Air pressure affects sea level

Higher air pressure gives lower sea levels: an increase in air pressure of 1 hPa lowers the water level by 1 cm. Sea level varies from day to day and week to week, depending on the weather situation. Air pressure has a direct influence on the sea level.

High air pressure exerts a force on the surroundings and results in water movement. So high air pressure over a sea area corresponds to low sea level and conversely low air pressure (a depression) results in higher sea levels. This is called the inverse barometer effect.

Seasonal variations

The average sea level during a year is 0 cm and the average air pressure is 1013 hPa. Since the air pressure normally varies between 950 and 1050 hPa during a year, the expected variation in sea level due to air pressure is between +63 cm and -37 cm around mean sea level.

Water levels at a particular location are not only affected by the local air pressure but also by other factors, so this simple correlation is rarely observed.

Sea levels in north-western Europe are often high during autumn and winter when there are frequent depressions and strong westerly winds, but low during the spring and summer when high pressure and gentle winds dominate.

The sea surface on the Baltic can slope significantly both from north to south and from west to east. Deep low pressure passages over the Bothnian Bay, combined with high pressure over the southern Baltic can create sea level differences of up to 2 m.

Baltic Sea: a giant bathtub

Strong winds can cause a set-up in the Baltic Sea level, pushing the water up against the coast, so that there is a (gentle) slope in sea level from one side of the sea to the other. As the wind changes, it can no longer maintain this sea level slope, and the water is released starting an oscillation – a seiche - like water in a bathtub.

The oscillations continue back and forth many times, being slowly dissipated by friction. The resulting effect is a standing wave. The period between two high water incidents is governed by the depth of water and the breadth of the sea area. Oscillations in the Baltic from north to south have a period of 4 days and may continue for several weeks. The amplitude in northern Sweden can reach 50 cm.

Another oscillation can form between the Gulf of Finland and the south western Baltic. This seiche has a period of 27 hours. These oscillations have least effect in the middle of the sea. The sea level station at Landsort is relatively central to the Baltic and so is not much affected by these oscillations. As a result, sea level measurements from Landsort are a good guide to the water level in the Baltic over longer time periods.

Write a new comment: