The Baltic Sea is located between two climate zones

The climate of the Baltic Sea is influenced by a temperate marine zone and a subarctic continental zone. That´s why the local climate is very variable.

The Baltic Sea weather is affected by two major atmospheric pressure systems: the low-pressure system in the North Atlantic and the high pressure one in the Azores. In addition, the temperature of the Baltic Sea region is also affected by the Eurasian air pressure system, where air pressure is high in winter and low in summer.

Weather conditions in the Baltic Sea region depend on the current location of the polar weather fronts and the strength of the western winds. These, in turn, vary considerably both seasonally and over longer periods. In the southern Baltic Sea, winter temperatures can be above zero, while there are freezing temperatures on the shores of the Gulf of Bothnia. In summer, with strong solar radiation throughout the Baltic Sea, temperature differences are much smaller.

In addition, there are local winds, generated due to the temperature differences between the land and the sea. At night, the land surface may cool more than the seawater, resulting wind blowing from the land out to sea. The opposite, onshore winds, arise due to the land warming faster than the sea.

 A view to a freezing sea in sunshine
Freezing sea

Storms from west

Storms which begin in the North Atlantic pass over the Baltic Sea. Westerly winds are most common in the autumn and winter. In April and May the winds are weak and variable in direction, while in summer, winds are usually light.

Storms are essential for the Baltic Sea as they strongly mix stratified layers of seawater, for example. At times, they drive much saltier and oxygenated water from the North Sea into the Baltic Sea. These so-called salt pulses bring oxygen to deep waters in the Baltic.

 Dark clouds and a sailboat
Frontal precipitation

The thermal memory of the sea is long

At sea, the air temperature depends on the temperature of the sea surface, which itself is dependent on the amount of radiation received from the sun. In autumn time the sea slowly releases its stored heat into the air. Conversely, in springtime the sea is cooler than the air and heat is transferred from the air to the sea.

The temperature of the near-surface air of the Baltic Sea region has generally increased significantly from the situation in the late 19th century to the present day. However, this temperature increase has not been continuous, and only the differences between decades have been great.


NAO index

The strength of the western winds is illustrated by NAO index (North Atlantic Oscillation). The NAO index is positive when there is high air pressure in the south and low air pressure in the north. At this time, temperate west winds dominate. In turn, the NAO index is negative when there is high pressure in the north and vice versa in the south and the winds blow from the north and east.

The NAO index variates also in a long turn. For example, in the 1960s, when the NAO index was strongly negative, winds were exceptionally weak. When the NAO index was very high in the early 1990s, winter storms were both widespread and severe. Wind conditions were average in the first decade of the 2000s.

Changes in climate over the past few decades have also had a clear impact on the amount of salt pulses entering the Baltic Sea. The NAO index changed from negative to positive in the mid-1970s and salt pulses declined significantly at this time. As a result, oxygen levels in the deeps of the central Baltic Sea began to remain extremely poor for years at a time.

 Sunset in the horizon

Precipitation increases, climate is warming

Global climate change also affects the climate in the Baltic Sea region. In addition, climate change affects, among others, water levels and ice conditions, as well as the chemical and biological conditions of the Baltic Sea.

Precipitation has varied in the Baltic Sea region, both seasonally and regionally. However, the general trend is that precipitation seems to have increased during the winter and spring in the second half of the 20th century, probably due to climate change.

Global warming is expected to have a major impact on the seasonal variation of river flows as the length of both snow and ice periods decreases. The amount of water flowing from rivers into the Baltic Sea has a significant impact on both its physical and ecological processes.