The Baltic Sea is flooded with fresh water from the surrounding rivers, with the longest rivers being the Vistula and Oder rivers.

Strong North Sea tides do not reach the Baltic Sea, so the Baltic Sea lacks tidal currents and the tidal waves are small.

The water surface flowed due to the storm, and strong northeasterly winds caused large waves on the south coast, contributing to the high water level along the coast; The dominant southwesterly winds helped to accumulate sand dunes along the German and Polish coasts, while raising the water level on the northern Baltic coast.

The surface circulation of the Baltic Sea is cyclonic, running from east to north along the southern coast and then south along the east coast of Sweden to the Øresund Strait, from which it flows. The flow rate is about 5 cm/s—20 cm/s.

Under the action of strong winds, the flow direction and velocity of local sea areas will change. It can sometimes reach more than 80 cm/s near shore and can increase to 30 cm/s in open water.

When the runoff into the sea is large, the surface water enters the North Sea through the Øresund Strait; The Baltic Sea, on the other hand, flows from the depths of the Great Belt Strait and flows eastward along the south shore and then northward along the east coast, forming a counterclockwise current.

The tides in the Baltic Sea are divided into irregular semi-diurnal tides, irregular all-day tides, and regular all-day tides. The tidal range varies from only 4 cm (Klaipeda) to 10 cm (Gulf of Finland). However, due to the influence of wind, pressure, runoff and the confluence of seawater, non-tidal cycle fluctuations (from a few hours to a few days and nights) can cause large changes in coastal water levels.

Water levels can fluctuate by more than 50 centimetres in the nearshore areas of the open sea area and up to 1.5-3 metres in the inshore part of the bay as a result of cyclone migration. In November 1824, the water level in Leningrad reached 4.1 meters.

The altering effect of the North Atlantic current is hardly felt in the Baltic Sea.

The reason why the water of the Baltic Sea is cold is that the salt content of the sea water here is only 1/4 of the salt content of the water of other oceans, so it is easier to freeze. One-fifth of Europe's surface water flows into the Baltic Sea through more than 250 rivers, and a large amount of fresh water pours in, and the water of the Baltic Sea is discharged into the North Sea through narrow channels is limited, so the water of this sea is almost light.

The prolonged freezing of spring slows down the departure of the spring climate, while the slow expansion of the surface ice prolongs autumn, and even when the Baltic Sea is navigable, it is dangerous to sail due to frequent strong storms and sudden changes in wind.

The Baltic Sea is the least salinous sea in the world, because the Baltic Sea was not formed for a long time, it was still an ocean of ice water at the end of the ice age, and then the glaciers retreated northward, leaving the lowest-lying valley to form the Baltic Sea, and the water quality was already better.

Secondly, the Baltic Sea is closed, and the channel with the open sea is shallow and narrow, and the seawater with high salinity is not easy to enter. In addition, the Baltic Sea has a high latitude, low temperature, and weak evaporation; Influenced by the westerly wind belt, the climate is humid, there is more rain, surrounded by 250 rivers of different sizes such as the Vistula, Oder, Neman, Western Dvina and Neva, and the average annual river runoff is 437 cubic kilometers, which is about 4 times the catchment area of the Baltic Sea.

Therefore, the waters of the Baltic Sea are very thin. The salinity of seawater is only 0.7%-0.8%, with the highest recorded salinity of 1.0% at sea surface and 1.5% on the seafloor, which is well below the average salinity of seawater worldwide (3.5%).

The salinity of the Baltic Sea gradually decreases from the outlet to the sea, the salinity of the sea water in the Great Belt Strait and the Little Belt Strait is 15‰, 8‰-11‰ in the west, 8‰-8‰ in the east of the island of Murne, 6‰-8‰ in the middle, 3‰-6‰ in the Gulf of Finland (only 2‰ near the inland), and 4‰-5‰ in the Gulf of Bothnia (2‰ in the far north).

The salinity of the deep and near-bottom layer is 16‰ in the west, 12‰-13‰ in the middle, and about 10‰ in the north. When the inflow of Atlantic water increases, the salinity in the west can increase to 20‰. The high salinity of the deep Baltic Sea is due to the inflow of North Sea water with high salinity.

The development of industry and shipping has made the strategic position of the Baltic Sea more and more important, but it has also caused the Baltic Sea to suffer more and more serious pollution.

A 2007 report by experts from the Baltic Sea Council for the Helsinki Commission found that the Baltic Sea region was warming faster than the global average this century, which had affected the region's fisheries and extended the growing season.

The study notes that the region is already warming faster than the global average, with the average annual temperature across the Baltic Sea rising by 3-5°C this century.

The pollution left by the former Soviet Union in the Baltic Sea has already caused great damage to the habitats of marine life, and the warming climate is even worse. In addition, the potential for more precipitation due to warmer temperatures will reduce the salinity content of the Baltic Sea and continue to destroy the habitat of marine life.

The northern region is likely to warm faster than the global average, as the dark soils and water in the north absorb much more heat than the reflective ice and snow.

A warming climate also threatens to prolong the growing season in the Baltic region. By the end of the 21st century, temperatures will rise by 3°C to 5°C in the Baltic Sea, extending the growing season by 20-50 days in the north and 30-90 days in the south, which will benefit crops and forests. The ice period in the Baltic Sea will be significantly shortened, by 1-2 months in the northern part and by 2-3 months in the central region.

In the Baltic Sea, where a significant number of passing ships leak or discharge waste oil into the sea every year, the Baltic Sea is becoming increasingly polluted, with as many as 150,000 seabirds dying each year.

Researchers surveyed the coastal environment of the southern 50-kilometre island of Gotland in the Baltic Sea and found that about 20,000 seabirds die each year due to oil pollution. It is inferred that about 150,000 of the millions of seabirds that winter in the Baltic Sea every year die from the oil.

In addition, the survival of grey seals along the Baltic Sea coast has also been seriously threatened by the surrounding environmental pollution, and many grey seals are affected by diseases such as intestinal ulcers. There are about 10,000 grey seals in the Baltic Sea, and in 1900 there were about 100,000 grey seals.

In the 70s of the 20th century, due to polychlorinated biphenyls (PCBs), the number of grey seals fell to 40,000 at one point.

Due to the severe lack of oxygen, in many areas of the Baltic Sea, large swaths of the seabed have evolved into "underwater deserts", and a large number of seabed plants and animals have died. Out of a total of 47 surveyed areas, the scientists found only 4 areas with high animal and plant activity, while in 37 areas there were almost no signs of life.

In the Baltic Sea 50-60 meters below the surface, oxygen levels are close to zero, while hydrogen sulfide, nitrogen and phosphorus are relatively abundant. The automatic purification of seawater in the waters of the Baltic Sea is very slow.

Ye Chao heard that if this situation continues, the creatures in this area are very likely to face extinction.