately 0.1 metres per second. It can also be related to wind speed such that the former is often about 1-2 % of the latter.

Seawater is much heavier than air and thus, when it starts moving it does not stop very easily. Therefore, after strong winds calm, seawater can continue to flow noticeably quickly, even in totally calm weather.

Deep-water currents bring new saltier water

Fresh water introduced to the sea by rivers and rainfall and saline water flowing into the Baltic Sea from the Danish straits cause density differences. Compared to surface currents, deep off-bottom flows are slower. In addition, their flow direction depends on local seafloor formations. Deep-water current speeds generally reach about a few centimetres per second.

Such deep-water current flows have a particular significance to the health status of the Baltic Sea. Occasionally and during suitable weather conditions, a large pulse of saltier seawater enters from the North Sea via the Danish straits.

Saltwater flows mainly along the bottom of the Baltic Sea, filling one basin after another. These water masses flow into the main basin of the Baltic Sea as far as the Gulf of Finland. As they do so, they renew the deep, isolated waters at the bottom of the basins.

After entering the Baltic Sea, the salt pulse must travel almost 1,000 kilometres from the Danish straits to reach the southwestern shores of Finland. Flowing slowly at about five centimetres per second or 0.18 kilometres per hour, the saltwater mixes with the older, less saline water of the Baltic Sea and becomes diluted.

It takes six months for the salt pulse to reach the Gotland Deep and several further months to reach Finnish marine areas.


The sea level of the Baltic Sea changes due to various natural phenomena

The sea level of the Baltic Sea is dependent on the movement and density of the water masses, as well as the air pressure, but very little on the tides. The water of the Baltic Sea is lighter the farther one travels into the Gulfs of Finland- and Bothnia. Therefore, in principle, the water level is sloped, i.e. it is higher at the ends of the bays than in the middle of the Baltic or in the southern Baltic Sea.

The Baltic Sea exchanges water with the Atlantic Ocean and thus, the water volume of the Baltic Sea varies somewhat. Strong winds tilt the water, forcing it into the bay ends, from where it flows back again once the wind direction and speed change. Atmospheric pressure presses down on the water and therefore, fluctuations in atmospheric pressure also affect the water level.

In the Baltic Sea, particularly in the Quark area, land uplift has a decreasing effect on water levels. Conversely, both the thermal expansion of the ocean and the melting of continental glaciers are increasing the level of the world's seas, which is also evident in the Baltic Sea.

Sea levels are mainly affected by wind, air pressure and ice cover

High air pressure presses the sea surface downwards. Conversely, during periods of low air pressure, sea levels rise somewhat. A pressure change of one millibar corresponds roughly to a difference in sea level height of one centimetre. Therefore, normal changes in air pressure can cause sea level height differences of tens of centimetres.

Extensive ice cover can also affect short-term fluctuations in seawater level by preventing the wind from affecting the water surface. When the wind cannot push water against the coast, the highest extreme water levels are not as easily created as in open water.

Tidal effect in Finland is only few centimetres.

Seiche – a standing wave

When water that has been piled up against the coast by the wind returns to its equilibrium position, a standing wave - seiche is generated. It depends on the length of the seafloor basin.

A seiche is a phenomenon typically found in the basins formed by the Central Basin of the Baltic Sea and the Gulf of Finland, together with the Southern Baltic Sea basin. The seiche’s effect on water levels is accentuated at the inner end of bays and its duration lasts well over 24 hours.

On a much smaller scale, a similar back and forth oscillating motion can also be achieved, e.g. in a bathtub.

The total variation of Finnish sealevel is more than three metres

On the Finnish coastline, sea level variations are highest in the northern Bay of Bothnia and the eastern Gulf of Finland and lowest in Åland.

Sea level changes most during stormy periods in autumn and winter. The average sea level on the Finnish coast is highest in December and lowest in April to May. However, individual years may differ greatly from each.

Floods and low sea level

The greatest floods can occur if the water volume of the Baltic Sea is large, the water levels have started to oscillate, and the target area is characterized by both extremely low air pressure and strong winds. The wind blows from a suitable direction while simultaneously, a seiche pushes water into the end of a bay.

This was almost the case during the record storm surge on the Gulf of Finland and the eastern coast of the Baltic Sea on the 9th of January 2005.

Low sea level is problematic for shipping, for example.


The salt pulse is a phenomenon anticipated in many areas of the Baltic Sea

The water of the Baltic Sea is brackish, being formed from a mixture of salty water from the North Sea and fresh water from the catchment area of the Baltic Sea. However, saltwater can only enter the Baltic Sea via a single route, i.e. through the Danish straits.

Moreover, saltwater does not flow constantly through the straits in large volumes. Indeed, due to both geographical- and weather-related conditions, saltwater enters the Baltic Sea deeps only on rare occasions as a salt pulse. When it does enter, it brings both oxygenated and salty water into the Baltic Sea.

Water mixing is prevented by a halocline

The water in the Baltic Sea is not always equally saline throughout. The saltier the water, the denser and heavier it becomes. This causes saltwater to sink to the bottom, while the less saline and lighter water remains close to the surface. This creates a saltwater transition layer, or halocline, where the salinity varies with depth.

In the Baltic Proper a permanent halocline is in the depth from 40 to 80 metres. The water below the halocline is heavier than above it.

A cubic metre of surface water weighs just under 1,005 kg, whereas a cubic metre of deep water weighs approximately 1,010 kg. Although the difference seems small, it is large in terms of mixing water masses.

Although the cooling of the surface water in autumn increases its density, it is not enough for the halocline to disappear. Even then, the less saline surface water and more saline and heavier deep water cannot mix.

Not even powerful autumn storms are capable of breaking the halocline barrier. For this reason, the water in the Gotland Deep never receives oxygenation from surface water.

 Two graphs about oxygen concentration.
The depth profile of the oxygen concentration changes from the Gotland Deep to the Gulf of Finland (left) and from the Gotland Deep to the Bay of Bothnia (right). Oxygen values have been measured in situ in August 2012.

The last of the oxygen on the seabed is consumed by the decomposition of organic matter

The decomposition of dead organisms and other organic matter which sink to the bottom consumes oxygen on the seabed. When the water below the permanent halocline does not receive oxygen from the surface, the oxygen will inevitably run out.

This situation is called the standing water phase or stagnation. When deep-water oxygen has been consumed, the decomposition of organic matter continues without oxygen. This is called anaerobic degradation, during which, toxic hydrogen sulphide is formed on the seabed.

The depletion of oxygen and the formation of hydrogen sulphide destroy all higher organisms on the seabed. At this time, all benthic animals die, and the fish flee.

The depletion of oxygen also accelerates the release of nutrients, especially phosphorus, from the sediments into the water. This phenomenon increases the nutrient content of bottom water. This release of nutrients stored in the substrate over time as a result of internal water processes is called internal loading.

The oxygen content of Finnish sea areas varies regionally

In the Gulf of Bothnia, the oxygen in the water column is consistently at good levels throughout the year. The oxygen level drops only slightly towards the bottom. In the absence of a halocline, the entire mass of water is mixed every year and oxygen is “pumped” from the surface to the bottom.

However, in the Gulf of Finland, oxygen depletion occurs occasionally in the both open sea and in the archipelago. Bacterial decomposition activity consumes all the oxygen from the seabed and new oxygen cannot be mixed from the surface.

In the open sea, a deep halocline prevents water from mixing. Oxygen depletion during summer in the Archipelago Sea is caused by a temperature stratification or thermocline. Unlike a halocline, which may be permanent, the thermocline is temporary. Therefore, this temperature transition layer disappears every autumn as the water cools, mixing the water mass all the way to the bottom, bringing oxygen as it does so.

The salt pulse is only created under favourable conditions

In the long, narrow, and shallow threshold area between the Baltic Sea and the ocean, water flows outwards from the Baltic. Outflowing water mixes with the surface waters of the Skagerrak and the North Sea. Some of this mixed water flows back into the Baltic Sea again.

However, it is difficult for the deep salty water in the North Sea to rise over the sills of the Danish straits and gain access to the Baltic Sea. Only under very favourable weather conditions will a situation occur where a large amount of markedly saltier water can at one time penetrate over the sills and flow into the basins of the southern Baltic Sea.

Such an event is called a salt pulse because of its relatively short duration, i.e. a few weeks. Only a large salt pulse, also called Major Baltic inflow, can break the stagnation phase and replace the deep anoxic standing water with oxygenated saltwater.

The pulse is a very large amount of water, i.e. 200 to 300 cubic kilometres.

 A graph about a salt pulse
Stages of the Baltic Sea saltwater pulse in chronological order: A: Brackish water, which is less saline than North Sea seawater, flows constantly out of the Baltic Sea. At the same time, smaller amounts of saline North Sea water flow into the Baltic Sea to the deeper layers. This weak inflow does not reach into the large depressions and thus does not oxygenate them. B: The North Sea often produces moderately intense saltwater inflows that do not extend to the Bornholm Basin. C: Instead, a strong inflow, i.e. a large saltwater pulse, is able to displace the old nutrient-rich and anoxic water within the Gotland Deep. When displaced by oxygenated and saline water, the deep water is set in motion, eventually reaching the shallow coast where saline and nutrient-rich old water rises to the surface.

The affect is positive for many organisms

When a major Baltic inflow penetrates the Baltic Sea, it raises the salinity of almost the entire area. The distribution of many plant and animal species changes according to their salinity requirements. Many planktonic marine species spread north- and eastwards.

As deeper oxygen conditions improve, benthic organisms may reconquer areas previously dominated by macroscopic fauna. In addition, cod can also spawn further north, even in the Gotland Deep, which is an important spawning ground when oxygen conditions allow.

The influence may be negative for eutrophication

Major Baltic inflow may also have negative effects. Eutrophication intensifies when nutrient-rich deep-water mixes with well-lit surface layers in which primary production occurs.

On being displaced by more saline the deep water which is low in oxygen is pushed onwards and can eventually settle into the basin of the Gulf of Finland, even to its easternmost parts. In such cases, this displaced low-oxygen water strengthens the halocline within the deep basins.

The halocline forms a floor that prevents the wind from mixing oxygenated surface water with bottom water. Thus, a powerful halocline can lead to the deoxygenation and internal loading of the seabed as nutrients are released from the bottom sediment.

There have been only a few major Baltic inflows in the 2000s

Due to the structure of the Baltic Sea basins, the anoxic condition of deep seabeds is a natural phenomenon. The conditions for a major Baltic inflow are most favourable during winter storms. In 1951, a very strong and large salt pulse entered the Baltic Sea. The pulses of 1975, 1976, and 1993 were also large.

Until the 1980s, a few moderate influxes regularly entered the Baltic Sea, followed by a stagnant water phase. This stagnation was broken by the extremely large inflow that entered the Baltic Sea in 1993. The next major Baltic inflow occurred ten years later in 2003.

Since then, stagnation continued and oxygen conditions worsened in the Baltic Proper, as well as in the Gulf of Finland, until the third largest ever inflow occurred in 2014, followed by medium pulses in both 2015 and 2016.

However, by this time, the condition of the deep waters of the Baltic Proper had already become so poor that even this inflow only improved the situation slightly and then only for a short period of time. A large amount of oxygen is needed to neutralise the amount of hydrogen sulphide produced during the current stagnation period, which has been constantly increasing. Only then can the situation improve.

Upwelling brings deep water layers to the surface

Many have experienced that seawater has suddenly become colder, even on a warm summer’s day. What in the world could have happened?

This phenomenon is quite common and is due to upwelling, i.e. the transport of cold deep layers to the water’s surface. In the Baltic Sea, water wells up from depths of a few tens of metres.

Upwelling is created by wind-generated water currents

Water currents created by winds cause upwelling. A typical upwelling situation occurs when the wind blows along the coast, i.e. from right to left when viewed from the beach. The Coriolis phenomenon of the Earth's rotation, which turn currents to the right in the northern hemisphere, causes coastal waters to drift offshore.

In turn, this offshore drifting water is replaced by upwelling water flowing from the depths to the surface. To be clear, this is not a case of rapid cooling, rather the warmer water has simply been replaced by cooler water. A typical upwelling zone extends along the coastline extends outwards from about 5 to 20 kilometres from the shore to the open sea. This area of cold water is easily visible in satellite images too.

Upwelling requires time

Upwelling is not instantaneous because the large mass of the seawater opposes movement. For example, in the Gulf of Finland, it is necessary for moderate winds to blow along the coast for at least a couple of days before upwelling can begin.

After an upwelling event, currents and solar radiation offset the biggest temperature differences within a few days. However, traces of upwelling sometimes continue to linger for a couple of weeks.

Upwelling teases morning swimmers

Summer upwelling events can dramatically affect life on a swimming beach. The water near the shore may become strongly chilled by more than ten degrees during the night, which in turn poses an unpleasant surprise for the first swimmers taking their morning dip.

Upwelling can promote algal growth

By bringing nutrients to the surface, upwelling can promote the growth of planktonic algae, including the formation of blue-green algal blooms. Although the surface water itself may be too cold immediately after upwelling for algae growth, it will accelerate during the next warm and calm period.

This scenario occurs often in August in the Gulf of Finland. In the Gulf of Finland, nutrients in the surface water are low and winds are low until around midsummer. After midsummer, possibility of higher wind speed increases and upwelling may occur and supply nutrients to the surface.

 Temperature profile about sea temperature.
Tallinn is on the left and Helsinki on the right in the pictures. Upper: Normal situation in summer: warm water on the surface, in between is a temperature transition layer or thermocline, below is cold water. Lower: Due to upwelling, cold water has risen to the surface on the Estonian coastline. Warm water floats on the surface off the Finnish coastline.

Baltic Sea waves are created by the wind

Waves are perhaps the most impressive form of sea motion that people normally associate with the sea. They occur when the wind blows over a calm water surface, causing the water to flow and creating turbulence.

Turbulence creates pressure differences which disrupt the smooth water surface. The wind is able to “grab” the ripples created on the surface and makes them larger.

Small waves are initially steep-sided but then begin to increase their length faster than their height. Shorter waves move forward more slowly than long ones and they interact with each other, exchanging energy as they do so.

Soon there are many waves of different heights, lengths, and directions. Such wind-generated waves are highly irregular in nature.

The sea surface is not only a passive playground for the wind. Instead, this mobile surge of different-sized and intermittent breaking waves affects the interaction of the water and the atmosphere, e.g. by changing the wind momentum and also affecting gas exchange. Thus, waves also modify the winds that caused their growth.

In addition to wind speed, wave growth is primarily influenced by wind duration and fetch length. Fetch refers to the distance over which the wind effects on the water

When the duration of the wind does not limit growth, the fetch length determines the wave height. Therefore, in the northern part of the Baltic Proper, the waves grow to the highest level in the entire Baltic Sea region.

Waves are measured in height and length

The height of a wave is the distance between its trough and crest. The significant wave height has been scientifically determined to correspond to the wave height visually perceived by a trained observer.

The highest individual waves can be about twice as high as the significant wave height. In the Baltic Sea significant wave heights greater than eight metres have been recorded, with over 14 metres high individual waves.

The largest waves in the Bothnian Sea are almost as large as those in the northern Baltic Proper, whereas those on the Bothnian Bay and the Gulf of Finland are slightly smaller. Wave heights have been measured in Finnish sea areas since the 1970s.

The shape of the water area and its depth also influence wave growth. Waves begin to feel the effect of the bottom when the water depth is less than half the wavelength.

The wavelength is the distance between two successive crests. When waves arrive in shallow water, their wavelengths become shorter, steeper, and they finally break. In sufficiently shallow water, waves can remove material from the bottom, including sediments and nutrients.

 Little waves  A big wave

Salinity, temperature and wave growth

Neither water salinity nor temperature have much effect on waves. Only temperature differences in air and water have been proven to affect wave growth. When the seawater is warmer than the air, waves increase in size somewhat faster than under the opposite conditions.

In autumn and winter, the seawater is usually warmer than the air. However, higher waves at this time are mainly due to the occurrence of more frequent strong winds than in the summer period.

Swells travel long distances

As the wind calms, wave speed becomes higher than wind speed and the wind can no longer supply energy to the waves. The smallest waves die down first and the surge turns into a swell, which is transported away.

Swell is also known as waves transported from a distant storm. In the world oceans, swells can travel thousands of kilometres almost unabated before breaking on the shallow waters of the coast, sometimes creating impressive breakers.